Constructing a Lithium-Ion/Polymer Battery with an Imbedded Protection Circuit Module (PCM)

Michael KovalcikTeam 10

ECE_480_FS08

Executive Summary:

Rechargeable lithium- ion (Li-Ion) and lithium-polymer (Li-Po) cells have a higher energy density and greater nominal voltage than conventional, onetime use batteries. Li-Ion and Li-Po batteries are the ideal power source for many portable electronic devices. In order to maximize their lifecycle and increase safety, both Li-Ion and Li-Po cells require the use of a Protection Circuit Model (PCM) when assembled together to form a battery. The PCM will generally provide the battery with overcharge/discharge protection, short circuit protection, current limitation, and voltage and current balancing in each cell. All of these factors are important in obtaining the maximum output and longest lifecycle from a rechargeable lithium battery. These notes will discuss the need for and configuration of a PCM, the advantages and disadvantages of these configurations, and step-by-step instructions for the construction of a rechargeable lithium battery with an onboard PCM.

Keywords:

Battery, Cell, Lithium-Ion (Li-Ion), Lithium-Polymer (Li-Po), Protection Circuit Module (PCM), Energy Density, Nickel-Metal-Hydride (Ni-MH), Nickel-Cadmium (NiCad), Lead-AcidIntroduction:

When compared to other types of rechargeable batteries, the high energy density associated with lithium-ion and lithium-polymer cells makes them the ideal power source for portable electronic devices. Because of this, rechargeable lithium batteries can be found in a wide variety of consumer electronic devices such as cellular telephones, laptop computers, digital cameras and cordless power tools. In order to maximize their lifecycle and reduce safety concerns, both lithium-ion (Li-Ion) and lithium-polymer (Li-Po) cells require the use of a Protection Circuit Model (PCM) when assembled together to form a battery (See Figure 4.). The PCM may be imbedded in the charger or permanently attached to each individual battery pack (See Figures 1a. & 1b.).

Figure 1a. Figure 1b.

Objective:

The purpose to these application notes is to explain the advantages and disadvantages of rechargeable lithium battery technology, explain the need for protection circuitry, describe the two main methods for implementing a PCM, and instruct the reader on how to construct a Li-Ion or Li-Po battery that includes an onboard PCM.

Warning! These application notes are intended for theoretical use in the Michigan State University, ECE 480 course only. While the author does have experience in this field and for certain types of electrical systems discussed here in, the reader is asked not to build any such battery or circuit discussed in this paper without a thorough, professional education in electrical engineering and having read all data available from the manufacturer of the cells, PCM, Charger, and any other equipment or devices used on, in, or near the circuit and/or system. Furthermore, the author accepts no liability for

any bodily harm or destruction that may occur from an individual, other than the author, attempting to construct an apparatus as described in these notes. If you have not met the above requirements; DO NOT BUILD THIS DEVICE!Background:

As Electrical Engineers, it is likely at some point in our careers we will have some level of professional involvement with battery operated systems. Rechargeable Nickel-Metal-Hydride (Ni-MH) and Nickel-Cadmium (NiCad) cells and Lead-Acid batteries are no longer the standard rechargeable battery technologies. These three technologies suffer from lower energy densities (50-112, 33-75, & 18-55 J/kg respectively) than conventional, one time use Zink-Carbon or Alkaline cells. This has made these onetime use batteries and cells more desirable for many consumer applications.

The new generation rechargeable cells have made it possible to combine higher energy density (95-335 J/kg), relatively high nominal voltage (between 3 & 4 volts), and the ability to hold a charge for years, into a compact rechargeable battery that outperforms onetime use batteries. The ability of lithium ion (Li-Ion) and lithium polymer (Li-Po) cells to charge and discharge more than 1000 times, without a reduction in performance, makes them environmentally friendly as well. Increased mass production of these cells has made this technology more economical. The culmination of these factors is quickly making rechargeable lithium cells the preferred power source for portable electronic devices

Rechargeable lithium cells are not without their drawbacks. The primary ingredient, lithium, is highly reactive, making cells that contain it more dangerous than those utilizing another rechargeable cell technology. Because of the increased safety risk, both lithium ion and lithium polymer batteries require the inclusion of four safety features to reduce the risk of fire or explosion:

- Shutdown Separator (Disables a cell if its temperature is too high)- Tear-Away Tab (Disables a cell if its internal pressure is too high)- Vent (Releases built up internal pressure in a cell)- Thermal Interrupt (Disables a cell if it is being overcharged or if the current

level used to charge it is too high)

Aside from increasing the overall complexity of these cells, these safety features take up space, add weight, and limit design options. Because the activation of a safety feature can permanently and irreversibly disable a cell, rechargeable lithium batteries usually include active circuit technology for protection, especially during the charging cycle. This protection circuitry is often contained in a PCM (Protection Circuit Module, See Figure 4.). The PCM may be imbedded in the charger or permanently attached to an individual battery pack. The remainder of these notes will discuss the advantages and disadvantages to each method of implementation and will go over the details for the construction of a Li-Ion or Li-Po battery that contains a permanently attached PCM.

Comparing Methods:

There are two methods for implementing a lithium ion or lithium polymer battery utilizing a Protection Circuit Module or PCM. First, the PCM may be imbedded in the charger pack (See Figure 1a.). While on the charger, the battery will have the standard protection offered by a PCM including:

- Overcharge Protection- Short Circuit Protection- Charge Current Limiting- Balancing Functions for Each Cell.

