Management of Battery Life

[Hengus]

I am on my second i3. A new 120Ah BEV arrived last week. The bigger battery capacity has got me thinking about the management of battery life. With a 180 mile range (UK), I will only need to charge about once a week unless we are away visiting family or on holiday. My objective is to maintain long-term battery life so do I:

A. Charge up to 100% once a week?

B. Maintain a charge of say 80% max by charging once a week for three weeks followed by a 100% charge once a month (for cell balancing purposes)?

C. Just charge as and when I feel like it or when I need to?

I should add that most of my charging will be on a 240volt/7kW/32 amp charger.

PS I am loving my new 120Ah BEV as much as my old 60Ah Rex.

 

[alohart]

I usually keep cars for 15 or more years, so I would choose B. which should statistically result in a slower battery cell degradation rate. I would think but don't know for sure that monthly charging to 100% would be sufficient to maintain cell charge level balance, particularly if the battery pack remains at >90% charge level for several days per month. Apparently, cell balancing occurs at charge levels less than 100% but at a slower pace.

Many feel that managing the charge level is more effort than they wish to exert, so they would choose C.

 

[Hengus]

Thanks. The UK Jury on another forum is of the opinion that owners should just charge to 100% irrespective of the starting SOC. The great unknown for me is how much of the battery headroom is being consumed through bad charging practice? This may not be enough to impact on the BMW battery warranty but could it become an issue in later years.

 

[gt1]

I try not charge above 85% or so. Usually I set the EVSE timer to 2 hours and it does the trick. (I have a 94AH model). My only concern with this strategy is that it might interfere with the automatic cell balancing.

 

[Oleksiy]

I charge lower than 100% routinely, checked my battery cell balancing several times using ISTA+ software. No issues - 0.1-0.2V difference between min and max values. Other screenshots of battery voltage page from ISTA+ of other cars on the net show the same. I'm of the opinion that all this cell balancing thing is overblown.

 

[jadnashuanh]

If you believe the e-book on the i3, he says that the wear on the battery to charge it from 0-100% is the same as charging it 10x from 90-100%.

I like to have the battery full when I get in. I've not noticed a degradation of the battery in my 2014 i3 while doing this.

England may not see some of the extreme weather that I see in NH, USA where it can get to -15F on occasion. You can damage the cells if it gets super cold while the SOC is low.

That may not be an issue in say HI.

FWIW, the battery management has a buffer at both the top and bottom, so what you're seeing on the display isn't the absolute SOC.

 

[alohart]

I'm of the opinion that all this cell balancing thing is overblown.

I suspect that cell balancing will become more important as cells age and their internal resistances and capacities diverge. For newer cells, cell balancing is probably not needed as much.

 

[Oleksiy]

I'm of the opinion that all this cell balancing thing is overblown.

I suspect that cell balancing will become more important as cells age and their internal resistances and capacities diverge. For newer cells, cell balancing is probably not needed as much.

This makes sense, I'll keep checking the state of cell balancing. The battery has gone from 96% to 90% SOH for almost one year of use, so the internal resistance should be increasing as well.

 

[gt1]

If you believe the e-book on the i3, he says that the wear on the battery to charge it from 0-100% is the same as charging it 10x from 90-100%.

He is probably right, but would it be the the same for 10 charges from 40 to 50%?

 

[jadnashuanh]

If you believe the e-book on the i3, he says that the wear on the battery to charge it from 0-100% is the same as charging it 10x from 90-100%.

He is probably right, but would it be the the same for 10 charges from 40 to 50%?

Don't know for sure, but my gut says yes.

 

[Oleksiy]

Cycling the battery in the middle, avoiding too high or too low voltage, significantly improves longevity of li-ion batteries. This has been proven by numerous studies. In practice, this is why EVs with larger batteries are less prone to degradation, especially if their owners don’t follow ABC rule.

As to the BMW manual, the “always be charging” nonsense included there is sufficient for me to disregard their advice on the battery and revert to science instead.

 

[alohart]

Cycling the battery in the middle, avoiding too high or too low voltage, significantly improves longevity of li-ion batteries. This has been proven by numerous studies. In practice, this is why EVs with larger batteries are less prone to degradation, especially if their owners don’t follow ABC rule.

