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BQ40Z50-R1: BQ40Z50-R1

Victor Park
Intellectual 2550 points
Part Number: BQ40Z50-R1

Hello,

Can I ask your help for below questions?

 

1. 3S2P battery full charge

2. Discharge Bank3 only with 200mAh h

3. Checked Bank voltage max-min = 26mV

CellVolt1

CellVolt2

CellVolt3

4267

4265

4241

 

4.Connect all Bank and discharge with 0.2C(=1.598A) <= Logging start

5. Rest 5hr

6. Charge to RSOC 100% (500mA)

 

As Bank3 has less capacity, Bank1 and Bank2 CB time has been calculated. CB time1_4536s=1.26hr / CB time2_11607s=3.2hr

According to datasheet, internal cell balancing current is about 9mA. So, for 200mAh cell balancing, it might require about 22.2hour (200mAh/9mAh). However, calculated CB time looks not enough(just  3.2hour).

After 3.2hour, CB bit is turned off and when charge is terminated, voltage max – min = 25mV. It is just 1mV difference before/after cell balancing. I’m curious cell balancing works correctly or not. Can I ask your comments on it? I’ve attached log file and gg file.

17G90Q.gg.csv 

I’m also curious that how cell balancing works. When Cell1 internal FET turns on, it wastes charge to GND. Then what about Cell2 internal FET on? Does it also waste charge to GND together with Cell1 FET turn on? Or, does it accumulate charge on Cell1?

 

  • 0
    TI__Intellectual 2146 points

    Hi Victor,

    I see the log but I don't see a valid .gg.csv file. Do you mind sending that again?

    As for your questions on how cell balancing works, yes, Cell 2 will waste charge to GND if the Cell 1 FET is also on.

  • 0 in reply to Albert Anwar
    TI__Intellectual 2550 points

    Hello Albert,

    Thanks for reply!

    so sorry. I was confused. Here is gg file.

    8875.gg.csv

    By the way, talking about your comments below, is there specific condition for Cell1 FET is also on? I mean, is it something controllable by customer not to waste charge to GND?

    "Cell 2 will waste charge to GND if the Cell 1 FET is also on"

  • 0 in reply to Victor Park
    TI__Intellectual 2146 points

    Hi Victor,

    Thanks for the file, I'll get back to you next week regarding the issue.

    I should clarify my previous response here - the Cell 1 FET should only be on if the device is balancing Cell 1 only. If the device is balancing Cell 2 it shouldn't turn the Cell 1 FET on, because the device cannot balance multiple cells at once. So the above situation (where Cell 2 wastes charge to ground because the Cell 1 FET is also on) should never happen unless there's an error with the device. If the device is balancing Cell 2, the extra charge will accumulate on Cell 1.

  • +1 in reply to Albert Anwar
    TI__Intellectual 2146 points

    Victor,

    Here are my thoughts after looking at the log: the cell balancing feature is activating not when the battery is relaxed, but when the battery enters charging mode. Section 7.4 in the TRM explains how cell balancing works in charge mode, but the essential bit is that it doesn't check what the max cell voltage delta is like it does when the device does cell balancing in relax mode. So the voltage delta doesn't matter for cell balancing occuring during charge - the cells can still be a bit imbalanced at the end of balancing.

    From the logs though, it looks like the max cell voltage delta when balancing finishes is 4mV, which is much less than the 23mV at the end of charging. This suggests that the cell voltages are rising unevenly as they are being charged with cell balancing off, which could be due to manufacturing differences of the cells. Again the device won't deal with these imbalances during charging because it doesn't check this delta, but it'll wait until after the battery completes charging to deal with it.

    The only question now is why the cell balance time is so low. I suspect it's because the balancing is occuring during charging, so not only is the cell with the highest voltage being bypassed, but the cell with the lowest voltage is also being charged, leading to faster balancing times than you would expect. In any case the cell balancing feature brings the max cell voltage delta to 4mV which is fairly close, so I would say that cell balancing works correctly here.

    You can also try triggering cell balancing during relax mode. If the max cell voltage delta is still high after that, it could more strongly indicate that maybe cell balancing isn't working since minimal charge is entering the battery from the charger.

  • 0 in reply to Albert Anwar
    TI__Intellectual 2550 points

    Hello Albert,

    Thanks for kind help!

    By the way, can I ask what you meant for 'The only question now is why the cell balance time is so low.'? According to your comments, Cell Balancing looks work well as voltage gap was 4mV when CB is finished. Why did you say CB time is so low?

  • 0 in reply to Victor Park
    TI__Intellectual 2146 points

    Victor,

    As you mentioned before the cell balance time for cell 2 is around 3.2 hours. The TRM shows how to calculate the cell balance time by hand, and doing that with the data you provided would seem to lead to a higher cell balance time than what was actually calculated. At the end of the day the actually calculation does seem correct, so my hand calculations may be inaccurate.