Battery Management for Maximum Performance in Plug-In Electric and Hybrid Vehicles P. T. Krein Dept. of Electrical and Computer Engineering University

Battery Management for Maximum Battery Management for Maximum Performance in Plug-In Electric Performance in Plug-In Electric

and Hybrid Vehiclesand Hybrid Vehicles

P. T. KreinP. T. KreinDept. of Electrical and Computer EngineeringDept. of Electrical and Computer Engineering

University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign

Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-ChampaignGrainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign

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AcknowledgementsAcknowledgements

• Thanks to Ryan Kroeze for literature work Thanks to Ryan Kroeze for literature work and analysis contributions.and analysis contributions.

• A version of this presentation was delivered A version of this presentation was delivered at the IEEE Vehicle Propulsion Power at the IEEE Vehicle Propulsion Power symposium in September.symposium in September.


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OutlineOutline

• Performance requirementsPerformance requirements• Present situationPresent situation• Lead-acid cellsLead-acid cells• NiMH cellsNiMH cells• Li-ion cellsLi-ion cells• Battery management componentsBattery management components• ConclusionConclusion


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Performance RequirementsPerformance Requirements

• Hybrid vehiclesHybrid vehicles– High power density, meaning:High power density, meaning:– High charge acceptance for brakingHigh charge acceptance for braking– High power delivery for accelerationHigh power delivery for acceleration– Cycle life – tens of thousands of shallow cyclesCycle life – tens of thousands of shallow cycles– Adequate energy density, but this is secondaryAdequate energy density, but this is secondary– Wide ambient temperature rangeWide ambient temperature range

• Electric vehiclesElectric vehicles– High energy densityHigh energy density– Fast, reliable chargingFast, reliable charging– Cycle life – thousands of deep cyclesCycle life – thousands of deep cycles

1 cycle/5 miover 100,000 miles


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Plug-In HybridsPlug-In Hybrids

• Require the power capabilities and cycling Require the power capabilities and cycling capabilities of hybrids.capabilities of hybrids.

• Benefit from high energy density and good Benefit from high energy density and good recharge properties.recharge properties.

• In other words: must satisfy everyone and In other words: must satisfy everyone and everything.everything.

• This motivates work on “hybrid storage” that This motivates work on “hybrid storage” that combines batteries (high energy density) with combines batteries (high energy density) with ultracapacitors (high power density).ultracapacitors (high power density).

• Here we explore the batteries.Here we explore the batteries.


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Present SituationPresent Situation

• EVs and HEVs require thousands of battery EVs and HEVs require thousands of battery cycles with minimal degradation.cycles with minimal degradation.

• Typical strategy derates batteries:Typical strategy derates batteries:use a narrow state of charge (SOC)use a narrow state of charge (SOC)regime.regime.

• This results in a low “effective energyThis results in a low “effective energydensity” in exchange for power density.density” in exchange for power density.

• Space applications get much more.Space applications get much more.• The presentation emphasizes ways to The presentation emphasizes ways to

maximize battery capabilitiesmaximize battery capabilities

UoSat-5University of Surrey


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Present SituationPresent Situation

• NiMH cells today are being usedNiMH cells today are being usedin about a 15% SOC range. in about a 15% SOC range. Reasons are explored here.Reasons are explored here.

• Lead-acid cells provide a similarLead-acid cells provide a similarrange.range.

• Li-ion cells are more promising.Li-ion cells are more promising.• Active balancing that worksActive balancing that works

throughout the SOC range isthroughout the SOC range isan important enabler.an important enabler.


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Lead-Acid CellsLead-Acid Cells

• Operating results from starting-lighting-Operating results from starting-lighting-ignition (SLA) batteries.ignition (SLA) batteries.

• Consistent with float operation in telecom.Consistent with float operation in telecom.• Best life results above 85% SOC.Best life results above 85% SOC.• But the top end involves gassing reactions But the top end involves gassing reactions

and sacrifices efficiency.and sacrifices efficiency.• Energy density is about 35 W-h/kg given Energy density is about 35 W-h/kg given

100% discharge cycles.100% discharge cycles.• Effective energy density (15%) isEffective energy density (15%) is

5.3 W-h/kg.5.3 W-h/kg.• Ultracapacitors can do as well.Ultracapacitors can do as well.


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Lead-Acid CellsLead-Acid Cells

• Cells show damage from sulfation when Cells show damage from sulfation when operated at lower SOC.operated at lower SOC.

