1. Comparing the Battery with other Power Sources

EfficiencyThe battery is highly efficient. Below 70 percent charge, the charge efficiency is close to 100 percent and the discharge losses are only a few percent. In comparison, the energy efficiency of the fuel cell is 20 to 60 percent, and the thermal engines is 25 to 30 percent. (At optimal air intake speed and temperature, the GE90-115 on the Boeing 777 jetliner is 37 percent efficient.)InstallationThe sealed battery operates in any position and offers good shock and vibration tolerance. This benefit does not transfer to the flooded batteries that must be installed in the upright position. Most IC engines must also be positioned in the upright position and mounted on shock- absorbing dampers to reduce vibration. Thermal engines also need air and an exhaust.Operating costLithium- and nickel-based batteries are best suited for portable devices; lead acid batteries are economical for wheeled mobility and stationary applications. Cost and weight make batteries impractical for electric powertrains in larger vehicles. The price of a 1,000-watt battery (1kW) is roughly $1,000 and it has a life span of about 2,500 hours. Adding the replacement cost of $0.40/h and an average of $0.10/kWh for charging, the cost per kWh comes to about $0.50. The IC engine costs less to build per watt and lasts for about 4,000 hours. This brings the cost per 1kWh to about $0.34. [BU-1101, Battery Against Fossil Fuel] MaintenanceWith the exception of watering of flooded lead batteries and discharging NiCds to prevent “memory,” rechargeable batteries require low maintenance. Service includes cleaning of corrosion buildup on the outside terminals and applying periodic performance checks.Service lifeThe rechargeable battery has a relatively short service life and ages even if not in use. In consumer products, the 3- to 5-year lifespan is satisfactory. This is not acceptable for larger batteries in industry, and makers of the hybrid and electric vehicles guarantee their batteries for 8 to 10 years. The fuel cell delivers 2,000 to 5,000 hours of service and, depending on temperature, large stationary batteries are good for 5 to 20 years.Temperature extremesLike molasses, cold temperatures slow the electrochemical reaction and batteries do not perform well below freezing. The fuel cell shares the same problem, but the internal combustion engine does well once warmed up. Charging must always be done above freezing. Operating at a high temperature provides a performance boost but this causes rapid aging due to added stress. [BU0502, Discharging at High and Low Temperatures] Charge timeHere, the battery has an undisputed disadvantage. Lithium- and nickel-based systems take 1 to 3 hours to charge; lead acid typically takes 14 hours. In comparison, filling up a vehicle only takes a few minutes. Although some electric vehicles can be charged to 80 percent in less than one hour on a high-power outlet, users of electric vehicles will need to make adjustments.DisposalNickel-cadmium and lead acid batteries contain hazardous material and cannot be disposed of in landfills. Nickel-metal-hydrate and lithium systems are environmentally friendly and can be disposed of with regular household items in small quantities. Authorities recommend that all batteries be recycled. 

Select between maximum runtime, long service life, small size and low cost.

Rechargeable batteries play an important role in our lives and many daily chores would be unthinkable without the ability to recharge an empty battery. As the battery improves, an increasing number of devices are fitted with rechargeable batteries. The points of interest are good runtimes, economical price, long life, large loading capabilities, safe operation, ease of

storage and disposal.

The most common rechargeable batteries are Lead acid, NiCd, NiMH and Li-ion. Lead Acid  – This is the oldest rechargeable battery system. Lead acid is rugged,

forgiving if abused and is economically priced; but it has a low specific energy and limited cycle life. Lead acid is used for wheelchairs, golf cars, personnel carriers, emergency lighting and uninterruptible power supply (UPS). Lead is toxic and cannot be disposed in landfills. 

Nickel-cadmium  – Mature and well understood, NiCd is used where long service life, high discharge current and extreme temperatures are required. NiCd is one of the most rugged and enduring batteries; it is the only chemistry that allows ultra-fast charging with minimal stress. Main applications are power tools, medical devices, aircraft batteries and UPS. Due to environmental concerns, NiCd is being replaced with other chemistries. Cadmium is toxic. 

Nickel-metal-hydride  – A practical replacement for NiCd, NiMH has only mild toxic metals and provides higher specific energy. NiMH is used for medical instruments, hybrid cars and industrial applications. NiMH is also available in AA and AAA cells for consumer use. 

Lithium-ion  – Most promising battery chemistry, Li-ion is replacing many applications that were previously served by lead and nickel-based batteries. More delicate than most other chemistries, Li-ion needs a protection circuit for safety. Li-ion is more expensive than most other batteries but high cycle count and low maintenance reduce the cost per cycle over many other chemistries. 

Battery research gravitates towards lead- and lithium-based batteries. AGM (Absorbent Glass Mat) is a major battery type in the lead acid family and lead acid with carbon additives are making progress by allowing faster charge and increasing cycle life. (See BU-202: New Lead Acid Systems). Lead acid is estimated to grow in market share, but the bulk of battery growth is with Li-ion. (See BU-204: How do Lithium Batteries Work?) 

Table 1 compares the characteristics of the four commonly used rechargeable battery systems showing average performance ratings at time of publication. Li-ion is divided into many different types, each is described in more detail in BU-205: Types of Lithium-ion. The batteries are so named by their active materials that in most cases are the cathode. The traditional Li-ion systems are cobalt, manganese and phosphate.

Missing in the list is the popular lithium-ion-polymer. This battery gets its name from the unique separator and electrolyte system that energizes the battery. Once fully developed and refined, the polymer architecture has a large potential.

Also missing is the rechargeable lithium-metal. This battery is subject to more development in controlling dendrite growth, which can compromise safety. Once resolved, Li-metal has the prospect of becoming an alternative battery choice with extraordinary high specific energy and good specific power. Other promising lithium-based batteries are making advancements, albeit very incremental. These are described under BU-211: Alternative Battery Systems and BU-212: Experimental Rechargeable Batteries.Unique packaging has made Li-ion available and affordable. The most common format is the 18650, a cell that measures 18mm in diameter and is 65mm long. This format offers the largest variety, highest specific energy, lowest cost per Wh and perhaps also provides the most reliable service. Over 2.5 billion 18650 cells have been produced in 2013. (See BU-301: A look at Old and New Battery Packaging.)

Three unique categories of Li-ion have emerged, each addressing different applications. Available in 18650 (and other formats) these categories are the Energy Cell energy with high capacity, the Power Cell with high loading capabilities and the Hybrid Cells that satisfies both

requirements at compromised performance levels. Other cells are made for extended temperature range and extremely long cycle counts.

Table 1: Characteristics of commonly used rechargeable batteries. The figures are based on average ratings of commercial batteries at time of publication. Specialty batteries with above-average ratings are excluded.

1. Combining cobalt, nickel, manganese and aluminum raises energy density up to 250Wh/kg.

2. Cycle life is based on the depth of discharge (DoD). Shallow DoD prolongs cycle life.3. Cycle life is based on battery receiving regular maintenance to prevent memory.4. Ultra-fast charge batteries are specially made (See BU-401a: Fast and Ultra-fast

Chargers)

5. Self-discharge  is highest immediately after charge. NiCd loses 10% in the first 24 hours, then declines to 10% every 30 days. High temperature and age increase self-discharge.

6. 1.25V is traditional; 1.20V is more commonly. (See BU-303: Confusion with Voltages).

7. Manufacturers may rate voltage higher because of low internal resistance (marketing).8. Capable of high current pulses; needs time to recuperate.9. Do not charge Li-ion below freezing. See BU-410: Charging at High and Low

Temperatures.10. Maintenance may be in the form of equalizing or topping charge to prevent sulfation.11. Protection circuit cuts off below about 2.20V and above 4.30V on most Li-ion;

different voltage settings apply for lithium-iron-phosphate.12. Li-ion may have lower cost-per-cycle than lead acid.

2. Types of Lithium-ion

Lithium-ion is named after their active material, written in full or specified by their chemical symbols. A series of letters and numbers strung together can be hard to pronounce and remember and battery chemistries are also given in abbreviated letters. 

For example, lithium cobalt oxide, one of the most common Li-ion, has the chemical symbols of LiCoO2 and abbreviation LCO. For reason of simplicity, a short form as been assigned to a chemistry which for this battery is Li-cobalt. Cobalt is the main active material that gives this battery character.

