JoeS said:
Can anyone suggest a specific test to perform when the pack is opened up on the bench to verify the cell balancer is operational?
Before even opening the pack I would try running the "cell smoothing" diagnostic operation to attempt to force the cells back into balance.
This can be done with Diagbox (which I suspect Martin has access to) or it can also apparently be activated by recent beta versions of Hobdrive.
Although I have not used this diagnostic mode before (my cells balance perfectly during a charge) I believe what it does is attempt to balance the cells
without charging. Normally balancing occurs towards the end of the charging cycle however if the imbalance is too large it could take multiple cycles to balance.
The smoothing operation will work at any SoC and activate the balancers on all high cells to bleed them down to the lowest cell's voltage without charging, and as a result should be able to correct even a large imbalance. As the bleeders are small this could take many many hours but in theory should bleed the high cells down until they match the low cell and then a charge could be performed to charge all cells back up again.
Assuming this succeeded I would then watch to see whether the cells get out of balance again.
A persistent cell voltage imbalance at a low SoC indicates a difference in Ah capacity between cells, however at a high SoC it indicates either a problem with the balancing circuitry, or a parasitic load that is discharging one cell more than others. (Such as a fault on the CMU board) In theory a cell with high internal leakage could also cause this but it is unlikely.
We've seen a failure of quite a few LTC chips on the CMU boards now - a faulty LTC chip could easily cause the balancer for a cell to remain on all the time, or the LTC chip itself could present a parasitic load since it is individually connected to each cell.
If one cell is subject to a parasitic load/discharge path which exceeds the discharge rate of the cell balancers, then the balancing system will not be able to maintain balance of the cells. There has been a reported case of this happening on a Leaf over on Speakev.com.
