Reason will only charge to 100%

[Gonville]

What use case(s) cause the pack to be so far out of balance that it can take a few months of charging to 100% to rectify?
I checked mine a while ago after only charging to 80% (via home charging) for quite a while and it was still perfectly balanced according to Electrified app. I don't use it daily however so I have assumed, rightly or wrongly, that balancing may occur at < 80% and the pack is balancing during the days it's not in use.

Does frequent fast charging to lets say only 60% and using the car daily or many times per day cause the pack to become out of balance over time? as not resting for long enough at a high(er) state of charge where balancing occurs more quickly?

Seems like charge level is only one part of maintaining a healthily balancedbattery pack

Agree with all that. My use case was the car was the daily driver almost every day for 5-15 miles only. Over 2 year. Once every month maybe a 150 miles trip with 3 rapid charges (60Ah) only ever topped up on free (gratis) 7kW in town during the daily driver session so never using any paid electricity such that the SOC was probably always between 25%-75%. Come the colder weather I get 35m on a full charge - so maybe 12 months of that use case resulted - at the time I had no measurement capability. But it recovered after charging at home when the ‘free’ civic 7kW started fee based charging.

It wasn’t until attending the meeting in BMW HQ Farnborough that the realisation that a top cells was actually reduced ‘after’ charging completed to aid pack balancing that a little light bulb came on. I had for months been stopping the charge as soon as our retail excursions were complete. Never getting to 100%.

 

[Gonville]

That is a terrific explanation of the impact of multi cell systems and the need for balancing. The fact we only learned in 2016 that the BMW BMS was doing this was an eye opener - the how and exactly what the conditions are seems still in doubt but the early adopters in our group achieved much better ranges than I did because they all charged to 100% at home whereas I was in Yorkshire and too stingy. I only ever charged off solar or the free local authority posts ( now long gone). I didn’t need 100% for range except on those DC charged forays where we stopped well before 100 as it took too long past 80% in the 60Ah. Splash and dash. So most other folks were using the fine ABC algorithm that prevented anyone discovering this little ‘out of balnce’ symptom so ably described by @alohart with the 2000 Honda Insight.

 

[Gonville]

I also found this to add to the debate on degradation. Has been in my archive but someone was doing these tests in the USA. AVT-INL? Anyone? Static capacity test? It looks like the format from David’s book but I haven’t been back to check and why there are only 4 cars?

 

[Dutchi3]

Different car, but my Kangoo EV has 50 bms cell-modules, each equipped with a 1 Watt dissipating resistor when balancing. It takes at least a few hours to balance the pack. Of course not every resistor need to dissipate electrons in the form of heat, but per 200Ah cell, it burns about 4Wh of energy per balancing cycle. Just a rough estimate of course, but if you have only one resistor balancing per full charge, it can take a while, like in a i3.