Battery Management for Maximum Performance in Plug-In Electric and Hybrid Vehicles P. T. Krein Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign
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.
Outline • Performance requirements • Present situation • Lead-acid cells • NiMH cells • Li-ion cells • Battery management components • Conclusion
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
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.
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-5 University of Surrey
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 worksthroughout the SOC range isan important enabler.
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%) is5.3 W-h/kg. • Ultracapacitors can do as well.
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.
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%.
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?
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.
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%.
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 hybridelectric vehicle applications,” in Ann. Battery Conf. Appl. Advances, 1998, pp. 13-18.
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.
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
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.
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%.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.