This section summarizes six of the most common Li-ion: Lithium Cobalt Oxide (LiCoO2), LithiumManganese Oxide (LiMn2O4), Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC), LithiumIron Phosphate (LiFePO4), Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2), and Lithium Titanate. (Li4Ti5O12). All readings are average estimates at time of writing. Lithium Cobalt Oxide(LiCoO2)Its high specific energy makes Li-cobalt the popular choice for cell phones, laptops and digital cameras. The battery consists of a cobalt oxide cathode and a graphite carbon anode. The cathode has a layered structure and during discharge, lithium ions move from the anode to the cathode. The flow reverses on charge. The drawback of Li-cobalt is a relatively short life span, low thermal stability and limited load capabilities (specific power). Figure 1 illustrates the structure.

Figure 1: Li-cobalt

structure

The cathode has a layered

structure. During discharge

the lithium ions move from

the anode to the cathode; on

charge the flow is from

cathode to anode.Courtesy of Cadex

Li-cobalt cannot be charged and discharged at a current higher than its rating. This means that an 18650 cell with 2,400mAh can only be charged and discharged at 2,400mA. Forcing a fast charge or applying a load higher than 2,400mA causes overheating and undue stress. For optimal fast charge, the manufacturer recommends a C-rate of 0.8C or 1920mA. See BU-402: What is C-rate). The mandatory battery protection circuit limits the charge and discharge rate to a safe level of about 1C.The hexagonal spider graphic (Figure 2) summarizes the performance of Li-cobalt in terms of specific energy or capacity that relates to runtime; specific power or the ability to deliver high current; safety; performance at hot and cold temperatures; life span reflecting cycle life and longevity; and cost. Other characteristics of interest not shown in the spider webs are toxicity, fast-charge capabilities, self-discharge and shelf life.

Figure 2: Snapshot of an average

Li-cobalt battery

Li-cobalt excels on high specific

energy but offers only moderate

performance specific power, safety

and life span.

Courtesy of Cadex

Summary Table

Lithium Cobalt Oxide: LiCoO2 cathode (~60% Co), graphite anode                                      

Short form: LCO or Li-cobalt.                                                                                                             Since

1991

Voltage, nominal 3.60V

Specific energy (capacity)150–200Wh/kg. Specialty cells provide up to

240Wh/kg.

Charge (C-rate)

0.7–1C, charges to 4.20V (most cells); 3h charge

typical. Charge current above 1C shortens battery

life.

Discharge (C-rate)1C; 2.50V cut off. Discharge current above 1C

shortens battery life.

Cycle life500–1000, related to depth of discharge, load,

temperature

Thermal runaway150°C (302°F). Full charge promotes thermal

runaway

Applications Mobile phones, tablets, laptops, cameras

Comments

Very high specific energy, limited specific power.

Cobalt is expensive. Serves as Energy Cell. Market

share has stabilized.

Table 3: Characteristics of Lithium Cobalt Oxide Lithium Manganese Oxide (LiMn2O4)Li-ion with manganese spinel was first published in the Materials Research Bulletin in 1983. In 1996, Moli Energy commercialized a Li-ion cell with lithium manganese oxide as cathode material. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improved current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life are limited.Low internal cell resistance promotes fast charging and high-current discharging. In an 18650 package, Li-manganese can be discharged at currents of 20–30A with moderate heat buildup. It is also possible to apply one-second load pulses of up to 50A. A continuous high load at this current would cause heat buildup and the cell temperature cannot exceed 80C (176F). Li-manganese is used for power tools, medical instruments, as well as hybrid and electric vehicles.

Figure 4 shows the crystalline formation of the cathode in a three-dimensional framework.

This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation.

Figure 4: Li-manganese structureThe cathode crystalline formation of lithiummanganese oxide has a three-dimensional framework structure that appears after initial formation. Spinel provides low resistance but has a more moderate specific energy than cobalt. Courtesy of Cadex

Li-manganese has a capacity that is roughly one-third lower than Li-cobalt. Design flexibility allows engineers to maximize the battery for either optimal longevity (life span), maximum load current (specific power) or high capacity (specific energy). For example, the long-life version in the 18650 cell has a moderate capacity of 1,100mAh; the high-capacity version is 1,500mAh.

Figure 5 shows the spider web of a typical Li-manganese battery. The characteristics appear marginal but newer designs have improved in terms of specific power, safety and life span.

Figure 5: Snapshot of a pure Li-

manganese battery

Most modern manganese-based

Li-ion systems include a blend of

nickel and cobalt. Typical

designations are LMO/NMC for

lithium manages oxide/nickel-

manganese-cobalt.

Courtesy of BCG research

Most Li-manganese batteries “partner” with Lithium Nickel Manganese Cobalt Oxide (NMC) to improve the specific energy and prolong the life span. This combination brings out the best in each system and the so-called LMO (NMC) is chosen for most electric vehicles, such as the Nissan Leaf, Chevy Volt and BMW i3. The LMO part of the battery, which is about 30 percent on the Chevy Volt, provides high current boost on acceleration, the NMC part gives the long driving range.

Summary Table

Lithium Manganese Oxide: LiMn2O4 cathode. graphite anode                                                              

Short form: LMO or Li-manganese (spinel structure)                                                                    Since

1996

Voltage, nominal 3.70V (some may be rated 3.80V)

Specific energy (capacity) 100–150Wh/kg

Charge (C-rate)0.7–1C typical, 3C maximum, charges to 4.20V

(most cells)

Discharge (C-rate)1C; 10C possible with some cells, 30C pulse

(5s), 2.50V cut-off

Cycle life300–700 (related to depth of discharge,

temperature)

Thermal runaway250°C (482°F) typical. High charge promotes

thermal runaway

ApplicationsPower tools, medical devices, electric

powertrains

Comments

High power but less capacity; safer than Li-

cobalt; commonly mixed with NMC to improve

performance.

Table 6: Characteristics of Lithium Manganese Oxide

 

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC)

Leading battery manufacturers focus on a cathode combination of nickel-manganese-cobalt (NMC). Similar to Li-manganese, these systems can be tailored to serve as Energy Cells or Power Cells. For example, NMC in an 18650 cell for moderate load condition has a capacity of about 2,800mAh and can deliver 4–5A; NMC in the same cell optimized for specific power has a capacity of only about 2,000mWh but delivers a continuous discharge current of 20A. A silicon-based anode will go to 4,000mAh but at reduced loading capability and shorter cycle life.

The secret of NMC lies in combining nickel and manganese. An analogy of this is table salt in which the main ingredients sodium and chloride are toxic on their own but mixing them

serves as seasoning salt and food preserver. Nickel is known for its high specific energy but poor stability; manganese has the benefit of forming a spinel structure to achieve low internal resistance but offers a low specific energy. Combining the metals enhances each other strengths.

NMC is the battery of choice for power tools, e-bikes and other electric powertrains. The cathode combination is typically one-third nickel, one-third manganese and one-third cobalt. This offers a unique blend that also lowers the raw material cost due to reduced cobalt content. Other combinations, such as NCM, CMN, CNM, MNC and MCN are also offered in which the metal content of the cathode deviates from the 1/3-1/3-1/3 formula. Manufacturers keep the ratio a well-guarded secret. Figure 7 demonstrates the characteristics of the NMC.

Figure 7: Snapshot of NMC

NMC has good overall

performance and excels on

specific energy. This battery is the

preferred candidate for the electric

vehicle and has the lowest self-

heating rate.

Courtesy of BCG research

Summary Table

Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2. cathode, graphite anode             Since 2008

Short form: NMC (NCM, CMN, CNM, MNC, MCN similar with different metal combinations)

Voltage, nominal 3.60V, 3.70V

Specific energy (capacity) 150–220Wh/kg

Charge (C-rate)0.7–1C, charges to 4.20V, some go to 4.30V; 3h charge typical.

Charge current above 1C shortens battery life.

Discharge (C-rate) 1C; 2C possible on some cells; 2.50V cut-off

Cycle life 1000–2000 (related to depth of discharge, temperature)

Thermal runaway 210°C (410°F) typical. High charge promotes thermal runaway

Applications E-bikes, medical devices, EVs, industrial

CommentsProvides high capacity and high power. Serves as Hybrid Cell.

Favorite chemistry for many uses; market share is increasing.

Table 8: Characteristics of Lithium Nickel Manganese Cobalt Oxide (NMC)

 Lithium Iron Phosphate(LiFePO4)In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries. Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused.

Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a pronged time. (See BU-808: How to Prolong Lithium-based Batteries). As trade-off, the lower voltage of 3.2V/cell reduces the specific energy to less than Li-manganese. As with most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception. Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. Figure 9 summarizes the attributes of Li-phosphate.

Li-phosphate is often used to replace the lead acid starter battery. Four cells in series produce 12.80V, a similar voltage to six 2V lead acid cells in series. Vehicles charge lead acid to 14.40V (2.40V/cell). With four Li-phosphate cells in series, each tops at 3.60V, which is the correct full-charge voltage. At this point, the charge should be disconnected but Li-phosphate is tolerant to some overcharge, however keeping the voltage at 14.40V for a prolonged time, as most vehicle do on a long drive, could stress Li-phosphate. Cold temperature operation could also be an issue with Li-phosphate as starter battery.

