A recent 30 kWh import from Japan is showing one cell pair quite low - for example at full charge: 95 pairs - average = 4.077V, max = 4.086V;
1 pair - 3.823V; 263mV max variation. All 95 higher pairs were closely balanced.
As the SoC reduces the low cell causes the car reported SoC to drop suddenly - e.g. at 67% SoC, dash says 25% - this is probably expected when the low cell drops is around the voltage knee - the low cell was at 3.617V.
There seems to be an issue if the cars are left unused, especially cold. It is reportedly usually a problem of balance, not bad cells, and has been seen only on the 30 kWh packs. One solution has been to open the packs and manually balance the low cell pair but this is quite intrusive.
Today we started with a slow process of leaving the car on with a light load (approx 5 to 6A), all cells except the lowest shunting, and noticed this pattern of voltage reduction over a period of 8 hours:
Min Avg Diff
3.701 3.968 0.267
3.689 3.946 0.257
3.617 3.853 0.236
3.568 3.816 0.248
3.519 3.766 0.247
3.436 3.71 0.274
3.234 3.678 0.444
Note that the low cell voltage is reducing at a slower rate than the rest above 3.6V which implies reasonable capacity but it depends on the effective shunt current. I had understood it was around 1A - is this correct?
Over the next few days some 100% charges and further discharge cycles will be tried. The top end balancing process does not last that long and I am hoping that using the shunts over an extended period may help balance the pack.
Has anyone successfully recovered this amount of poor balance in a 30 kWh pack? Any suggestions welcomed. Thanks for reading.