Re: Is my battery dying ?
Posted: Tue Oct 23, 2018 5:31 am
kiev wrote:i suspect the cell may have been okay since it didn't act like as weak or failed cells as in your case (the quick rise to the top voltage during charging)--my guess was that something on the CMU board failed and was always discharging that cell. Over time the voltage difference kept getting larger until a "full" charge was showing less than 8 bars.
That's what happened to my car a few years ago. The affected cell was consistently lower in voltage than the others (and dropping by the day), but it wasn't showing signs of being weak, as during heavy acceleration or regen, it would drop or rise the same as the other cells. In my case, pack SoC followed the weak cell down. It didn't show any real signs of internal short, either. None of the temperature sensors were reading high and it drove totally fine despite the reduced range.
As for the charge cutoff on level 1 or 2, I forget what the number was. The car ramps down charging as the highest cell(s) reach(es) 4.105 volts to hold them at that voltage. However, if a cell sits at 4.11 volts and doesn't come back down while the charger ramps down, then the car will stop charging. Whatever the SoC is at that moment is what the car considers "full" as far as charging goes. This is what prevents the car from reaching 100% charge.
In my case, with a lower but otherwise apparently healthy cell, the other cells in the pack all reached 4.105 volts, and the charger ramped down to avoid overcharging them. At some point, the cells wanted to go higher in voltage, which the car stopped by ending the charge. Day 1 of the failure, this was at 76% charge. In my case, SoC of the entire pack followed the low cell. Since it couldn't reach 4.105 volts without over-charging the other cells, it never reached 100%. SoC followed the cell down to a total range of about 15 miles by the time I took the car to the dealer.
kiev wrote:i think you do have some truly bad or weak cells, but they are manifesting as reduced range or capacity degradation, and not the "lack of reaching full" on the fuel gauge. But it seems that those 2 cells would be getting over-charged if they were held high while waiting for the others to catch up?
If their internal resistance is high, they would be slower at absorbing the charge. With terminal voltage being held constant, the resistance of the weak cells is limiting charge current for the whole pack. The pack will only charge as fast as the slowest cell can absorb a charge. They would only be over-charged if the car didn't limit per-cell terminal voltage (eg. if the car would ignore the weak cells and try to QC at 361 pack volts).