Charging to 80% capacity

[alohart]

While battery chemistry itself is to some degree responsible, power density tends to be the more significant factor. I.e., because the 60Ah cars must use 2x the capacity of the cells in the car vs a 120Ah one, they are essentially getting cycled twice as often - which is a major driver of decay.

Our new 2014 60 Ah BEV lost ~25% of its usable capacity after having been driven only 13k miles over 7 years. Our used 2019 120 Ah BEV had lost <5% of its usable capacity after having been driven 32k miles over 3 years. Our 60 Ah battery pack had experienced fewer charge-discharge cycles and more aging than our 120 Ah battery pack. So in my very limited experience, age was a much more important predictor of degradation than the number of charge-discharge cycles. However, there are way too many factors and way too few examples to draw any general conclusions from my experience.

There is one oddity about our 2014 BEV that I've always wondered about its relevance: it was stored every year for a total of 3 years out of our 7 years of ownership. I always stored it at a charge level between 40% and 60% and at an average temperature of ~77 ºF (25 ºC). I've never been able to find any suggestion that a battery cell that isn't being charged or discharged regularly degrades more rapidly. However, maybe this is true.

 

[Gonville]

Our new 2014 60 Ah BEV lost ~25% of its usable capacity after having been driven only 13k miles over 7 years. Our used 2019 120 Ah BEV had lost <5% of its usable capacity after having been driven 32k miles over 3 years. Our 60 Ah battery pack had experienced fewer charge-discharge cycles and more aging than our 120 Ah battery pack. So in my very limited experience, age was a much more important predictor of degradation than the number of charge-discharge cycles. However, there are way too many factors and way too few examples to draw any general conclusions from my experience.

There is one oddity about our 2014 BEV that I've always wondered about its relevance: it was stored every year for a total of 3 years out of our 7 years of ownership. I always stored it at a charge level between 40% and 60% and at an average temperature of ~77 ºF (25 ºC). I've never been able to find any suggestion that a battery cell that isn't being charged or discharged regularly degrades more rapidly. However, maybe this is true.

What level was it charged too? The 2014 60Ah? The manual always used to say store it fully charged? Pretty sure cell balancing would have been significant factor over that length of time. Still as seen 7 years at 2% a year should have been around 14%.

 

[Dutchi3]

Still as seen 7 years at 2% a year should have been around 14%

I can confirm this. I lost about 20% in 10 years. Still able to do 75-80 miles with a velvet right foot and in moderate weather.

 

[alohart]

What level was it charged too?

Typically 80% with occasional full charges when more range might be needed soon.

The 2014 60Ah?

Yes

The manual always used to say store it fully charged?

The U.S. Owner's Manual still does, but that doesn't make sense for lithium ion battery cell longevity. It also recommends leaving an EVSE plugged in the entire storage period. I turned off the electrical power while we were away to avoid potential problems with power surges, so I wasn't comfortable leaving an EVSE plugged in. The manual suggests asking a BMW service department for recommendations when storing longer than 3 months. I stored for 3 months thrice, 6 months twice, and 9 months once. The recommendation was to store at a charge level ~50% with the 12V battery disconnected which I followed.

Pretty sure cell balancing would have been significant factor over that length of time.

The mi3 app showed excellent cell charge level balance with the minimum and maximum cell voltages varying by less than 0.1V at a full charge. At one point, I left the BMW Occasional Use Cable plugged in for a week whenever our i3 was parked at home at a full charge which didn't reduce the minimum and maximum cell voltage difference. I realize that cell charge level balancing is a very slow process, but I would think that I would have noticed a slight improvement over a week if cell charge level balance was a problem.

Still as seen 7 years at 2% a year should have been around 14%.

Possibly our year-round warm ambient temperatures might have led to faster cell degradation which several local i3 owners experienced. Or maybe a batch of lower-quality cells (poor quality control) was installed in our i3's since excessive degradation hasn't been a problem for all 60 Ah battery pack owners including some in warm climates.

Anyway, our 2014 i3 is long gone. Our 120 Ah BEV's haven't experienced similar degradation under the same conditions and charging protocols over the past 3 years, so the cell chemistry must be improved.