How is your battery holding up?

[Fisher99]

I have a 2014 i3/REX with 51k miles. I'm the third owner, with the first being a lease and the second being a friend of mine who owned it until I purchased it in June of 2019. The battery pack is 22.6 kwh and the "secret menu" is showing 16.1 kwh, or 71% of the original capacity. That means that the battery has lost 29% of its capacity in roughly 6 years. That seems excessive to me, but then again I'm new to electric cars and am a total neophyte when it comes to battery technology and things electrical. Would be interested to know what the experts on this forum think. And what other people are seeing on their i3's.

[richs]

22.6 kWh is the full capacity of the 2014 model battery, including a buffer on the top end that is not used to protect the battery, even when the battery is brand new. The stated user-available capacity of the battery is 18.8 kWh, so your reading of 16.1, or 85%, is actually pretty good for the mileage on the car.

Source for my numbers:

https://insideevs.com/news/334057/bmw-i ... -expected/

Also, as others have pointed out, the Batta. Kappa. value from the service menu is just an estimate and it will vary up and down a over time, slowly trending downwards as the battery ages.

[Fisher99]

Well, that's certainly an encouraging bit of information. Makes me feel considerably less nervous about my battery situation!

[flynt]

I also have a 14 REX. My batt kappa max went to ~13.1, or 70.2%. I took it to the dealer and they "updated the software" and I'm back to 16.1ish.
Here's a thread with a little real-world info. viewtopic.php?f=11&t=5651

I'm nervous about my battery capacity. I envy yours, and most others I've read about online. I wouldn't sweat it if I were you, but I'd keep an eye on range and batt kappa max - mine dropped pretty quickly.

[richs]

I took a look at my own 2017 i3. It has about 17k miles on it. According to several web articles from 2016, the 33 kWh battery in the 2017 model has "27.2 kWh available". That's interesting, because my car reports a Batt. Kapa. max of 29.5 KWh!

If the service menu is to be believed, my car will charge up to 108.5% of its rated capacity, even after 17k miles and almost three years. Maybe that's one magic battery pack that has gotten better with time, like fine wine. More likely, however, is that BMW just fibbed and made up the number of 27.2. BMW may simply underrate the battery to avoid warranty replacements near the end of the warranty period. They don't want to replace batteries for free; they want to sell new cars.

I'd love to hear from one of the battery pack engineers about this, but I'm sure that they had to sign NDAs, unfortunately.

[symonray]

The current Batt. Kapa. Max reading on my 2017 i3 (with 24k miles) is also 29.5 KWh.

[3pete]

the 2017 model has "27.2 kWh available". That's interesting, because my car reports a Batt. Kapa. max of 29.5 KWh!

One thing that's interesting about this is that depending on what measure you look at, the 2017 vs. the 2014 battery may have:
1) 157% capacity (94ah / 60ah),
2) 150% capacity (33kwh total capacity/ 22kwh total capacity)
2) 145% capacity (27.2kWh usable / 18.8 kWh usable)

Picking the middle number, if it was 150%, the usable capacity would be 28.2kwh on the 94ah battery, so that essentially "buys" BMW another kWh of capacity to use for the upper or lower buffer. That wouldn't explain consistent readings over 28.2 but it gives them a possible opportunity to help with battery management.

Potentially understating the capacity to 27.2 also means your battery capacity would have to drop below 19.04 kWh to trigger the warranty as opposed to 19.74.

These aren't massively different, but I find it interesting. Both factors play in BMWs favor: giving an extra kWh to battery management and lowering the warranty threshold a bit might combine to be a meaningful difference in warranty claims.

[alohart]

2) 150% capacity (33kwh total capacity/ 22kwh total capacity)

To be a bit more precise:

2) 154% capacity (33.2kwh total capacity/ 21.6kwh total capacity)

which, when compared to the usable capacity of the 60 Ah battery pack, would make the usable capacity of the 94 Ah battery pack 28.9 kWh. For some reason, BMW rated the nominal voltage of the 94 Ah battery pack to be a bit lower than that of the 60 Ah battery pack. I have no idea what the implications of this are.

It appears that the 94 Ah cell degradation rate is significantly lower than that of the 60 Ah cell with few 94 Ah battery pack owners complaining of lost range compared with 60 Ah battery pack owners. Part of this is certainly the older average age, the higher average number of charge/discharge cycles, and the greater average depth of discharge of the 60 Ah battery packs, the last 2 being due to their lower capacity.

None of us knows whether the battery management system (BMS) is artificially reducing 60 Ah battery pack usable capacity to reduce the cell degradation rate in hopes of reducing the number battery pack capacity warranty claims. Many 60 Ah owners have reported that BMW mechanics have been able to do something that increases the usable capacity which avoids a capacity warranty claim, at least temporarily. To me, this is strong evidence that the 60 Ah cell degradation rate might not be as fast as it can seem and that the BMS is partially to blame for a reduction in usable capacity.

[jadnashuanh]

Given the same number of miles driven, a smaller capacity battery will accumulate more discharge cycles than a larger capacity battery.

[richs]

I think that we shouldn't discount the fact that BMW (and every EV maker) has gotten a lot more experience with battery management over the past half decade. The sudden increase in user-available capacity for 60 Ah batteries after servicing may reflect some of the experience that has been gained. Perhaps BMW now believes that they were overly conservative with the top/bottom buffers in the first few i3 model years and that they can "release" (in a software update/parameter tune) some of that capacity for use on older vehicles. Will reducing the buffer size degrade the battery faster? Maybe it will, or maybe they've determined that as long as the buffers are "big enough" it doesn't really matter.

Battery technology has changed a lot in the seven-year life of the i3. First 60 Ah, then 94 Ah, now 120 Ah, all in the same form factor, and without much increase in price. BMW says that the i3 will get one more battery upgrade in its life, even beyond the 120 Ah. That's a lot of technological churn, and I'm sure that BMW has learned a huge amount over the years. Given that Li-ion batteries are still a rapidly evolving technology, I'm willing to give BMW (and everyone else) a bit of a break here. It's hard to determine how long a battery pack will last, and I wouldn't blame them for hedging their bets a little and underrating the pack just to make sure they didn't lose a ton of money in warranty work.