• Present designs should be able to support an Present designs should be able to support an SOC range of 50% to 100%, but only if the SOC range of 50% to 100%, but only if the batteries are stored full.batteries are stored full.

• Promising future designs are likely to correct Promising future designs are likely to correct the most severe damagethe most severe damagemechanisms.mechanisms.

• Do not favor HEV and EVDo not favor HEV and EVapplications except on aapplications except on a“use, park, charge” cycle.“use, park, charge” cycle.


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NiMH CellsNiMH Cells

• Extensive data in preparation for and from Extensive data in preparation for and from experience with commercial hybrids.experience with commercial hybrids.

• Toyota has had fewToyota has had fewproblems with Priusproblems with Priustraction batteries –traction batteries –routine replacementroutine replacementhas has notnot been required. been required.

• Limited SOC swing – about 50% to 65%.Limited SOC swing – about 50% to 65%.


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• Given density of 70 W-h/kg for full discharge, Given density of 70 W-h/kg for full discharge, the effective density is less than 10 W-h/kg.the effective density is less than 10 W-h/kg.

• The argument can be made that these The argument can be made that these designs use nickel-metal-hydride batteries for designs use nickel-metal-hydride batteries for the functions of ultracapacitors.the functions of ultracapacitors.

• What aspects is this application attempting to What aspects is this application attempting to optimize?optimize?


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• At the high end, positive electrode At the high end, positive electrode degradation and electrolyte loss occurs.degradation and electrolyte loss occurs.

• Positive pressure can transfer hydrogen Positive pressure can transfer hydrogen among adjacent cells but amplifies among adjacent cells but amplifies degradation and imbalances cells.degradation and imbalances cells.

• At the low end, the negative electrode At the low end, the negative electrode experiences irreversible oxidation.experiences irreversible oxidation.

• Impedance rises for discharge. Impedance rises for discharge.


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• High-end effects are minimized if SOC is High-end effects are minimized if SOC is limited well below 80%.limited well below 80%.

• Low-end effects are strong below 20% SOC, Low-end effects are strong below 20% SOC, but performance degrades to some degree but performance degrades to some degree below 40% SOC.below 40% SOC.

• External active balancing helps maintain External active balancing helps maintain discharge performance between 20% and discharge performance between 20% and 40% SOC, and limits degradation above 80%.40% SOC, and limits degradation above 80%.


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• Differential power density is the remaining Differential power density is the remaining issue. (Here DOD = 100% - SOC.)issue. (Here DOD = 100% - SOC.)

From Menjak, Gow, Corrigan, Venkatesan, Dhar, Stempel, Ovshinsky, “Advanced Ovonic high-power nickel-metal hydride batteries for hybrid

electric vehicle applications,” in Ann. Battery Conf. Appl. Advances, 1998, pp. 13-18.


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• The reduction in charge power density as the The reduction in charge power density as the high end has been treated as a limiting factor: high end has been treated as a limiting factor: regeneration energy acceptance drops regeneration energy acceptance drops rapidly above 60% SOC.rapidly above 60% SOC.

• The SOC range from 20% to 80% can be The SOC range from 20% to 80% can be utilized ifutilized if– Active balancing over the whole range prevents Active balancing over the whole range prevents

local limitations from pulling cells out of balance local limitations from pulling cells out of balance between 20% and 40% SOC, and between 60% between 20% and 40% SOC, and between 60% and 80% SOC. and 80% SOC.

– Braking strategy limits charge power at the high Braking strategy limits charge power at the high end.end.


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• Thus the SOC range from 20% to 80% can Thus the SOC range from 20% to 80% can be used for plug-in operation.be used for plug-in operation.

• Increases effective energy density to 42 W-Increases effective energy density to 42 W-h/kg – factor of 4 improvement.h/kg – factor of 4 improvement.