Figure 9: Snapshot of a typical

Li-phosphate battery

Li-phosphate has excellent safety

and long life span but moderate

specific energy and a lower

voltage than other lithium-based

batteries. LFP also has higher self-

discharge compared to other

lithium-ion systems.

Courtesy of Cadex

Summary Table

Lithium Iron Phosphate: LiFePO4 cathode, graphite anode                                                   

Short form: LFP or Li-phosphate                                                                                                      

Since 1996

Voltage, nominal 3.20V, 3.30V

Specific energy (capacity) 90–120Wh/kg

Charge (C-rate) 1C typical, charges to 3.65V; 3h charge time typical

Discharge (C-rate)1C, 25C on some cells; 40A pulse (2s); 2.50V cut-off

(lower that 2V causes damage)

Cycle life1000–2000 (related to depth of discharge,

temperature)

Thermal runaway 270°C (518°F) Very safe battery even if fully charged

ApplicationsPortable and stationary needing high load currents

and endurance

Comments

Very flat voltage discharge curve but low capacity.

One of safest

Li-Ions. Used for special markets. Elevated self-

discharge.

Table 10: Characteristics of Lithium Iron Phosphate

 Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2)Lithium Nickel Cobalt Aluminum Oxide battery, or NCA, has been around since 1999 for special application and shares similarity with NMC by offering high specific energy and reasonably good specific power and a long life span. These attribute made Elon Musk choose NCA for the Tesla EV’s. Less flattering are safety and cost. Figure 11 summarizes the six key

characteristics. NCA is a further development of lithium nickel oxide; adding aluminum gives the chemistry greater stability.

Figure 11: Snapshot of NCA

High energy and power densities,

as well as good life span, make the

NCA a candidate for EV

powertrains. High cost and

marginal safety are negatives.

Courtesy of Cadex

Summary Table

Lithium Nickel Cobalt Aluminum Oxide: LiNiCoAlO2 cathode (~9% Co), graphite anode               

Short form: NCA or Li-aluminum.                                                                                                      Since 1999

Voltage, nominal 3.60V

Specific energy (capacity) 200-260Wh/kg; 300Wh/kg predictable

Charge (C-rate)0.7C, charges to 4.20V (most cells), 3h charge typical, fast

charge possible with some cells

Discharge (C-rate)1C typical; 3.00V cut-off; high discharge rate shortens battery

life

Cycle life 500 (related to depth of discharge, temperature)

Thermal runaway150°C (302°F) typical, High charge promotes thermal

runaway

Applications Medical devices, industrial, electric powertrain (Tesla)

Comments Shares similarities with Li-cobalt. Serves as Energy Cell.

Table 12: Characteristics of Lithium Nickel Cobalt Aluminum Oxide

 Lithium Titanate (Li4Ti5O12)Batteries with lithium titanate anodes have been known since the 1980s. Li-titanate replaces the graphite in the anode of a typical lithium-ion battery and the material forms into a spinel structure. The cathode is graphite and resembles the architecture of a typical lithium-metal battery. Li-titanate has a nominal cell voltage of 2.40V, can be fast-charged and delivers a high discharge current of 10C, or 10 times the rated capacity. The cycle count is said to be higher than that of a regular Li-ion. Li-titanate is safe, has excellent low-temperature discharge characteristics and obtains a capacity of 80 percent at –30C (–22F). However, the battery is expensive and at 65Wh/kg the specific energy is low, rivalling that of NiCd. Li-titanate charges to 2.80V/cell, and the end of discharge is 1.80V/cell. Figure 13 illustrates the characteristics of the Li-titanate battery. Typical uses are electric powertrains and UPS. 

Figure 13: Snapshot of Li-

titanate

Li-titanate excels in safety, low-

temperature performance and life

span. Efforts are being made to

improve the specific energy and

lower cost.

Courtesy of BCG research

 

Summary Table

Lithium Titanate: Graphite cathode; Li4Ti5O12 (titanate) anode                                                            

Short form: LTO or Li-titanate                                                                                                          

Since 2008

Voltage, nominal 2.40V

Specific energy (capacity) 70–80Wh/kg

Charge (C-rate) 1C typical; 5C maximum, charges to 2.85V

Discharge (C-rate) 10C possible, 30C 5s pulse; 1.80V cut-off  on LCO/LTO

Cycle life 3,000–7,000

Thermal runaway One of safest Li-ion batteries

Applications UPS, electric powertrain (Mitsubishi i-MiEV, Honda Fit EV)

CommentsLong life, fast charge, wide temperature range but low specific

energy and expensive. Among safest Li-ion batteries.

Table 14: Characteristics of Lithium TitanateFigure 15 compares the specific energy of lead, nickel- and lithium-based systems. While NCA is the clear winner by storing more capacity than other systems, this only applies to specific energy. In terms of specific power and thermal stability, Li-manganese and Li-phosphate are superior. Li-titanate may have low capacity but this chemistry outlives most other secondary batteries in terms of life span. It has also the best cold temperature performance. As we move towards electric powertrains, safety and cycle life are becoming more important than capacity alone.

Figure 15: Typical specific energy of lead, nickel- and lithium-based batteries

NCA enjoys the highest specific energy; however, manganese and phosphate are superior in

terms of specific power and thermal stability. Li-titanate has the best life span.

3. Finding the Optimal Runtime and Power Ratio of Li-ion

Optimizing the selection of a Li-ion system that includes specific energy, specific power and runtime.

Batteries can be made to perform as an Energy Cell that stores a large amount of energy, or a Power Cell that is capable to deliver high load currents. An analogy is a water flask that is designed to hold a large volume of liquid while offering a wide opening to permit quick pouring.

The physical dimensions of a battery are specified by volume in liter (l) and kilogram (kg). Adding dimension and weight provides specific energy in Wh/kg, power density in Wh/l and specific power in W/kg. Most batteries are rated in Wh/kg, revealing how much energy a given weight can generate. Wh/l denotes watt/hours per liter. See Battery Definition and what they mean.)

The relationship between energy and power of a battery can best be represented in a Ragone plot. This plot places energy in Wh on the horizontal x-axis and power in W on the vertical y-axis. The diagonal lines across the field disclose the time the battery cells can deliver energy at various loading conditions. The derived power curve provides a clear demarcation line of what level of power a battery can deliver. The Ragone plot is logarithmic to display performance profiles of very high and low values.

Figure 1 illustrates the Ragone plot reflecting the discharge energy and power of four classic lithium-ion systems packaged in 18650 cells. The battery chemistries featured are the most common power-based lithium-ion systems, which include lithium-iron phosphate (LFP), lithium-manganese oxide (LMO), and nickel manganese cobalt (NMC).

Figure 1: Ragone plot reflects Li-ion 18650 cells. Four Li-ion systems are compared for discharge power and energy as a function of time.Courtesy of Exponent

Legend: The A123 APR18650M1 is a lithium iron phosphate (LiFePO4) with 1,100mAh and a continuous discharge current of 30A. The Sony US18650VT and Sanyo UR18650W are manganese–based Li-ion cells of 1500mAh each with a continuous discharge current of 20A. The Sanyo UR18650F is a 2,600mAh cell for a moderate 5A.discharge. This cell provides the highest discharge energy but has the lowest discharge power.The Sanyo UR18650F [4] has the highest specific energy and can power a laptop or e-bike for many hours at a moderate load. The Sanyo UR18650W [3], in comparison, has a lower specific energy but can supply a current of 20A. The A123 [1] has the lowest specific energy but offers the highest power capability by delivering 30A of continuous current.

The Ragone plot helps choosing the best Li-ion system to satisfy optimal discharge power and energy as a function of discharge time. If an application calls for very high discharge current, the 3.3 minute diagonal line on the chart points to the A123 (Battery 1) as a good pick; it can deliver up to 40 Watts of power for 3.3 minutes. The Sanyo F (Battery 4) is slightly lower and delivers about 36 Watts. Focusing on discharge time and following the 33 minute discharge line further down, Battery 1 (A123) only delivers 5.8 Watts for 33 minutes before the energy is depleted whereas the higher capacity Battery 4 (Sanyo F) can provide roughly 17 Watts for the same time; its limitation is lower power.

For best results, battery manufacturers take the Ragone snapshot on new cells, a condition that is only valid for a short time. When calculating power and energy thresholds, design engineers must include battery fade that will develop as part of cycling and aging. A battery operated systems should still provide full function with a battery that has faded to 70 or 80 percent. A further consideration is temperature as a battery loses power when cold. The Ragone plot does not include these discrepancies.  