This system, however, will not be able to protect the battery once it is removed from the charger. This means that the battery its self has few or no attached active circuit elements which will monitor its operation. The advantages and disadvantages of this system are shown below in Table 1.

Advantages DisadvantagesReduced Battery Weight No Active Battery Protection During UseUses Only One PCM Increased Safety RiskReduced Cost For Multiple Batteries Reduced Battery Performance & Shorter Life

Table 1. Advantages and disadvantages of charger imbedded PCM.

Next, the PCM can be permanently attached to each individual battery pack (See Figure 1b.). This system is generally used in rechargeable lithium batteries that have multiple cells, are more expensive, and where the small amount extra weight from a PCM will not effect performance. The advantages and disadvantages of this system are shown below in Table 2.

Advantages DisadvantagesFulltime Active Battery Protection Increased Battery Weight & ComplexityIncreased System Safety Each Battery Requires Its Own PCMOptimal Battery Performance & Life Increased Cost For Multiple BatteriesAbility to Recharge Battery In-PlaceUse of Unspecialized Power Sources for RechargingUse of Onboard battery Fuel Gauge

Table 2. Advantages and disadvantages PCM onboard battery.

The advantages of the onboard PCM are usually worth the initial increased cost and weight. The cost increase is often recouped by the extended battery life, in-place charging, and increased battery performance.

How to:

Battery/Cell Size & Rating: This section includes instructions on how to assemble the second rechargeable Li-Ion/Po battery system with an onboard PCM. In order to construct a battery system that is the correct size for the intended application, and assuming you have already determined the voltage needed, you must calculate the amp hours that will be needed by using the formula below:

(Current Required for Operation) X (Duration of Operation) = AH

After determining the amp hours the item to be powered will use, it is usually a good idea to add 10 to 20% to the calculated amp hour result.

Next you will need to select a lithium-ion or lithium-polymer cell that has an amp hour rating nearest the level calculated. If you cannot find the exact rating you require, you should select a cell that has a higher amp hour rating.

Next you will need to determine how many of these cells you will require to obtain the voltage level your system requires. Both types of rechargeable lithium cells usually have a nominal voltage of 3.7 volts. The choice of battery voltages is usually limited to some multiple of this number.

# of Cells 1 2 3 4 5 6 7 10 13 nVoltage 3.7

v7.4v

11.1v

14.8v

18.5v

22.2v

25.9v

37v 48.1v

n x 3.7

Select the appropriate number of cells in series required to reach the voltage level you require.

Where to Order: Next you will need to select a PCM that is designed to utilize this voltage and rated for the current that you require. A useful website for finding both batteries and a PCM is:

http://www.batteryspace.com/

Assembly: Once you have received the cells and the PCM it is a simple matter of soldering the cells together in series and then connecting it to the PCM as shown in Figure 4.

Figure 4. Connection schematic for a three cell (11.1v) Li-Ion/Po PCM.

The PCM in Figure 4. utilizes three Li-Ion/PO cells in series to produce a combined voltage of 11.1v. On the left you can see the positive and negative terminals (P- & P+) and the connector for the fuel gauge. On the right you can see the points where the battery and individual cells are to be connected. The individual cells are also connected so that the PCM can perform balancing functions to ensure that each cell maintains an equivalent voltage level, and does not exceed the individual cells overcharge and overdischarge limits (Usually ranges from about 4.2 - 4.35v and 2.4 – 2.5v respectively).

While the nominal voltage level of the battery will usually be equal to the number of cells times the nominal voltage of each cell (N x 3.7v), you should be aware that this is an average. The maximum and minimum voltages of the battery will be the number of cells times the overcharge and overdischarge limits respectively. This may or may not be the same number indicated in the instructions for the PCM you have selected. For the 11.1v system in Figure 4. the maximum battery voltage may be calculated to be higher then the PCM specification. This is fine because the onboard PCM system is also used when charging the battery, so as long as it is charged through the P+ and P- terminals of the PCM, it will never reach the higher overcharge and overdischarge limits of the cells.

Charging: In order to charge your PCM onboard battery simply connect the appropriate PCM terminals to a DC power supply, ensuring that the current level used is in accordance with the level specified by the manufacturer of the individual battery cells used or the PCM, which ever is lower.

Warning! These application notes are intended for theoretical use in the Michigan State University, ECE 480 course only. While the author does have experience in this field and for certain types of electrical systems discussed here in, the reader is asked not to build any such battery or circuit discussed in this paper without a thorough, professional education in electrical engineering and having read all data available from the manufacturer of the cells, PCM, Charger, and any other equipment or devices used on, in, or near the circuit and/or system. Furthermore, the author accepts no liability for any bodily harm or destruction that may occur from an individual, other than the author, attempting to construct an apparatus as described in these notes. If you have not met the above requirements; DO NOT BUILD THIS, OR ANY OTHER DEVICE mentioned in this paper!

Sources:

http://www.mpoweruk.com/lithiumS.htmhttp://en.wikipedia.org/wiki/Lithium_ion_polymer_batteryhttp://www.nexergy.com/battery-density.htmhttp://en.wikipedia.org/wiki/Lithium_ion_batterieshttp://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4345