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BQ77915: BQ77915 Cell Balancing ?

Craig Bennit
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ24133,

Hello I have done the above layout with a BQ7791502PWR (Digikey) for 3 x 18650 Cells with cell balancing and it is charged via a BQ24133 Charger chip.

1)   What I am experiencing is that when the batteries are charging they all remain at about the same level as they charge up but they only

      Stop charging when their voltage reaches 4.50Volts and Not at 4.20Volts?

2)  If I switch the unit Off after this and leave it like this for the next 8 Days (Charger Disconnected)  then the battery voltages all seem to slowly on a daily bases Reduce

     their voltages all remaining at more or less the same level until they reach 3.70Volts ?

3) I originally had (OCDP) - R92 @ 100K but, this was not working correctly as the batteries were charging all at different voltages until the pack reached 12.6Volts then Charging stops.

    I changed this to 604K and that fixed the various voltages to all be at around the same voltage as they charge up but, It seem to take the Individual batteries up to 4.50Volts

   and the pack total to 13.5Volts then charging stops?

4) I have Not tried changing the OCDP Resistor R92 to a different value but, this issue seems to be linked to R92 Value, Current Value of R92 = 604K?

Can you Please look at my circuit diagram and then Please give me some in site into this matter?

Kind Regards

Craig

   

  • 0
    TI__Guru 54715 points

    Hi Craig,

    BQ7791502  VOV - VFC is 100 mV, VOV is 4.2V so VFC is expected to be 4.1V.  I don't know the BQ24133 but see it is fixed at 4.2V/cell.  Working normally it may charge a cell to the OV threshold due to tolerances or any mismatch per cell. 

    You may want to check the connection of the charger, it should connect at PAC-BATTERY- so the charge FET can stop current.  Also check how the charger might be influenced by R91.  R91 should have no effect on the charger.  If you have not you might inquire with the charger experts what to check for overcharge voltage.

    With the battery charged to OV including 4.5V the BQ77915 would see the OV and should balance cells down to the VOV- OVhysteresis, or to 4V.

    Please let us know what you find.