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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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 2 Outline • • • • • • • Performance requirements Present situation Lead-acid cells NiMH cells Li-ion cells Battery management components Conclusion Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 3 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 1 cycle/5 mi over 100,000 miles – High energy density – Fast, reliable charging – Cycle life – thousands of deep cycles Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 4 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 5 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 energy density” in exchange for power density. • Space applications get much more. UoSat-5 University of Surrey • The presentation emphasizes ways to maximize battery capabilities Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 6 Present Situation • NiMH cells today are being used in about a 15% SOC range. Reasons are explored here. • Lead-acid cells provide a similar range. • Li-ion cells are more promising. • Active balancing that works throughout the SOC range is an important enabler. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 7 Lead-Acid Cells • Operating results from starting-lightingignition (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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 8 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 damage mechanisms. • Do not favor HEV and EV applications except on a “use, park, charge” cycle. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 9 NiMH Cells • Extensive data in preparation for and from experience with commercial hybrids. • Toyota has had few problems with Prius traction batteries – routine replacement has not been required. • Limited SOC swing – about 50% to 65%. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 10 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? Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 11 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 12 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%. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 13 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 14 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 15 NiMH Cells • Thus the SOC range from 20% to 80% can be used for plug-in operation. • Increases effective energy density to 42 Wh/kg – factor of 4 improvement. “Harding Handbook for Quest Batteries,” Fig. 3.7.2, available http://www.hardingenergy.com/pdfs/NiMH.pdf Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 16 Li-Ion Cells • Lithium-ion cells in general have much better reversibility than other common secondary chemistries: Energy reversibility can exceed 90%. of Charge • Discharge curves indicateState regimes of reduced reversibility. 4.2 Pack Type 1 Pack Type 2 OPEN Cell Voltage 4.1 Pack Type 3 4 3.9 3.8 3.7 3.6 3.5 100 80 60 40 20 0 Capacity (pct) Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 17 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%. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 18 Li-Ion Cells • The curve shown earlier shows rapid imbalance and capacity reduction below 20% SOC. • Key problem: cell balancing – no inherent mechanism in Li-ion. • Typical systems use resistive limiters to enforce the upper voltage limit. www.popularmechanics.com • Limiters add system nonlinearity that drives (lossy) cell balancing at the top end of SOC, Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 19 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 20 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 21 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 22 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 auxiliaries when SOC upper limit is reached. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 23 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 24 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 25 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 26 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 27 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 28 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. Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 29 Questions and Discussion Grainger Center for Electric Machines and Electromechanics University of Illinois at Urbana-Champaign 30