It should be noted that loading a battery to its full power capability increases stress and

shortens life. When a high current draw is needed continuously, the battery pack should be made larger. Tesla does this with their Model S cars by doubling and tripling the battery. An analogy is a heavy truck fitted with a large diesel engine that provides long and durable service as opposed to installing a souped-up engine of sports car with similar horsepower.

The Ragone plot also calculates power requirements of other energy sources and storage devices, such as capacitors, flywheels, flow batteries and fuel cells. As fuel cells and internal combustion engines draw fuel from a tank, a conflict develops because energy-delivery can be made continuous. The Ragone plot may also be deployed to establish the optimal energy/power ratio and loading condition of a renewable power source, such as solar cells and wind turbines.

 Cycling Performance

Learn about the rugged old NiCd and compare with NiMH and Li-ion

To compare older and newer battery systems, Cadex tested a large volume of nickel-cadmium, nickel-metal-hydride and lithium ion batteries used in portable for communication devices. Preparations included an initial charge, followed by a regime of full discharge/charge cycles at 1C. The tables show the capacity in percent, DC resistance measurement and self-discharge obtained from time to time by reading the capacity loss incurred during a 48-hour rest period. The tests were carried out on the Cadex 7000 Series battery analyzers.

Nickel-cadmium

In terms of life cycling, nickel-cadmium is the most enduring battery. Figure 1 illustrates the capacity, internal resistance and self-discharge of a 7.2V, 900mA pack with standard NiCd cells. Due to time constraints, the test was terminated after 2,300 cycles. The capacity remained steady; the internal resistance stayed low at 75mWand the self-discharge was stable. This battery receives a grade “A” rating for almost perfect performance.

Figure 1: Performance of standard NiCd (7.2V, 900mAh)This battery receives an “A” rating for a stable capacity, low internal resistance and moderate self-discharge over many cycles.Courtesy of Cadex

The ultra-high-capacity nickel-cadmium offers up to 60 percent higher specific energy compared to the standard version, however, this comes at the expense of reduced cycle life. In Figure 2 we observe a steady drop of capacity during 2,000 cycles, a slight increase in internal resistance and a rise in self-discharge after 1,000 cycles.

Figure 2: Performance of ultra-high-capacity NiCd (6V, 700mAh)This battery offers higher specific energy than the standard version at the expense of reducedcycle life.Courtesy of Cadex

Nickel-metal-hydride

Figure 3 examines NiMH, a battery that offers high specific energy but loses capacity after the 300-cycle mark. There is also a rapid increase in internal resistance after cycle count 700 and rise in self-discharge after 1000 cycles. The test was done on an older generation NiMH. 

Figure 3: Performance of NiMH (6V, 950mAh).This battery offers good performance at first but past 300 cycles, the capacity, internal resistance and self-discharge start to increase rapidly. Newer NiMH has better results.Courtesy of Cadex

Lithium-ion

Figure 4 examines the capacity fade of a modern Li-ion Power Cell at a 2A, 10A 15A and 20A discharge. Stresses increase with higher load currents, and this also applies to rapid and ultra-fast charging. (SeeBU-401a: Ultra-fast charging of Li-ion.)

Li-ion manufacturers often do not specify the rise of internal resistance and self-discharge as a function of cycling. Advancements have been made with electrolyte additives to keep the resistance low through most of the battery life. The self-discharge of Li-ion is low and is in par with lead acid.

Figure 4: Cycle characteristics of IHR18650C by E-One Moli. (3.6V, 2,000mA). 18650 Power Cell was charged with 2A and discharged at 2, 10, 15 and 20A. The internal resistance and self-discharge are N/A.Courtesy of E-One Moli Energy

Batteries tested in a laboratory tend to provide better results than in the field. Elements of stress in everyday use do not always transfer well into test laboratory. Aging plays a minimal role in a lab because the batteries are cycled over a period of a few months rather than the expected service life of several years. The temperature is often moderate and the batteries are charged under controlled charging condition and with approved chargers.The load signature also plays a role, and the nickel-based batteries were discharged with a DC load. All batteries deliver a slightly lower cycle life if discharged with pulses. (See BU-501: Basics About Discharging.) If a battery must repeatedly be loaded to peak currents, it is advised to install a pack with higher Ah rating.

Types of Battery Cells

Compare the pros and cons of the cylindrical cell, button cell, prismatic cell and pouch

As batteries were beginning to be mass-produced, the jar design changed to the cylindrical format. The large F cell for lanterns was introduced in 1896 and the D cell followed in 1898. With the need for smaller cells, the C cell followed in 1900, and the popular AA was introduced in 1907. See BU-301: Standardizing Batteries into Norms.

Cylindrical Cell

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.

Most lithium and nickel-based cylindrical cells include a positive thermal coefficient (PTC) switch. When exposed to excessive current, the normally conductive polymer heats up and becomes resistive, acting as short circuit protection. Once the short is removed, the PTC cools down and returns to conductive state.

Most cylindrical cells also feature a pressure relief mechanism and the most simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and dry-out may occur after the membrane breaks. Re-sealable vents with a spring-loaded valve are the preferred design. Some Li-ion cells connect the pressure relief valve to an electrical fuse that opens the cell if an unsafe pressure builds up. Figure 1 shows a cross section of a cylindrical cell.

Typical applications for the cylindrical cell are power tools, medical instruments, laptops and e-bikes. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.

Figure 1: Cross section of alithium-ion cylindrical cellThe cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities.Courtesy of Sanyo

Nickel-cadmium provided the largest variety of cell choices and some spilled over to nickel-metal-hydride, but not to lithium-ion as this chemistry established its own formats. The 18650 illustrated in Figure 2 remains one of the most popular cell packages. Typical applications are power tools, medical devices, laptops and e-bikes.

Figure 2: Popular 18650 lithium-ion cellThe metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter. Courtesy of Cadex

In 2013, 2.55 billion 18650 cells were produced; earlier with 2.2Ah and now mostly with a capacity of 2.8Ah. Some newer 18650 Energy Cells are 3.1Ah and the capacity will grow to 3.4Ah by 2017. Cell manufacturers prepare for the 3.9Ah 18650, a format that they hope will be made available at the same cost as the lower capacity versions.

The 18650 is the most optimized cell and offers the lowest cost per Wh. As consumers move to the flat designs, the 18650 is peaking and there is over-production. Batteries may eventually be made with flat cells but experts say that the 18650 will continue to lead the market. Figure 3 shows the over-supply situation that has been corrected thanks to the demand of the Tesla electric vehicles. 

Figure 3: Demand and supply of the 18650.

The demand for the 18650 would have peaked in 2011 had it not been for Tesla. The switch to a flat-design in consumer products and larger format for the electric powertrain will eventually saturate the 18650.

Courtesy Avicenne Energy

The larger 26650 cell with a diameter of 26mm instead of 18mm did not gain the same popularity as the 18650. The 26650 is commonly used in load-leveling systems with Li iron phosphate.

Some lead acid systems also borrow the cylindrical design. Known as the Hawker Cyclone, this cell offers improved cell stability, higher discharge currents and better temperature stability compared to the conventional prismatic design. The Hawker Cyclone has its own format.

Even though the cylindrical cell does not fully utilize the space by creating air cavities on side-by-side placement, the 18650 has a higher energy density than a prismatic/pouch Li-ion cell. The 3Ah 18650 delivers 248Wh/kg, whereas a modern pouch cell has only 143Ah/kg. The higher energy density of thecylindrical cell compensates for its less ideal stacking characteristics. The empty space can be used for cooling to improve thermal management.

Cell disintegration cannot always be prevented but propagation can. Cylindrical cells are often spaced apart to stop propagation should one cell take off. Spacing also helps in the cooling. In addition, a cylindrical design does not change size. A 5mm prismatic cell, in comparison, can expand to 8mm with use and allowances must be made. Button Cell

The button cell, also known as coin cell, enabled compact design in portable devices of the 1980s. Higher voltages were achieved by stacking the cells into a tube. Cordless telephones, medical devices and security wands at airports used these batteries.

Although small and inexpensive to build, the stacked button cell fell out of favor and gave way to more conventional battery formats. A drawback of the button cell is swelling if charged too rapidly. Button cellshave no safety vent and can only be charged at a 10- to 16-hour charge; however, newer designs claim rapid charge capability.

Most button cells in use today are non-rechargeable and are found in medical implants, watches, hearing aids, car keys and memory backup. Figure 4 illustrates the button cells with accompanying cross section. 