“Harding Handbook for Quest Batteries,” Fig. 3.7.2,available http://www.hardingenergy.com/pdfs/NiMH.pdf


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Li-Ion CellsLi-Ion Cells

• Lithium-ion cells in general have much better Lithium-ion cells in general have much better reversibility than other common secondary reversibility than other common secondary chemistries: Energy reversibility can exceed 90%.chemistries: Energy reversibility can exceed 90%.

• Discharge curves indicate regimes of reduced Discharge curves indicate regimes of reduced reversibility.reversibility.

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3.7

3.8

3.9

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0 20 40 60 80 100

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• Experience with laptop computers is showing Experience with laptop computers is showing that Li-ion cells degrade under float that Li-ion cells degrade under float conditions: extended operation when held at conditions: extended operation when held at 100% SOC decreases operating life.100% SOC decreases operating life.

• Life testing in telecom applications shows that Life testing in telecom applications shows that limiting the upper end charge voltage reduces limiting the upper end charge voltage reduces degradation substantially.degradation substantially.

• The effect is similar to limiting SOC to less The effect is similar to limiting SOC to less than 90%.than 90%.


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• The curve shown earlier shows rapid The curve shown earlier shows rapid imbalance and capacity reduction below 20% imbalance and capacity reduction below 20% SOC.SOC.

• Key problem: cellKey problem: cellbalancing – no inherent balancing – no inherent mechanism in Li-ion.mechanism in Li-ion.

• Typical systems useTypical systems useresistive limiters toresistive limiters toenforce the upper voltage limit. enforce the upper voltage limit.

• Limiters add system nonlinearity that drives Limiters add system nonlinearity that drives (lossy) cell balancing at the top end of SOC, (lossy) cell balancing at the top end of SOC,

www.popularmechanics.com


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• Balancing is more important at the low end, Balancing is more important at the low end, where discharge effects begin to pull cells where discharge effects begin to pull cells apart.apart.

• In reality, a method is needed that can balance In reality, a method is needed that can balance over the entire useful SOC range.over the entire useful SOC range.

• When this is done, the possible range of SOC When this is done, the possible range of SOC becomes 20% to 90%.becomes 20% to 90%.

• If the cells achieve 200 W-h/kg for 100% If the cells achieve 200 W-h/kg for 100% discharge, the effective energy density is 140 discharge, the effective energy density is 140 W-h/kg – more than triple the best NiMH W-h/kg – more than triple the best NiMH results. results.


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Battery Management ComponentsBattery Management Components

• Vehicle system-level control strategy must Vehicle system-level control strategy must focus on a limited SOC range, as present focus on a limited SOC range, as present hybrids do.hybrids do.

• The proven long-life SOC range is The proven long-life SOC range is considerably wider than in present practice.considerably wider than in present practice.

• Components:Components:– Strategies with active top-end and bottom-end Strategies with active top-end and bottom-end

SOC limits.SOC limits.– Active cell balancing over the full range.Active cell balancing over the full range.– Techniques to limit or mitigate power density Techniques to limit or mitigate power density

requirements at extremes.requirements at extremes.


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Choices for LimitsChoices for Limits

• Use established charge sustaining strategies, Use established charge sustaining strategies, but open the tolerance bands.but open the tolerance bands.– NiMH: 50% NiMH: 50% 30% SOC range 30% SOC range– Li-ion: 55% Li-ion: 55% 35% 35%

• Target a daily driving and charging profile.Target a daily driving and charging profile.– Seek to end the day at the low end, ready for Seek to end the day at the low end, ready for

charging.charging.– Allow a high SOC pack to decrease slowly during the Allow a high SOC pack to decrease slowly during the

daily drives.daily drives.

• Adaptive cycle intelligence.Adaptive cycle intelligence.


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Choices for MitigationChoices for Mitigation

• Divert power demand extremes to Divert power demand extremes to ultracapacitors – but only at the extreme SOC ultracapacitors – but only at the extreme SOC ends.ends.

