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Serial balancing of Li-Pos for bq24616

Zhang Hui
Intellectual 625 points
Other Parts Discussed in Thread: BQ24616, BQ76920

Hello,

In using bq24616, in some scenario the design discharges different amounts of capacity from batteries connected in series.

For example, we use five lipos to get 21V, but the lowest one is occasionally used as a low voltage sources for electronic modules (1.8, 3.3V processor, I/O circuits, etc.). Therefore the lowest battery almost always discharges at least slightly more than the upper ones.

We are questioning the design ourselves: when upper 4 cells are actually charged to each’s full 4.2V voltage, the lower one might still at its 4.0V; the bq charger of course has no access to individual cell’s voltage as it gets overall voltage feedback from VFB. As long as the lowest one is lower from 4.2V to cause VFB<2.1V, the upper ones are charged with unnecessary current alone with the lowest one.

1. Does there lies a potential danger of bulging or even explosion?

2. As a general question, is it advised against discharging cells of a serial connection at different rates?

3. If we already have several cells of different voltage (4 vs 4.2, e.g.), can we just connect them and then charge with bq? Or should we first charge each individually (with one cell 4.2V charger) to full, then assemble to series and then start the new charge/discharge cycles?

 

Hui

  • 0
    TI__Expert 5565 points

    Hui,

    1. It sounds like you are charging unprotected, individual cells rather than charging a pre-manufactured, protected battery pack, correct? If so, you need to have battery protection/monitoring for your cells to ensure that each individual cell in your series cell is not over-charged. As you pointed out, the bq24616 does not have access to the cell voltage of every cell. Therefore, you must rely on a separate battery protection/monitoring IC that disconnects the pack when a cell is over-charged. If you want to ensure that each cell is charged equally, you will need to have an IC that does "cell balancing". TI offers many products that do this. Take a look at our bq76920, for example, that provides protection as well as cell balancing for up to 5 series cells.
    2. It is not recommended to charge/discharge individual cells at different rates. This could cause one cell to degrade faster than the other cells in the series, leading to less charge capacity/voltage/etc over time. I would recommend that you do NOT run your low voltage sources from the lowest cell. Instead, consider putting a low-IQ LDO in your design that takes the 21V battery voltage as the input, and converts that to 1.8V/3.3V/etc for your application. TI offers both single channel (LM295x-N, for example) and multi-channel (LM2984C, for example) LDOs that could fit your application.
    3. See comments in #1. There is no need to charge each cell individually as long as you have a battery protection IC that monitors each cell voltage in your series configuration. The charger will charge the battery pack at 21V, and the protection/monitoring IC will monitor each cell to ensure there is no over-voltage. If the protection/monitoring IC also has cell balancing (like the bq76920), then it will try to make sure that all of your cells in the series configuration are charged to the same voltage.