CAUTIONKeep button cells to out of reach of children. Swallowing a cell can cause serious health problems. SeeConcerns with Batteries.

 

Figure 4: Button cellsButton cells, also known as coin cells, offer small size and ease of stacking but do not allow fast charging. Most commercial button cells are non-rechargeable.Courtesy of Sanyo and Panasonic

 

Prismatic Cell

Introduced in the early 1990s, the modern prismatic cell satisfies the demand for thinner sizes. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. Others designs are wound and flattened into a pseudo-prismatic jelly roll. These cells are predominantly found in mobile phones, tablets and low-profile laptops and range from 800mAh to 4,000mAh. No universal format exists and each manufacturer designs its own.

Prismatic cells are also available in large formats. Packaged in welded aluminum housings, the cells deliver capacities of 20 to 30Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 5 shows the prismatic cell.

Figure 5: Cross sectionof a prismatic cellThe prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design.Courtesy of Polystor Corporation

 

The prismatic cell requires a slightly thicker wall to compensate for decreased mechanical stability compared to the cylindrical design. Some swelling due to gas buildup is normal. Discontinue using the battery if the distortion presses against the battery compartment.

Bulging batteries compromise safety and can damage equipment. Pouch Cell

In 1995 the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through, conductive foil-tabs are welded to the electrodes and brought to the outside in a fully sealed way. Figure 6 illustrates a pouch cell.

Figure 6: The pouch cellThe pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.Courtesy of Cadex

The pouch cell makes the most efficient use of space and achieves a 90–95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some support in the battery compartment. The pouch pack finds applications in consumer, military and automotive applications. No standardized pouch cells exist; each manufacturer designs its own.

Pouch packs are commonly Li-polymer and serve well as Power Cells by delivery high current. The capacity is lower than Li-ion in the cylindrical package and the flat-cell may be less durable. Expect 8–10 percent swelling over 500 cycles and make provision in the battery compartment for expansion. It is best not to stack pouch cells on top of each other but to lay them flat side by side. Prevent sharp edges that can stress the pouch cells as they expand.

Extreme swelling is a concern. Users of pouch packs have reported up to three percent swelling incidents on a poor batch run. The pressure created can crack the battery cover and in some cases break the display and electronic circuit boards. Discontinue using an inflated battery and do not puncture the bloating cell in close proximity to heat or fire. The escaping gases can ignite. Figure 7 shows a swollenpouch cell.

 Figure 7: Swelling pouch cellSwelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package.Courtesy of Cadex

Pouch cells are manufactured by adding a temporary “gasbag” on the side. During the first charge, gases escape into the gasbag, the gasbag is cut off and the pack is resealed as part of the finishing process. Subsequent charges should no longer produce gases. Ballooning indicates that the manufacturing process may have been flawed.

The prismatic and pouch cells have the potential for greater energy than the cylindrical format but the technology to produce large formats is not yet mature. The cost per kWh in the prismatic/pouch cell format is still higher than with the 18650 cell. As a comparison, the cost for the Nissan Leaf with Pouch/ Prismatic cells is $455/kWh; best practice (DoE/AABC) with this format is $350/kWh. The lowest price per kWh is the Tesla EV with the 18650 cells. Greenwich Strategy estimates the cost of the Tesla Gen III battery at $290/kWh.  

Summary

Rechargeable batteries are either flooded with the electrodes awash in electrolyte as in the lead acid, or cylindrical as in NiCd, NiMH and Li-ion. Along came the pouch cell, a format that is similar to packaging perishable food. Intended to be cheaper to manufacture and more flexible in form factor, refinements are needed to bring this amazing cell concept to the same performance level of the cylindrical version. In summary:

Cylindrical cell has superior performance, good mechanical stability and lends itself to automated manufacturing. The cell cycles well, offers a long calendar life, is low cost, but is heavy and has a low packaging density.

Prismatic cell are encased in aluminum or steel for stability. Jelly-rolled or stacked, the cell is space-efficient but costlier to manufacture, less consistent in performance, harder to manage thermally and may have a shorter cycle life than the cylindrical cell.

Pouch cell uses laminated architecture in a bag. It is light and cost-effective but exposure to humidity and high temperature can shorten life. A swelling factor of 8 to 10 percent over 500 cycles is normal.

Figure 8 compares the price in $US/Wh of the cylindrical, prismatic and pouch cell, also known as the laminated. While the cylindrical cell has been most economical to manufacture, flat-cell designs are getting competitive and battery experts predict a shift towards these cell formats, especially if the performance criteria of the cylindrical cell can be met.

Learn how to arrange batteries to increase voltage or gain higher load currents.

Battery packs achieve the desired operating voltage by connecting several cells in series; each

cell adds its voltage to the total terminal voltage. Parallel connection attains higher capacity

for increased current handling; each cell adds to the ampere/hour (Ah) count.

Some packs may consist of a combination of serial and parallel connections. Laptop batteries

commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two in parallel to boost

the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning four

cells in series and two in parallel. Insulating foil between the cells prevents the conductive

metallic skin from causing an electrical short.

Most battery chemistries lend themselves to serial and parallel connection. It is important to

use the same battery type with equal voltage and capacity (Ah) and never mix different makes

and sizes. A weaker cell would cause an imbalance. This is especially critical in a serial

configuration because a battery is only as strong as the weakest link in a chain. An analogy is

a chain in which the links represent the cells of a battery connected in series (Figure 1).

Figure 1: Comparing a battery with a chain

Chain links represent cells in series to increase voltage, doubling a link denote parallel connection to

boost current loading.

A weak cell may not quit immediately but will get exhausted more quickly than the strong

ones when in continued use. On charge, the low cell fills up before the strong ones because

there is less to fill and remains in over-charge longer than the others. On discharge, the weak

cell is empty first and gets hammered by the stronger brothers. Cells in multi-packs must be

matched, especially when used for demanding industrial applications. (See BU-803a: Cell

Mismatch, Balancing).

Single Cell Applications

The single-cell configuration is the most simplistic battery pack; the cell does not need

matching and the protection circuit on a small Li-ion cell can be kept simple. Typical

examples are mobile phones and tablets with one 3.60V Li-ion cell. Other uses of a single cell

are wall clocks, which typically use a 1.5V alkaline cell, wristwatches and memory backup,

most of which are very low power applications.

The nominal cell voltage for a nickel-based battery is 1.20V, alkaline 1.50V; silver-oxide

1.60V and lead acid 2.00V. Primary lithium batteries range between 3.00V and 3.90V. Li-ion

is 3.60V; Li-phosphate 3.20V and Li-titanate 2.40V.

Li-manganese and other lithium-based systems often use voltages of 3.70V and higher. This

has less to do with chemistry than marketing. For operational purposes they serve as 3.60V

cells. Low internal resistance keeps the voltage high under load and this advantage is utilized

to increase the voltage and watt-hour (Wh). (See BU-303 Confusion with Voltages)

Serial Connection

Portable equipment needing higher voltages use battery packs with two or more cells

connected in series. Figure 2 shows a battery pack with four 1.20V nickel-based cells in series

to produce 4.8V. In comparison, a six-cell lead acid string with 2V/cell will generate 12V, and

four Li-ion with 3.60V/cell will give 14.40V.

Figure 2: Serial connection of four cells (4S)

Adding cells in a string increases the voltage; the current remains the same.

Courtesy of Cadex

If you need an odd voltage of, say, 9.50 volts, connect five lead acid, eight NiMH or NiCd, or

three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher

than what the device specifies. A 12V supply might work in lieu of 9.50V; most battery-

operated devices can tolerate some over-voltage but quit on low voltage. Higher voltage

applications often have less tolerance, however.

Portable devices using higher voltages have the advantage of keeping the conductor size

down. Consumer cordless power tools run on 12V and 18V batteries; high-end models use

24V and 36V. Most e-bikes come with 36V Li-ion, some are 48V.

The car industry wanted to increase the starter battery from 12V (14V) to 36V, better known

as 42V, by placing 18 lead acid cells in series. Logistics of changing electrical components

and arcing problems on mechanical switches derailed the move. Mild hybrid cars experiment

with 48V Li-ion and use DC-DC conversion to 12V to feed the electrical system. Early hybrid

cars run on a 148V battery; electric vehicles have packs with 450–500V. Such a battery needs

more than 100 Li-ion cells in series.

High-voltage batteries require careful cell matching, especially when drawing heavy loads or

when operating at cold temperatures. With multiple cells connected in a string, the possibility

of one cell failing is real and this would break the circuit. A solid state switch that bypasses

the failing cell would continue current flow, albeit at a lower sting voltage. Such a bypass is

seldom used because of cost.

