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Li-Ion Myth-Buster Poking holes into some common beliefs about Li-Ion cells and Li-Ion BMSs. Davide Andrea. Battery Power Conference 2010, 1 ,. Myths. A) Cells B) Batteries C) BMS D) Balancing. Battery Power Conference 2010, 2 ,. A: cell myths. A: Cell myths. Group A:
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Li-Ion Myth-Buster • Poking holes into some common beliefs • about Li-Ion cells and Li-Ion BMSs. • Davide Andrea Battery Power Conference 2010, 1,
Myths • A) Cells • B) Batteries • C) BMS • D) Balancing Battery Power Conference 2010, 2,
A: cell myths A: Cell myths • Group A: • Myths about Li-Ion cells • - Cell capacity myths • - Cell swelling myths Battery Power Conference 2010, 3,
A: cell myths Charge efficiency • Myth A1: • "A cell puts out less charge than you put into it" FALSE Battery Power Conference 2010, 4,
A: cell myths Charge efficiency Li-Ion charge efficiency = 100 %: Every electron that goes in can come back out (Though, energy efficiency is < 100 %) Battery Power Conference 2010, 5,
A: cell myths Charge efficiency 1 A in for 1 hour, 1 A out for 1 hour SOC: 100 %, to 0 %, back to 100 %:charge efficiency is 100 % Charge voltage is higher than OCV, lower during discharge Charge power is more than discharge power Energy doesn't go back down to 0: energy efficiency is < 100 % Battery Power Conference 2010, 6,
A: cell myths Capacity vs rate • Myth A2: • "Cell capacity depends on rate of discharge" FALSE Battery Power Conference 2010, 7,
A: cell myths Capacity vs rate Yes, at high discharge a fixed cutout will stop discharge sooner Battery Power Conference 2010, 8,
A: cell myths Capacity vs rate But the rest is still there; you can still get it out, with a constant voltage load. Battery Power Conference 2010, 9,
A: cell myths Capacity vs rate But the rest is still there; you can still get it out, with a lower cutout Battery Power Conference 2010, 10,
A: cell myths Capacity vs cycles • Myth A3: • "Cell capacity decreases with number of cycles" MIX Battery Power Conference 2010, 11,
A: cell myths Capacity vs cycles Degradation = capacity loss Higher resistance + fixed cutoffs = unused capacity Battery Power Conference 2010, 12,
A: cell myths Cell swelling • Myth #A5: "Cells swell due to charging" • Myth #A6: "Cells swell due to heat" • Myth #A7: "With a cell cooling system, there's no need for cell retention" FALSE FALSE FALSE Battery Power Conference 2010, 13,
A: cell myths Cell swelling Internal pressure: high at ~ 100 % SOC, whether sitting on a shelf or in use. It's not the process of charging It's not the temperature, not the I2R power It's the state of being fully charged. Battery Power Conference 2010, 14,
A: cell myths Cell swelling MUST use retention plates on prismatic or pouch cells. Battery Power Conference 2010, 15,
B: Battery myths B: Battery myths • Group B: • Myths about batteries • - Reliability myths Battery Power Conference 2010, 16,
B: Battery myths Paral. strings are better • Myth B1: "It's better to use multiple strings, connected in parallel" vs. MIX Battery Power Conference 2010, 17,
C: BMS myths C: BMS myths • Group C: • "I don't need a BMS" myths • - Cutoff myth • - Small SOC range myth • - Extra cells myth Battery Power Conference 2010, 18,
C: BMS myths No BMS: I use a cutoff • C1: "I don't need a BMSmy charger has a high voltage limit" • C2: "I don't need a BMSmy load has a low voltage cut-off" FALSE FALSE Battery Power Conference 2010, 19,
C: BMS myths No BMS: I use a cutoff There's no way of knowing the state of individual cells from the pack voltage. 13.2 Vtot 13.2 Vtot 13.2 Vtot 3.3 V 2.9 V 3.8 V 3.3 V 4.5 V 1.8 V 3.3 V 2.9 V 3.8 V 3.3 V 2.9 V 3.8 V Battery Power Conference 2010, 20,
C: BMS myths No BMS: I use a cutoff Top balancing won't help Fully charged, Top balanced Discharged. One has low capacity Discharged. All same capacity 14.4 Vtot 11.2 Vtot = LV cutoff 11.2 Vtot = LV cutoff 3.6 V 3.2 V 2.8 V 3.6 V 1.6 V 2.8 V 3.6 V 3.2 V 2.8 V 3.6 V 3.2 V 2.8 V Battery Power Conference 2010, 21,
C: BMS myths No BMS: I use a cutoff Yes: a CCCV will protect a top-balanced pack: when the pack voltage is at the max, all the cell voltages will be equally at the max. However, when discharging without a BMS, the voltage of the least capacity cell will drop too far and be damaged. Battery Power Conference 2010, 22,
C: BMS myths No BMS: I use a cutoff Bottom balancing won't help Fully discharged, Bottom balanced Charged. 1 has low capacity Charged. All same capacity 12.0 Vtot 14.4 Vtot = Charger CV 14.4 Vtot = Charger CV 3.0 V 3.3 V 3.6 V 3.0 V 4.5 V 3.6 V 3.0 V 3.3 V 3.6 V 3.0 V 3.3 V 3.6 V Battery Power Conference 2010, 23,
C: BMS myths No BMS: I use a cutoff Yes: a LV cutoff will protect a bottom-balanced pack: when the pack voltage is low, all the cell voltages will be equally low. However, when charging without a BMS, the voltage of the least capacity cell will go too high. And that's a fire danger! Battery Power Conference 2010, 24,
C: BMS myths No BMS: I mind the SOC • Myth #C3: • "I'll won't usethe entire SOC range,so I don't need a BMS" FALSE Battery Power Conference 2010, 25,
C: BMS myths Nice and easy: no BMS The assumptions are that: 1) The pack SOC is known 2) All the cells are at that same SOC Not so. Without a BMS, the pack SOC is not known (and even a BMS doesn't always know the SOC) Battery Power Conference 2010, 26,
C: BMS myths More cells: no BMS • Myth #C4: • "Extra cellsare cheaperthan a BMS" FALSE Battery Power Conference 2010, 27,
C: BMS myths More cells: no BMS The assumption is that a pack without a BMS will simply slowly degrade. So, instead of buying a $ 1000 BMS, I'll buy 10 extra cells. Not so. A SINGLE overcharge or over-discharge event can kill a cell. And it will keep on happening after you replace that cell. Battery Power Conference 2010, 28,
D: balancing myths D: Balancing myths • Group D: • Myths about cell balancing • - Balance purpose myths • - Balance point myths Battery Power Conference 2010, 29,
D: balancing myths Balancing purpose • D1: "Balancing protects a battery" • D2: "Balancing compensates for variations in cell capacity" • D3: "Balancing allows use of the entire charge in a battery" FALSE FALSE FALSE Battery Power Conference 2010, 30,
D: balancing myths Balancing purpose • The point of balancing is to maximize battery capacity. • Balancing brings all the cells to the same SOC at ONE point. • The SOC is balanced. Battery Power Conference 2010, 31,
D: balancing myths Bottom balancing • Myth D4: • "Bottom balancing protects the cells" MIX Battery Power Conference 2010, 32,
D: balancing myths Bottom balancing • No BMS & limited charge: • bottom balancemay work Battery Power Conference 2010, 33,
D: balancing myths Bottom balancing • Otherwise, only top balancemakes sense Battery Power Conference 2010, 34,
"Battery Management Systems for Large Lithium Ion Battery Packs" Davide Andrea book.LiIonBMS.com elithion.com Thank you