BQ20Z95: Cell Balancing

Part Number: BQ20Z95

We have a 3S1P 18650 Li-ION pack using a bq20Z95. On a recent batch we have been experiencing a number of packs becoming un-balanced whereby one cell is 0.5V lower than the other two cells e.g. cell1=3.2V, cell2=3.7V, cell3=3.7V. We have not seen this issue previously.

My understanding was that the bq20z95 automatically performs cell balancing by switching in discharge FETs to selectively discharge cells and maintain balance.

I have access to the bQ20z95 via a EV2400. Any suggestions on what to check?

over 6 years ago

0 Batt over 6 years ago

TI__Mastermind 42535 points

Cell balancing is only enabled during charge. So, when your battery is discharged it can have a large variance in voltage. Please check to see if your cell contacts are clean. Any increase in impedance there can reduce your balancing current and lead to leakage or reduced charge.

0 dh over 6 years ago in reply to Batt

Prodigy 220 points

Thanks for the reply. The pack is actually bought in from a UK supplier with the cells tagged (spot welded) together. We have been using this pack (same construction) for a number of years (approx. 3000 packs) and haven't seen this issue previously.

However, due to cell obsolescence, this latest batch has changed cell manufacturer to use LG INR18650HG2. The recent cell imbalances could be related to this change, or it could be a co-incidence?

A 500 mV imbalance seemed 'large'. We have an embedded MCU that (among other functions) monitors the battery and refuses to use/charge the battery with this level of cell imbalance within the pack. Perhaps we are being too cautious?

I have an imbalanced pack on my desk at the moment, so I'll try some charge / discharge cycles it to see if it re-balances.

0 dh over 6 years ago in reply to dh

Prodigy 220 points

Additionally, this post by

Doug Jensen

175 Points

Mark Ackerson posted BQ20Z90: BQ20Z90EVM Software in Power management > Power management forum. over 5 years ago

https://e2e.ti.com/support/power-management/f/196/t/712588?BQ20Z655-Cell-Balancing-FETS-Is-it-possible-these-remain-on-when-not-charging-What-can-cause-single-cell-discharging-when-mated-to-a-655-#pi320995filter=all&pi320995scroll=false

seems to describe my exact issue (albeit with a bq20z655 as opposed to my bq20z95). Unfortunately the reply doesn't answer the question as to what might have caused the imbalance in the first place!

0 Batt over 6 years ago in reply to dh

TI__Mastermind 42535 points

Hi DH,

Please try your solution of doing cycles to see if it resolves. It is also possible if your cell balance timers are off that can affect the cell balancing time. But if you are using the internal defaults, that should not be an issue.