Cell matching is a challenge when replacing a faulty cell in an aging pack. A new cell has a

higher capacity than the others, causing an imbalance. Welded construction adds to the

complexity of the repair and this gives reason why battery packs are replaced as a unit. High-

voltage batteries in electric vehicles, in which a full replacement would be prohibitive, divide

the pack into blocks, each consisting of a specific number of cells. If one cell fails, the

affected block is replaced. A slight imbalance will also occur here.

Figure 3 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full

1.20V. With depressed operating voltage, this battery reaches the end-of-discharge point

sooner than a normal pack. The voltage collapses and the device turns off with “Low Battery”

message.

Figure 3: Serial connection with a faulty cell

Faulty cell 3 lowers the voltage and cuts the equipment off prematurely.

Courtesy of Cadex

Parallel Connection

If higher currents are needed and larger cells are not available or do not fit the design

constraint, one or more cells can be connected in parallel. Most battery chemistries allow

parallel configurations with little side effect. Figure 4 illustrates four cells connected in

parallel. The voltage of the illustrated pack remains at 1.20V, but the current handling and

runtime are increased fourfold.

Figure 4: Parallel

connection of four cells

(4P)

With parallel cells, current

handling and runtime

increases while the voltage

stays the same.

Courtesy of Cadex

A cell that develops a high-resistance or opens is less critical in a parallel circuit than in serial

configuration but this will reduce the total load capability. It’s like an engine only firing on

three cylinders instead of all four. An electrical short, on the other hand, is more serious as the

faulty cell drains energy from the other cells, causing a fire hazard. Most so-called electrical

shorts are mild and manifest themselves as elevated self-discharge.

A total short can occur and this happens when a cell receives reverse polarization. Large

packs often include a fuse that disconnects the failing cell from the parallel circuit if it were to

short. Figure 5 illustrates a parallel configuration with one faulty cell.

Figure 5: Parallel/connection

with one faulty cell

A weak cell will not affect the

voltage but will provide a low

runtime due to reduced current

handling. A shorted cell could

cause excessive heat and

become a fire hazard.

Courtesy of Cadex

Serial/Parallel Connection

The serial/parallel configuration shown in Figure 6 enables design flexibility and achieves the

desired voltage and current ratings with a standard cell size. The total power is the product of

voltage-times-current; four 1.20V cells multiplied with 1000mAh produce 4.8Wh. High

energy-dense Li-ion has an advantage as four 18650 cells with 3,000mAh produce 12Wh.

Figure 6: Serial/ parallel

connection of four cells (2S2P)

This configuration provides

maximum design flexibility.

Courtesy of Cadex

Li-ion lends well to serial/parallel configurations but the cells need monitoring to stay within

voltage and current limits. Integrated circuits (ICs) for various cell combinations are available

to supervise up to 13 Li-ion cells. Larger packs need custom circuits, and this also applies to

the Tesla Model 85 that devours over 7000 18650 cells to make up the 85kWh pack.

Simple Guidelines for Using Household Primary Batteries

Keep the battery contacts clean. A four-cell configuration has eight contacts and each

contact adds resistance (cell to holder and holder to next cell).

Never mix batteries; replace all cells when weak. The overall performance is only as

good as the weakest link in the chain.

Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.

Remove batteries from the equipment when no longer in use to prevent leakage and

corrosion. This is especially important with carbon-zinc.

Do not store loose cells in a metal box. Place individual cells in small plastic bags to

prevent an electrical short. Do not carry loose cells in your pockets.

Keep batteries away from small children. In addition to a choking hazard, the current-

flow of the battery can ulcerate the stomach wall if swallowed. The battery can also

rupture and cause poisoning.

Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an

explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Household Secondary Batteries

Observe polarity when charging a secondary cell. Reversed polarity can cause an

electrical short, leading to a hazardous condition.

Remove fully charged batteries from the charger. A consumer charger may not apply

the optimal trickle charge when fully charged and the cell could overheat.

Battery Management Systems (BMS)

 

BMS means different things to different people. To some it is simply Battery Monitoring, keeping a check on the key operational parameters during charging and discharging such as voltages and currents and the battery internal and ambient temperature. The monitoring circuits would normally provide inputs to protection devices which would generate alarms or disconnect the battery from the load or charger should any of the parameters become out of limits.

For the power or plant engineer responsible for standby power who's battery is the last line of defence against a power blackout or a telecommunications network outage BMS means Battery Management Systems. Such systems encompass not only the monitoring and protection of the battery but also methods for keeping it ready to deliver full power when called upon and methods for prolonging its life. This includes everything from controlling the charging regime to planned maintenance.

For the automotive engineer the Battery Management System is a component of a much more complex fast acting Energy Management System and must interface with other on board systems such as engine management, climate controls, communications and safety systems.

There are thus many varieties of BMS.

Designing a BMS

In order to control battery performance and safety it is necessary to understand what needs to be controlled and why it needs controlling. This requires an in depth understanding of the fundamental cell chemistries, performance characteristics and battery failure modes particularly Lithium battery failures. The battery can not simply be treated as a black box.

BMS Building Blocks

There are three main objectives common to all Battery Management Systems

Protect the cells or the battery from damage Prolong the life of the battery Maintain the battery in a state in which it can fulfil the functional requirements of the

application for which it was specified.

To achieve these objectives the BMS may incorporate one or more of the following functions. (Follow the links to see how these functions are implemented.) 

Cell Protection Protecting the battery from out of tolerance operating conditions is fundamental to all BMS applications. In practice the BMS must provide full cell protection to cover almost any eventuality. Operating a battery outside of its specified design limits will inevitably lead to failure of the battery. Apart from the inconvenience, the cost of replacing the battery can be prohibitive. This is particularly true for high voltage and high power automotive batteries which must operate in hostile environments and which at the same time are subject to abuse by the user.

Charge control  This is an essential feature of BMS. More batteries are damaged by inappropriate charging than by any other cause.

Demand Management  While not directly related to the operation of the battery itself, demand management refers to the application in which the battery is used. Its objective is to minimise the current drain on the battery by designing power saving techniques into the applications circuitry and thus prolong the time between battery charges.

SOC Determination  Many applications require a knowledge of the State of Charge (SOC) of the battery or of the individual cells in the battery chain. This may simply be for providing the user with an indication of the capacity left in the battery, or it could be needed in a control circuit to ensure optimum control of the charging process.

SOH Determination  The State of Health (SOH) is a measure of a battery's capability to deliver its specified output. This is vital for assessing the readiness of emergency power equipment and is an indicator of whether maintenance actions are needed.

Cell Balancing  In multi-cell battery chains small differences between cells due to production tolerances or operating conditions tend to be magnified with each charge / discharge cycle. Weaker cells become overstressed during charging causing them to become even weaker, until they eventually fail causing premature failure of the battery. Cell balancing is a way of compensating for weaker cells by equalising the charge on all the cells in the chain and thus extending battery life.

History - (Log Book Function)  Monitoring and storing the battery's history is another possible function of the BMS. This is needed in order to estimate the State of Health of the battery, but also to determine whether it has been subject to abuse. Parameters such as number of cycles, maximum and minimum voltages and temperatures and maximum charging and discharging currents can be recorded for subsequent evaluation. This can be an important tool in assessing warranty claims.

Authentication and Identification  The BMS also allows the possibility to record information about the cell such as the manufacturer's type designation and the cell chemistry which can facilitate automatic testing and the batch or serial number and the date of manufacture which enables traceability in case of cell failures.

Communications  Most BMS systems incorporate some form of communications between the battery and the charger or test equipment. Some have links to other systems interfacing with the battery for monitoring its condition or its history. Communications interfaces are also needed to allow the user access to the battery for modifying the BMS control parameters or for diagnostics and test.

The following examples illustrate three very different applications of BMS in action.

Intelligent Batteries

The life of rechargeable NiCad and Nickel Metal Hydride batteries such as those used in power tools can be extended by the use of an intelligent charging system which facilitates communications between the battery and the charger. The battery provides information about its specification, its current condition and its usage history which is used by the charger to determine the optimum charging profile or, by the application in which it is used, to control its usage.

The prime objective of the charger/battery combination is to permit the incorporation of a wider range of Protection Circuits which prevent overcharging of, or damage to, the battery and thus extend its life. Charge control can be in either the battery or the charger. The objective of the application/battery combination is to prevent overloads and to conserve the battery. Similar to the charger combination, discharge control can be in either the application or in the battery.

 Although some special cells incorporating intelligence have been developed, the intelligence is more likely to be implemented in a battery pack.

 The system works as follows:

 The Intelligent Battery, or Smart Battery, provides outputs from sensors which give the actual status of voltages, currents and temperatures within the battery as well as the state of charge. It can also provide alarm functions indicating out of tolerance conditions.

