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Welcome
Connecting batteries in parallelUnexpected effects and solutions
Battery Power ConferenceSept. 18 2012
Davide Andrea, Elithion
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Elithion
Lithium-ion BMS for large batteriesTraction packsBattery modules for large arrays
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Overview
Paralleling at the factory vs. in the field Parallel cells, not strings Introducing “Short Discharge Time” Issues with paralleling batteries in the field Some solutions for those issues
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Factory vs field
At the factory: Parelleled once and
for all
Reason To get desired
capacity
In the field: Paralleled at any old
time
Reason For flexibility
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Cell in parallel vs. strings in parallel(at the factory)
Cells in parallel Strings in parallelCellis in parallel, then sets in series (lattice network)
Cells in series, then strings in parallel
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Cell in parallel vs. strings in parallelBMS cell boards or tap points
Cells in parallel Strings in parallel4 cell boards, or 5 tap points 16 cell boards, or 14 tap points
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Cell in parallel vs. strings in parallelLow cell capacity limitation
Cells in parallel Strings in parallel
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Cell in parallel vs. strings in parallelHi cell resistance limitation
Cells in parallel Strings in parallelBad cell reduces capacity NO LOAD UNDER LOAD
Bad cell shuts down battery NO LOAD UNDER LOAD
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Strings in parallelOne switch vs many switches
One switch per string String directly in parallelBad string can be isolated Bad cell shuts down battery
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Parallel connection in the field
Reasons: To carry only required capacity To add to depleted battery To service a battery To add redundancy
Issues: Inrush current with differing SOC Keeping track of SOC, capacity
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Parallel connection in the field
The fuel tank mindset: you can always add fuel tanks to increase range.
This is not applicable to batteries.
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The fuel tank analogyOne tank at a time
Bad analogy to batteries in parallel
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Adding a full batterywhen the first battery is empty
Charge rushes into the empty battery: heat!
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Initial connection (in field)Batteries as voltage sources
Batteries are voltage sources:Series: easyParallel: problematic If ideal voltage sources...
V1 - V2
0 Ω
Parallel ideal voltage sources = infinite current
= ∞A
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Initial connection (in field)Way to parallel voltage sources
Ideally, voltage sources are connected through current sources
Or, at least, through resistorsNever directly
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Initial connection (in field)Real world batteries
Resistance is non-0Voltage changes with SOC
Vb1 - Vb2
R1 + R2i(t) =
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Initial connection (in field)Damage from inrush current
Damage to interconnects Damage to cells? Possible if:
High dV/dSOC (standard Li-ion) Low Rseries
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Short discharge timeDefinition
Theoretical discharge time across a short circuit Constant, characteristic of each cell technology,
regardless of capacity or voltage Easy calculation of resistance
R = TShortDisch * Voltage / Capacity Easy calculation of efficiency
Eheat = Eout * TShortDisch / TActualDisch
Ranges from ~10 s to ~250 s
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Short discharge timefor various cell families
Lead acid
NiMHNiCd
Adv LA
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Specific Power vs. Short discharge time
Specific power Short discharge timeMarketing more than engineering?Achievable in reality
Mathematically preciseNot achievable in reality
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Short discharge timeShort discharge time vs energy density
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Initial connection (in field)Worst case
EIG F007 cells (LiPo, 7 Ah, 1.6 mΩ) Lowest resistance, high dV/dSOC
N-1 cells 100 % SOC + 1 cell 0 % SOC
(4.2 V – 2.7 V) / 1.6 mΩ = 940 A-pk = 134 C-pk In general 4 ~ 134 C peak
FULLCELLS
EMPTYCELL
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Initial connection (in field)2 cells
KOKAM SLPB....H5 cells (LiPo, 5 Ah, 3 mΩ) 1 cell 100 % SOC + 1 cell 0 % SOC2
2 kWhwasted
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Initial connection (in field)Energy and charge loss
Charge loss Energy lossNo charge is lost:Just as many electrons flow out of the most charged battery as flow into the least charged one.
A bit of energy is lost:The current through the connecting resistance produces heat.The energy loss is:~12 % for std Li-ion~8 % for LiFePO4Less for delta SOC < 100 %Independent of resistance
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Paralleling batteriesFactory vs. field
Paralleling at the factory: OKCells all have same SOC
Paralleling in the field: not idealPossible damage with low resistance cellsBMS's SOC value may become invalidBMS may be off, or current could exceed BMS's rangeEnergy loss ~10 % @ ΔSOC = 100 %
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Paralleling techniques (in field)To minimize inrush
Wait for equal voltages, or Charge lowest battery, or Discharge highest battery, or Transfer energy between batteries (DC-DC)
THEN you can connect in parallel
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Paralleling techniques (in field)SOC and capacity evaluation
Each battery requires its own BMS (& SOC)
SOC after connection: High inrush: each BMS estimates SOC from OCV Low inrush: each BMS calcs SOC from current
Master BMS computes SOC and capacity of entire pack from individual battery SOCs
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ConclusionsAt the factory
Paralleling at the factory is OK Parallel cells directly (not strings)
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ConclusionsIn the field
Paralleling in the field can be a problem Avoid if possible:
Damage, loss of energy, complex calculations But, if you must:
Use 1 BMS & 1 switch / string Prevent high inrush at connection by equalizing
voltages before connecting Calculate capacity and pack SOC from each
battery's capacity and SOC
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Thank you
Questions?