Download ECE 364 - Power Electronics

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.
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University of Illinois at Urbana-Champaign
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Outline
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Performance requirements
Present situation
Lead-acid cells
NiMH cells
Li-ion cells
Battery management components
Conclusion
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Performance Requirements
• Hybrid vehicles
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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
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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 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
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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.
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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.
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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 damage
mechanisms.
• Do not favor HEV and EV
applications 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 few
problems with Prius
traction batteries –
routine replacement
has 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 Wh/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%.
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)
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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: 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,
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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 auxiliaries
when 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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