The Intelligent Battery also contains a memory chip which is programmed by the manufacturer with information about the battery specification such as:-

Manufacturing data (Name, date, serial number etc) Cell chemistry Cell capacity Mechanical outline code Upper and lower voltage limits Maximum current limits Temperature limits

Once the battery is placed into use, the memory may also record :-

How many times the battery has been charged and discharged.

Elapsed time The internal impedance of the battery The temperature profile to which it has been subjected The operation of any forced cooling circuits Any instances when limits have been exceeded.

The system also requires devices which may be in either the battery or the charger or both which can interrupt or modify the charging according to a set of rules. Similarly, battery discharge can be controlled by the battery or demand management circuits in the application.

The Intelligent Battery also needs an Intelligent Charger it can talk to and a language they can speak.

The charger is programmed to respond to inputs from the battery, to optimise the charging profile, charging at the maximum rate until a preset temperature is reached, then slowing down or stopping the charge and or switching on a cooling fan so as not to exceed the temperature limit and thus avoid permanent damage to the battery. If a deterioration in the battery internal impedance indicates that reconditioning is necessary the charger can also be programmed to reform the battery by subjecting it to several deep charge, discharge cycles. Because the battery contains information about its specification which can be read by the charger, it is possible to build Universal Chargers which can automatically adapt the charging profile to a range of battery chemistries and capacities, so long as they comply with an agreed message protocol.

 A separate communications channel is needed to facilitate interactions between the battery and the charger. One example used for simple applications is the System Management Bus ( SMBus) which forms part of the Smart Battery System which is used mainly in low power applications. Batteries which comply with the SBS standard are called Smart Batteries. Intelligent batteries are however not limited to the SMS scheme and many manufacturers have implemented their own proprietary schemes which may be simpler or more complex, depending on the requirements of the application.

A 50% increase in battery life has been claimed by using such techniques.

Automatic Control System

This is an example of an Automatic Control System in which the battery provides information about its actual condition to the charger which compares the actual condition with the desired condition and generates an error signal which is used to initiate control actions to bring the actual condition into line with the desired condition. The control signals form part of a Feedback Loop which provides automatic compensation to keep the battery within its desired operating parameters. It does not require any user intervention. Some form of automatic control system is an essential part of all BMS

Battery Monitoring

As well as talking to the charger, the Intelligent Battery can also talk to the user or to other systems of which the battery may be a part. The signals it provides can be used to turn on warning lights or to inform the user about the condition of the battery and how much charge it has left.

Monitoring the battery condition is an essential part of all Battery Management Systems. In the first of the following two examples, the control actions are manual, - the power plant maintenance engineer fixes any deficiencies. In the second example the battery is part of an Automatic Control System made up from several interlinked feedback loops controlling the battery itself and its role as part of the overall vehicle energy management system.

Power Plant BMS

The battery management requirements are quite different for standby and emergency power installations. Batteries may be inactive for long periods topped up by a trickle charge from time to time, or as in telecommunications installations they may be kept on float charge to keep them fully charged at all times. By their nature, such installations must be available for use at all times. An essential responsibility of managing such installations is to know the status of the battery and whether it can be relied upon to support its load during an outage. For this it is vital to know the SOH and the SOC of the battery. In the case of lead acid batteries the SOC of individual cells can be determined by using a hydrometer to measure the specific gravity of the electrolyte in the cells. Traditionally, the only way of determining the SOH was by discharge testing, that is, by completely discharging the battery and measuring its output. Such testing is very inconvenient. For a large installation it could take eight hours to discharge the battery and another three days to recharge it. During this time the installation would be without emergency power unless a back up battery was provided.

The modern way to measure the SOH of a battery is by impedance testing or by conductance testing . It has been found that a cell's impedance has an inverse correlation with the SOC and the conductance being the reciprocal of the impedance has a direct correlation with the SOH of the cell. Both of these tests can be carried out without discharging the battery, but better still the monitoring device can remain in place providing a permanent on line measurement. This allows the plant engineer to have an up to date assessment of the battery condition so that any deterioration in cell performance can be detected and appropriate maintenance actions can be planned.

Automotive BMS

Automotive battery management is much more demanding than the previous two examples. It has to interface with a number of other on board systems, it has to work in real time in rapidly changing charging and discharging conditions

as the vehicle accelerates and brakes, and it has to work in a harsh and uncontrolled environment. This example describes a complex system as an illustration of what is possible, however not all applications will require all the functions shown here.

The functions of a BMS suitable for a hybrid electric vehicle are as follows:

Monitoring the conditions of individual cells which make up the battery Maintaining all the cells within their operating limits Protecting the cells from out of tolerance conditions Providing a "Fail Safe" mechanism in case of uncontrolled conditions, loss of

communications or abuse Isolating the battery in cases of emergency Compensating for any imbalances in cell parameters within the battery chain

Setting the battery operating point to allow regenerative braking charges to be absorbed without overcharging the battery.

Providing information on the State of Charge (SOC) of the battery. This function is often referred to as the "Fuel Gauge" or "Gas Gauge "

Providing information on the State of Health (SOH) of the battery. This measurement gives an indication of the condition of a used battery relative to a new battery.

Providing information for driver displays and alarms Predicting the range possible with the remaining charge in the battery (Only EVs

require this) Accepting and implementing control instructions from related vehicle systems Providing the optimum charging algorithm for charging the cells Providing pre-charging to allow load impedance testing before switch on and two

stage charging to limit inrush currents Providing means of access for charging individual cells Responding to changes in the vehicle operating mode Recording battery usage and abuse. (The frequency, magnitude and duration of out of

tolerance conditions) Known as theLOG BOOK  function Emergency "Limp Home Mode" in case of cell failure.

 In practical systems the BMS can thus incorporate more vehicle functions than simply managing the battery. It can determine the vehicle's desired operating mode, whether it is accelerating, braking, idling or stopped, and implement the associated electrical power management actions.

Cell Protection

One of the prime functions of the Battery Management System is to provide the necessary monitoring and control to protect the cells from out of tolerance ambient or operating conditions. This is of particular importance in automotive applications because of the harsh working environment. As well as individual cell protection the automotive system must be designed to respond to external fault conditions by isolating the battery as well as addressing the cause of the fault. For example cooling fans can be turned on if the battery overheats. If the overheating becomes excessive then the battery can be disconnected.

Protection methods are discussed in detail in the section on Protection.

Battery State of Charge (SOC)

Determining the State of Charge (SOC) of the battery is the second major function of the BMS. The SOC is needed not just for providing the Fuel Gauge indication. The BMS monitors and calculates the SOC of each individual cell in the battery to check for uniform charge in all of the cells in order to verify that individual cells do not become overstressed.

The SOC indication is also used to determine the end of the charging and discharging cycles. Over-charging and over-discharging are two of the prime causes of battery failure and the BMS must maintain the cells within the desired DOD operating limits.

Hybrid vehicle batteries require both high power charge capabilities for regenerative braking and high power discharge capabilities for launch assist or boost. For this reason, their batteries must be maintained at a SOC that can discharge the required power but still have enough

headroom to accept the necessary regenerative power without risking overcharging the cells. To fully charge the HEV battery for cell balancing (See below) would diminish charge acceptance capability for regenerative braking and hence braking efficiency. The lower limit is set to optimise fuel economy and also to prevent over discharge which could shorten the life of the battery. Accurate SOC information is therefore needed for HEVs to keep the battery operating within the required, safe limits.

HEV Battery Operating Range

 

 

Methods of determining the SOC are described in the section on State of Charge.

The Battery Management System

The diagram below is a conceptual representation of the primary BMS functions. It

shows the three main BMS building blocks, the Battery Monitoring Unit (BMU), the Battery Control Unit (BCU) and the CAN bus vehicle communications network and how they interface with the rest of the vehicle energy management systems. Other configurations are possible with distributed BMS embedded in the battery cell to cell interconnections.

In practice the BMS may also be coupled to other vehicle systems which communicate with the BMS via the CAN bus (see below) such as the Thermal Management System or to anti theft devices which disable the battery. There may also be requirements for system monitoring and programming, and data logging using an RS232 serial bus.

Battery Monitoring Unit

The Battery Monitoring Unit is a microprocessor based unit incorporating three functions or sub-modules. These sub-modules are not necessarily separate physical units but are shown separately here for clarity.

Battery Model

The Battery Model characterises in a software algorithm, the behaviour of the battery in response to various external and internal conditions. The model can then use these inputs to estimate the status of the battery at any instant in time.

An essential function of the battery model is to calculate the SOC of the battery for the functions noted above.

