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BQ78350-R1: Issues on cell balancing

Felipe Marques
Prodigy 230 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQ76940, BQ76940EVM,

Hello everyone,

We have been developing a custom Battery Pack (48V/50Ah - 15 LFPs cells connected in series) with the bq76940 + bq78350 + MSP430 combo. But, before using the 50Ah cells, for testing purpose we are using 2500mAh LFPs cells and bq78350/bq76940evm. During experimental tests with 0.5C/0.5C discharge and charge regime, we noticed which there is some cell imbalance after battery charging (figures below). As balancing parameters, we are using:

Cell balance threshold: 3400mV (nominal cell voltage of 3600mV, which corresponds a charge voltage of 54V)
Cell balance window: 100mV
Cell balance min: 40mV
Cell balance interval: 20s

Do you have any sugestion how we can overcome this or to improve cell balance?

Thank you in advance.

  • 0
    TI__Guru 54715 points
    Hi Felipe,
    The rapid voltage rise is a characteristic of the LiFePO4 cells. With your settings it seems your pack should start balancing when the high cell goes above 3400V. The bq78350 will adjust the threshold up when all cells exceed the 3400mV threshold or the high cell goes above the threshold + window = 3500mV. From the curve it looks like it may start balancing the high cell then stop until the voltages rise sufficiently. You might try making the window smaller. You might also look at the balance current selected which may already be quite high for a 50Ah pack and the time the charger spends in taper where balance current is a more significant portion of the charge current into the pack. If the taper time is short there won't be much time for balancing each cycle.
  • 0 in reply to WM5295
    Prodigy 230 points
    Hello WM5295,

    Thank you for the response. This rapid voltage rise is very difficult to deal with, because, it is located at very final end of battery charging.

    Before finishing this thread, I would like to ask you some questions, if you do not mind:

    1-) Do you believe using active balance could improve this results, since, we could be able to balance any time we want, minimizing energy losses (because it is active)?

    2-) Keeping passive balance, if we control the charge current, decreasing it as soon as we notice rapid change in cell voltage, do you believe it would help cell balancing?

    3-) In this test we used all depth of discharge (DoD of 100%). Some researches says which if we change DoD to 80~90% it could improve cells lifetime. In this scenario, it seens we could avoid the part where cell voltage changes rapidly. Again, do you believe it could help cell balancing?

    Best regards,
  • 0 in reply to Felipe Marques
    TI__Guru 54715 points
    Hi Felipe,
    These are opinions, I do not have example data to demonstrate.
    1. Yes, active balancing should be able to adjust the cells at any point of operation. Your algorithm would need to remember or anticipate which cells rise to know how much to balance. You may need to check with your cell supplier to see if the voltage rise near full is strictly due to state of charge or if there are other factors. Active balancing is more expensive than passive, its normal attractions are efficiency and the ability to balance at any point of the discharge curve and even one way at one end and the other at the opposite, that is to balance out of a low capacity cell at the end of charge and balance into the low capacity cell at the end of discharge. Passive balancing could also adjust the voltage at the end of charge, but the balancing algorithm will likely need to anticipate the state of the cells at the end of charge to balance to meet that goal. The bq78350-R1 does only voltage balancing in its set of parameters, it does not predict the end point. With a limited balance time it will take more charge cycles to balance the pack. The bq78350-R1 does not share the bus to the bq76940 monitor, it will reset any registers it finds changed from its setting.
    2. Yes, decreasing the current to extend the balancing time should improve the balancing. The bq78350-R1 must see the voltage to maintain its balancing operation, so current would need to be controlled to keep the voltage difference. That does mean the charge time would be extended.
    3. I understand that from studies also. The general idea of the passive balancing with the bq78350-R1 is to align the cells at full charge based on voltage (its best understanding of full charge). When the cells discharge, near the end some will drop off faster than others due to capacity differences. The bq78350-R1 CEDV algorithm sees the EDV2 point where the cells start to drop to learn the capacity.
    With my limited understanding of the research the cell lifetime is improved by reducing the stress from high and low voltage. If you operate the cells from say 10% to 90% DOD, it should reduce the voltage peaking when full and the amount of drop at empty. If something changes the cells over time from environmental conditions such a some cells being warmer, you may end up with some cells operating perhaps 5% to 85% and some at 15% to 95%. When there is a voltage difference in charge meeting the conditions the bq78350-R1 would balance, this may be an easier condition to maintain over time than to equalize a pack which is mismatched in state of charge at assembly or charged to cell maximum.
    With a limit to the charge of the cells, at reset the bq78350-R1 estimates the remaining capacity for the chemistry based on voltage. If the voltage is lower due to the limited upper range, it will underestimate the remaining capacity.
    If the limit of the discharge removes the curve of the cell voltage at the end of discharge, it may be difficult to get a good fit of the CEDV parameters over the limited range. If using CEDV parameters for the full cell capacity, since learning is normally at 7% capacity, a discharge to only 10% would not learn capacity. Perhaps that is OK with the reduced range of the cells and capacity update is only needed when it decreases to allow learning. Fixed EDV operation is also available if that better suits your range of operation.

    Sorry for the long post, hopefully something is helpful.