• This leads to relatively small ultracapacitor This leads to relatively small ultracapacitor packs that absorb as little as 10% of a given packs that absorb as little as 10% of a given braking energy sequence or deliver just 20% braking energy sequence or deliver just 20% of peak acceleration powerof peak acceleration power

• Use resistive brake auxiliariesUse resistive brake auxiliarieswhen SOC upper limit is reached.when SOC upper limit is reached.


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Active Cell BalancingActive Cell Balancing

• In Li-ion packs, cell mismatch is not restored In Li-ion packs, cell mismatch is not restored by altering the charge process alone.by altering the charge process alone.

• The cells can be pulled apart at the low end of The cells can be pulled apart at the low end of SOC, especially for high power pulses.SOC, especially for high power pulses.

• Resistive or switched voltage limiters can only Resistive or switched voltage limiters can only function at the high end.function at the high end.

• In HEV applications, there is limited dwell time In HEV applications, there is limited dwell time at the high end.at the high end.

• In EV applications, limiters must follow the In EV applications, limiters must follow the SOC limit settings.SOC limit settings.


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Active Cell BalancingActive Cell Balancing

• Active balancing methods bring cells Active balancing methods bring cells together regardless of SOC.together regardless of SOC.– Switched capacitor types – low energy use, Switched capacitor types – low energy use,

efficiency is high as mismatch reduces.efficiency is high as mismatch reduces.

– Switched inductor types – drives current to Switched inductor types – drives current to match charge in a controller manner.match charge in a controller manner.

– Individual cell or monoblock chargers – the Individual cell or monoblock chargers – the ultimate, but expensive, solution.ultimate, but expensive, solution.


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DiscussionDiscussion

• Present lead-acid cells are comparatively Present lead-acid cells are comparatively weak for plug-in hybrid applications.weak for plug-in hybrid applications.

• NiMH cells can be used for swings between NiMH cells can be used for swings between 20% and 80% SOC, achieving effective 20% and 80% SOC, achieving effective energy densities of 40-50 W-h/kg in plug-in energy densities of 40-50 W-h/kg in plug-in applications. Based on known results from applications. Based on known results from commercial hybrids, this should be viable.commercial hybrids, this should be viable.

• Li-ion cells can be used for swings between Li-ion cells can be used for swings between 20% and 90% SOC, achieving effective 20% and 90% SOC, achieving effective energy densities of 140 W-h/kg or more.energy densities of 140 W-h/kg or more.


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• All can have efficiency enhanced with All can have efficiency enhanced with ultracapacitors as auxiliaries.ultracapacitors as auxiliaries.

• The application in the stated range is The application in the stated range is predicated on active battery management, predicated on active battery management, especially active balancing.especially active balancing.

• There There areare commercial Li-ion batteries that commercial Li-ion batteries that have been matching the claimed performance have been matching the claimed performance specs and should be able to perform to the specs and should be able to perform to the requirements.requirements.


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• Is it enough?Is it enough?• In city driving, a well-designed car needs no In city driving, a well-designed car needs no

more than 80 W-h/km (125 W-h/mile).more than 80 W-h/km (125 W-h/mile).• At 140 W-h/kg, 100 kg of Li-ion batteries At 140 W-h/kg, 100 kg of Li-ion batteries

could deliver 175 km of all-electric city range.could deliver 175 km of all-electric city range.


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ConclusionConclusion

• There is growing knowledge of considerations There is growing knowledge of considerations for maximum battery performance in the for maximum battery performance in the context of plug-in hybrids.context of plug-in hybrids.

• Li-ion cells should be able to deliver more Li-ion cells should be able to deliver more than ten-fold effective energy density than ten-fold effective energy density improvement compared to present hybrid improvement compared to present hybrid strategies.strategies.

• For all cell types, limiting the SOC range is For all cell types, limiting the SOC range is vital for longevity.vital for longevity.

• Cell balancing to permit arbitrary SOC levels Cell balancing to permit arbitrary SOC levels also appears to be vital.also appears to be vital.


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Questions and DiscussionQuestions and Discussion

Documents

Battery Management for Maximum Performance in Plug-In Electric and Hybrid Vehicles P. T. Krein Dept. of Electrical and Computer Engineering University