The SOC is determined essentially by integrating the current flow over time, modified to take account of the many factors which affect the performance of the cells, then subtracting the result from the known capacity of the fully charged battery. This is described in detail in the section on SOC.

 The battery model can be used to log past history for maintenance purposes or to predict how many miles the vehicle may run before the battery needs recharging. The remaining range, based on recent driving or usage patterns, is calculated from the current SOC and the energy consumed and the miles covered since the previous charge (or alternatively from a previous long term average). The distance travelled is derived from data provided by other sensors on the CAN bus (see below).

The accuracy of the range calculation is more important for EVs whose only source of power is the battery. HEVs and bicycles have an alternative "Get you home" source of power should the battery become completely discharged.

The problem of losing all power when a single cell fails can be mitigated at the cost of adding four more expensive contactors which effectively split the battery into two separate units. If a cell should fail, the contactors can isolate and bypass the half of the battery containing the failed cell allowing the vehicle to limp home at half power using the other (good) half of the battery.

Outputs from the model are sent to the vehicle displays also using the CAN bus.

Multiplexing

To reduce costs, instead of monitoring each cell in parallel, the Battery Monitoring Unit incorporates a multiplexing architecture which switches the voltage from each cell (input pairs) in turn to a single analogue or digital output line (see below). Cost savings can be realized by reducing the number of analogue control and/or digital sampling circuits and hence the component count to a minimum. The drawbacks are that only one cell voltage can be monitored at a time. A high speed switching mechanism is required to switch the output line to each cell so that all cells can be monitored sequentially.

 

The BMU also provides the inputs for estimating the SOH of the battery, however since the SOH changes only gradually over the lifetime of the battery, less frequent samples are needed. Depending on the method used to determine the SOH, sampling intervals may be as low as once per day. Impedance measurements for example could even be taken only in periods when the vehicle is not in use. Cycle counting of course can only occur when the vehicle is operational.

Demand or Personality Module

The Demand Module is similar in some respects to the Battery Model in that it contains a reference model with all the tolerances and limits relevant to the various parameters monitored by the Battery Model. The Demand Module also takes instructions from the communications bus such as commands from the BMS to accept a regenerative braking charge or from other vehicle sensors such as safety devices or directly from the vehicle operator. This unit is also used to set and to monitor the vehicle operating mode parameters.

This module is sometimes called the Personality Module since includes provision for programming into the system, all the custom requirements which may be specific to the customer's application. For example, the cell maker will recommend a temperature limit at which for safety reasons the battery must be automatically disconnected. However the car manufacturer may set two lower limits, one at which forced cooling may be switched on and another which lights up a warning light on the driver's instrument panel.

For HEV applications, the Personality Module interfaces with the engine Electronic Control Unit (ECU) via the CAN bus. Provision is made in this module for setting the desired system SOC operating range and the parameters for controlling the power sharing between the electric drive and the internal combustion engine .

The Demand Module also contains a memory block for holding all the reference data and for accumulating the historical data used for monitoring the battery SOH. Data to display the SOH or switch on warning lights can be provided to the vehicle instrumentation module via the CAN bus.

The outputs from the Demand Module provide the reference points for setting the operating conditions of the battery or triggering the action of protection circuits.

Test access to the BMS for monitoring or setting system parameters and for downloading the battery history is provided through a standard RS 232 or RS485 serial bus.

Decision Logic Module

The Decision Logic module compares the status of the measured or calculated battery parameters from the Battery Model with the desired or reference result from the Demand Module. Logic circuits then provide error messages to initiate cell protection actions or to be used in the various BMS feedback loops which drive the system to its desired operating point or isolate the battery in the case of unsafe conditions. These error messages provide the input signals for the Battery Control Unit.

System Communications

The BMS needs a communications channel for passing signals between its various internal functional circuit blocks. It must also interface with several external vehicle systems for monitoring or controlling remote sensors, actuators, displays, safety interlocks and other functions.

Automotive BMS therefore uses the CAN bus which was designed for this purpose as its main communications channel.

Battery Control Unit

The Battery Control Unit contains all the BMS power electronics circuitry. It takes control signals from the Battery Monitoring Unit to control the battery charging process and to switch the power connections to individual cells.

Some of the possible functions of this unit are:

Controlling the voltage and current profile of the charger output during the charging process.

Providing top up charge to individual cells to equalise the charge on all cells in the battery chain.

Isolating the battery during fault or alarm conditions Switching the regenerative braking charge into the battery as required Dumping excessive regenerative braking charges when the battery is fully charged Responding to changes in the vehicle operating mode

To provide these functions, each cell in the battery may require expensive high current switches capable of switching 200 Amps or more to provide the necessary interconnections.

Binary Control and Progressive Control

In its simplest form, the BMS privides a "binary" ON/OFF resonse to a fault or an out of tolerance condition such as an overload, merely isolating the battery completely by opening the main contactors. Progressive or variable control can however be provided in the case of an overload by utilising the CAN Bus to call for a reduced the demand on the battery.

Cell Balancing

This is another essential function of the automotive BMS. As noted above it is required to compensate for weaknesses in individual cells which could eventually cause the failure of the complete battery. The reasons for cell balancing and how this is implemented are explained on the Cell Balancing page.

Limp Home Mode

Although batteries are designed to be trouble free for 3 or more years, there is always the possibility that the battery could be disabled by the failure of a single cell. If a cell goes open circuit, the battery is essentially dead. However the BMS is designed to monitor the status of every cell and so the location of the faulty cell will automatically be identified. It is not difficult to split the battery into two sections in series, each of which can be independently bypassed by disconnecting the section of the battery containing the faulty cell and switching a conducting link in its place. This will allow the vehicle to get to home or to the nearest refuge on half power using the good section of the battery. As well as the links the system will need two more expensive high power contactors to implement this function, but this investment may be well justified when the alternative could be a costly and dangerous breakdown on the motorway.

System Enhancements

Automotive BMSs may also be required to provide various functions which are not necessarily essential to managing the battery. These may include remote monitoring of the battery from the fleet headquarters and this may also include the GPS location of the vehicle. The driver could thus be warned if the vehicle was getting low on charge or if he was straying too far from a charging station.

Fortunately not all BMS applications are as complex as this one.

Practical BMS Implementation

There are many ways of implementing the battery management system and two different examples for a 256 Volt battery made up from 80 Lithium Iron Phosphate cells are shown below.

Master and Slaves

The master and slaves, star topolgy, organises the cells into blocks or modules with one slave managing each module. In the example shown, 16 X 3.2 volt cells are arranged in modules each with an output voltage of 51.2 Volts but other module sizes and voltages are possible.

The Slaves - Each cell has a temperature sensor as well as connections to measure the voltage, all of which are connected to the slave which monitors the condition of the cell and implements the cell balancing.

The Master - Multiple slaves can be connected to the master which monitors the current and integrates it over time to calculate the net Coulomb flow and this is modified using voltage and temperature data from the slaves to calculate the battery SOC. The master controls the main battery isolation contactor(s) initiating battery protection in response to data from the main current sensor or voltage and temperature data from the slaves.The master also provides the system communications.

BMS Master and Slaves (Star Topology)

This configuration has the advantages that it does not need printed circuit boards connected to individual cells and high voltage batteries can be accommodated by adding more modules and since the main battery current does not pass through slaves, it can also be used for high current batteries. Signal processing is shared between the master and the slaves simplifying the management of the information processing load.

Disadvantages are that the communications between the sensors and the slaves are in analogue form, and thus susceptible to noise, and the very large number of sensor wires, four per cell, which are required. Opto-isolated connections between the slaves and the master are also required since the votages on the slaves would otherwise be progressively higher, up to the full battery voltage, as connections are taken from further up the cell chain.

BMS Daisy Chain

The daisy chain, ring topology, uses a small simple slave printed circuit board connected to each cell to accommodate the voltage and temperature sensors with an A to D converter, as well as a current bypass switch to enable cell balancing by charge shunting and an communications transceiver with built in capacitive isolation for receiving and transmitting data in digital form. The slave takes its power from the cell it is monitoring and a single RS 485 three wire data bus connects the nodes from all the slaves to the master which polls each node in turn and requests an update of its cell conditions. The slave does not carry out any signal processing, apart from the A to D conversion, as this is all carried out by the master

along with all the monitoring, protection and communications functions as in the example above.

BMS Daisy Chain (Ring Topology)

 

The main advantages of this topology are its simpler design and construction and its potential for higher reliability in an automotive environment.

The disadvantages are the large number of mini-slave printed circuit boards which are needed and the difficulty of mounting them on some cell types. In addition the master has a higher processing load.