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Original question:

BQ40Z60: Charging voltage tolerance

BQ40Z60: Problem during CV mode

Etienne Espitalier
Prodigy 10 points
Part Number: BQ40Z60

Tool/software:

Dear Support,

I have a problem during the CV mode when I try to charge a 4S 2P battery at 100%. Indeed, during the end of charge, the voltage is fluctuating and that causes the flag COV to trig. The thing is that we are producing a motherboard with bq40z60 and we have a different behavior during the CV phase. For example for one of them the problem appears at 60% of charge. For the most the current is not regulated properly so the VCT never occurs. After a few cycles of charge and discharge the battery is desequilibrated, so when we mesure the voltage for each cell in charge, it is fluctuating between 4.10V and 4.27V for the worst case. I think this is not acceptable for the battery health...

I think for our production to modify the schematic of the evaluation board by adding a numeric potentiometer instead of the feedback resistors or put and external charger and bypass the bq40z60 charge function. What do you think as a best solution ? Do you think it is possible to monitor the battery voltage and adjust the VFB pin (create an additionnal regulation loop) ? Do you have another schematic than the evalution board to use ? Or do you have a firmware revison to set the parameters in a more accurate way ?

I join some curves (not the worst) to better understand and also the configuration file (.gg.csv)HNF_8x660_v0.0.6.gg.csv

I would be grateful if you can provide any help for me.

Best regards,

Etienne Espitalier

  • 0
    TI__Mastermind 25855 points

    Hello Etienne,

    Before going into the software/firmware side could you replicate the same problem on the EVM and attached a schematic of just the battery gauge. You can send it through private message if you send a friend request in E2E or attach it directly in the trend as well.

    Thank you,
    Alan 

  • 0 in reply to Alan Frias
    Prodigy 10 points

    Hello Alan,

    Thank you for your reply. Here is the schematic that we use :
    sch 4 batt.pdf

    Here for example I used to try to limit the charge at 90%. We can see that the voltage is stable at 9400s and start to rise unexpectedly at 9600s. After that, the battery try to charging again with a current higher than before. At the end it just try to maintain 90% by sending current pics.
    I will try to plot another curve on my motherboard to illustrate the problem.

    See you later !

    Etienne

  • 0 in reply to Etienne Espitalier
    Prodigy 10 points

    I have replicate the problem on the EVM and the curve looks like this :

    However I am able to stabilize it by incresing the voltage register to 34 (the real value is 32.69) in the data flash but I think it's not a normal behaviour.

    I have wait a few moment and now it's again chopping the current :

    Yesterday I tried to charge it with an external charger (laboratory power supply) using VSYS pin to input the voltage and it's works perfectly :

    The CV phase is very long due to the high internal resistance of the battery (mesured around 200mΩ) and I think it's a part of the problem with the bq40z60 charger. Indeed the batteries that we use are VARTA 1/LPP 443441 S PCM W (660 mAh) in configuration 4S 2P and there is also a small BMS chip in each cell.

    Thank you for your help !