Time for an update on the dying cells situation.
Capacity has continued to drop at an alarming rate and it does indeed seem that cells 25 and 70 are slowly but surely dying, with cell 69 also showing signs of being halfway as bad as 25 and 70, with the other 85 cells still looking relatively normal. First the Ah degradation over time:
The blue dots and line are actual Ah readings taken from the car using either Diagbox or Canion. From 28k miles to 38k miles the degradation rate was linear and approximately 0.1Ah per 1000 miles, after which a series of three large drops has occurred, which I believe are a result of cells 25 and 70 losing capacity much more rapidly than all the others, limiting the effective usable capacity of the pack.
The red line is a linear extrapolation of where I think the Ah capacity would have been if cells 25 and 70 weren't prematurely dying, in other words it should still be at about 38.2Ah instead of the 35.6Ah it is now.
As can be seen from the straight red line, the degradation from 28k to 38k was completely linear before the sudden drops occured. The gradual linear decline will be the programmed in "cell degradation model" the BMS uses in between actual measurements, with the "sudden" drops representing occasions where the BMS has measured the capacity and found it to be well below the predicted level, forcing it to update it's Ah estimate.
Now let's look at the degree of cell imbalance after discharging. For a given Ah capacity mismatch between bad and good cells, the voltage will start off balanced between all cells at 100% and start to diverge at some point below about 50%. The closer you get to 0% SoC the more extreme the voltage difference will become so to get consistent results it should ideally be checked at the same SoC or some extrapolation should be applied.
Here is the voltage imbalance at a 10% SoC from one of my images earlier in this thread posted back in April:
Even as low as 10% SoC the peak difference is 50mV, which is certainly higher than it should be, and it's clear that cells 25 and 70 are weak, but at that point they are not really bad yet. Has it got worse since then ? Unfortunately yes! Here is a reading taken yesterday at 16.5% SoC:
If the condition of the cells was the same as April, you would expect the voltage difference to be less than 50mV because this was taken at a higher SoC of 16.5%. But instead it is much worse - 80mV. So without a doubt the imbalance between cells 25 and 70 and the rest of the cells has got considerably worse since April, meaning that those two cells have continued to degrade at an accelerated rate relative to the rest of the pack.
It's also apparent that cell 69 is becoming weak, although not as bad as the other two.
Now a snapshot taken during a rapid charge by which time the charge rate has dropped to 20kW. Rapid charging is also a good test of weak cells, because cells with either reduced capacity or higher internal resistance will show a higher
voltage during the rapid charge than other cells. Again you can clearly see cells 25 and 70 poking out well above the rest, and cell 69 also poking out significantly:
I don't rapid charge very often but something I've noticed now is that rapid charging speeds start tapering very early now. Even when I plugged in at 30% SoC during this test with a battery temperature of 25C (eg not cold) it only charged at 43kW for less than a minute before it started throttling back, with it down to 30kW by about 40% and 20kW by 60%. In other words throttling back a lot earlier and more than it used to.
I believe this is caused by the BMS limiting the charge rate to keep all cells below 4.105 volts, and if you have cells 25 and 70 peaking up to that voltage too soon due to being weak, the charge rate must be throttled back to avoid subjecting them to over voltage. So my rapid charging rates are significantly knobbled over what they were last year.
In short I think I'm in serious trouble here with the high mileage I do. I estimate that I've lost about 7 miles of summer range since the start of this year when the Ah capacity first started to drop suddenly.
More investigation into Peugeot's 8 year 60k mile battery warranty is giving conflicting results. I was originally told that older cars were retroactively increased to the new 8 years, but Peugeot have told other speakev members that have enquired that it only applies to cars sold after 2015 not older cars.
More importantly though there is no guarantee of minimum battery capacity like there is with many other EV's. So unless I have an outright cell failure that stops the car driving it would not be covered anyway, so the warranty is basically worthless.
So I've made the decision that I may have to do a cell swap of cells 25, 69 and 70 myself and am currently in the process of sourcing some cells and trying to talk my other half into the idea that I actually need to buy them even though the car is still currently driving!
I still have just over 2 years on a personal loan to pay the car off so a replacement pack out of warranty is not an option, and a cell failure putting the car off the road permanently would be a disaster. The good news is that I think I have found a good source of low mileage second hand cells so even though I don't plan to do the swap until next spring if I can last until then (it's getting too far into winter to voluntarily do major car repairs) if I do go ahead with buying some cells I'll get them now and store them at the correct voltage until I need them.
I'm planning to get four cells, swap 25, 69 and 70, and keep one "new" one as a spare for the future.