battery management for maximum performance in plug-in...

Battery Management for Maximum Performance in Plug-In Electric

and Hybrid Vehicles

P. T. KreinDept. of Electrical and Computer Engineering

University of Illinois at Urbana-Champaign

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

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Acknowledgements

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

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


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Outline

• Performance requirements• Present situation• Lead-acid cells• NiMH cells• Li-ion cells• Battery management components• Conclusion


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Performance Requirements

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

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

1 cycle/5 miover 100,000 miles


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Plug-In Hybrids

• Require the power capabilities and cycling capabilities of hybrids.

• Benefit from high energy density and good recharge properties.

• In other words: must satisfy everyone and everything.

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

• Here we explore the batteries.


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Present Situation

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

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

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

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

maximize battery capabilities

UoSat-5University of Surrey


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Present Situation

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

• Lead-acid cells provide a similarrange.

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

throughout the SOC range isan important enabler.


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Lead-Acid Cells

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

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

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

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

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


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Lead-Acid Cells

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

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

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

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


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NiMH Cells

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

• Toyota has had fewproblems with Priustraction batteries –routine replacementhas not been required.

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


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NiMH Cells

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

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

• What aspects is this application attempting to optimize?


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NiMH Cells

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

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

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

• Impedance rises for discharge.


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NiMH Cells

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

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

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


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NiMH Cells

• Differential power density is the remaining 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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NiMH Cells

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

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

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

– Braking strategy limits charge power at the high end.


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NiMH Cells

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

• Increases effective energy density to 42 W-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 Cells

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

• Discharge curves indicate regimes of reduced reversibility.

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Li-Ion Cells

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

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

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


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Li-Ion Cells

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

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

• Typical systems useresistive limiters toenforce the upper voltage limit.

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

www.popularmechanics.com


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Li-Ion Cells

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

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

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

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


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Battery Management Components

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

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

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

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

requirements at extremes.


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Choices for Limits

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

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

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

the daily drives.• Adaptive cycle intelligence.


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Choices for Mitigation

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

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

• Use resistive brake auxiliarieswhen SOC upper limit is reached.


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Active Cell Balancing

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

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

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

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

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


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Active Cell Balancing

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

efficiency is high as mismatch reduces.– Switched inductor types – drives current to

match charge in a controller manner.– Individual cell or monoblock chargers – the

ultimate, but expensive, solution.


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Discussion

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

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

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


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Discussion

• All can have efficiency enhanced with ultracapacitors as auxiliaries.

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

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


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Discussion

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

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

could deliver 175 km of all-electric city range.


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Conclusion

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

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

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

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


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Questions and Discussion

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