vh2q said:
For what it's worth, I ran battery down to 0 bars and when the turtle came on the bad cell was down to 3.48V, the rest were still at 4.0V. So it seems that the bmu software sets the charge gauge based on the weakest cell, but controls the charger based on the strongest cell.
This is essentially correct. The BMU will not allow any cell to charge above 4.1 volts or below a minimum limit which seems to be a bit more flexible - typically between 3.3 volts and 3.6 volts.
When there is a weak cell these limits will be reached much sooner and the effective usable capacity is reduced.
After two charging cycles, weakest cell still does not get above 3.9V when the rest are at 4.1
Two charging cycles is not enough to do anything of significance. It took nearly 40 cycles to correct a 50mV error on my car.
Hobdrive was useful in diagnosing this situation, but there are an awful lot of meaningless screens and data. A lot of the fields come up as zero and you can't even get a current draw off the aux or the dc to dc converter. You can get a draw off the MCU. Many of the temps default to 32F. You can turn off the trouble codes but the car will regenerate them so the serious ones just pop up again. The cell smoothing function does not work. There are a bunch of screens that display CMU voltage. These appear to be clusters of 8 battery cells. This data is pretty useless.
The user interface of Hobdrive is pretty poor, I agree. However it can only provide information that the ECU's in the car make available. Under many conditions some of those fields are not applicable.
Resetting a fault code is not a magic cure - if the fault returns its because the fault is still there! Not the problem of the diagnostic tool.
The cell smoothing process is just a diagnostic process that can be initiated in the BMU ECU - I have an official Lexia 3 diagnostic tool for my Ion (the same tool used by dealers) and the cell smoothing function in that also did not work for me.
For reading cell voltages the best tool to use by far is Canion. If you have an STN11xx based adaptor like an OBDLink LX you can use Canion. Canion is read only and cannot reset codes or make any changes but I found it the best tool to diagnose weak cells on my car.
And finally, a battery pack that is only as good as its weakest cell is an abomination. Especially when, all too often it seems, one cell goes bad.
Unfortunately that's just the way it is - with nearly all EV's! Most EV's use either a single series string or more commonly a xxs2p configuration where cells are connected in parallel in pairs then the pairs are connected in series. The Leaf uses this paired approach.
However even in a pared approach there will be a large drop in capacity of the pair of cells if one cells go bad and the same issue will occur. Plenty of Leaf's have had cell failures as well.
About the only cars not affected by individual cell failures are Tesla's, which use massively parallel groups of small cells which are independently fused. If one cell in a parallel cluster of >20 cells goes faulty the capacity loss is minimal and the usable capacity of the pack will only be affected a small amount. However with a large number of cells the likelyhood of failure of a given cell is higher of course.
At least in the i-Miev a faulty cell can be replaced somewhat easily - I've done it on mine. The hardest part is getting the pack out of the car and back in again. Once the pack is down on a trolley the cell swap is relatively easy if you are an experienced DIY'er and have some electrical knowledge. Cells and modules in a Tesla pack are not DIY serviceable.