Exactly!

As to the BMW manual, the “always be charging” nonsense included there is sufficient for me to disregard their advice on the battery and revert to science instead.

I totally agree.

I've always thought that BMW's "ABC" advice and the design of the i3's BMS combine the convenience of not having to manage charging with minimizing the risk of battery packs whose capacities decline more than 30% having to be replaced while under warranty. The i3 is the first EV for many i3 owners, so BMW doesn't want to make it seem like charging an i3 requires any thought. Also, with relatively low ranges, i3's charged to less than full would have even lower usable ranges which BMW doesn't want to suggest.

For most i3 owners who don't plan to keep their i3's very long, BMW's advice works well enough. However, for those of us who plan to keep our i3's past the battery pack warranty period, BMW's advice isn't very good.

 

[Joff]

"Cycling the battery in the middle, avoiding too high or too low voltage, significantly improves longevity of li-ion batteries."
Absolutely ! That is why BMW has set up the i3 to neither fully charge, nor fully deplete it's battery. when your i3 tells you the battery is at 100% the cells in the battery are not at the max voltage they can handle. When your i3 says you have o% battery, it still has some in reserve. I read that it only lets you use 80% of the rated capacity of the battery. That's why they can advise to leave it on the charger. I'm not saying they won't last even longer if you use less of that capacity, just that the engineers at BMW do understand the batteries they are using. Tesla has a "trip mode" that allows you to fully charge the battery. It would be handy to have that feature sometimes, but on the other hand it is nice to know that the batteries on the used I3 i bought , have not been abused.

 

[Oleksiy]

8% lower limit reserve and 6% upper limit reserve, to be exact, in a healthy i3 60Ah battery. Later, once SOH goes down, it's reflected in the upper reserve - it goes down as well, the lower one seems to stay untouched on paper. See my pack's 85.5% SOC upper charge limit on a screenshot below - it roughly corresponds with the SOH of my battery at the time of 16.6-17 kWh (22.5% of total reserve equals 4.9 kWh of the total 21.7 kWh capacity). The lower limit seems to have shrunk as well - the real and displayed state of charge of 15% and 9% imply a 6% lower buffer, not 8% as shown.

Regardless of the size of the reserve (i3 battery pack has one of the largest among other EVs), it's still better to cycle closer to the middle or even to the bottom of the battery, leaving about 10-20% if possible, to extend the life of the battery. E.g., in extreme cases of 100 kWh batteries and short daily commute patterns it's better to cycle the pack in 40%-20% rather than in 60%-40% or 80%-60% window. And it's better to top up this 20% charge every day rather than 60% once in three days.

All these strategies are next to impossible, as Art correctly mentioned above, for small i3 packs (and BMW were aware of this), so what we could do instead is to try and charge the pack to lower than 100% instead, once in awhile charging up to 100% to balance the cells, if they are balanced this way, that is. Or alternatively adhere to ABC if the car is leased for a relatively short period of time and we don't care about potential battery degradation.

 

[Anonymous]

Not sure what the specs are on the earlier batteries, but Samsung claims that the 94 Ah i3 battery cells can be charged through 4,600 cycles before degrading to 80% 'end of life' capacity - which if charged every day, that would be about 12.5 years.

https://insideevs.com/lets-look-at-the-specs-of-the-samsung-sdi-94-ah-battery/

Not sure how well this translates to real-world use. Would be nice if Samsung followed up with real-world stats over time.

 

[Hengus]

That equates to over 750000 in a 120Ah i3. I am not sure that a 80% battery life equates to EOL in an EV that can do 165 - 180 UK miles at 100% battery life.

 

[Oleksiy]

Not sure what the specs are on the earlier batteries, but Samsung claims that the 94 Ah i3 battery cells can be charged through 4,600 cycles before degrading to 80% 'end of life' capacity - which if charged every day, that would be about 12.5 years.

https://insideevs.com/lets-look-at-the-specs-of-the-samsung-sdi-94-ah-battery/

Not sure how well this translates to real-world use. Would be nice if Samsung followed up with real-world stats over time.

This is not the only area where lab tests fail to capture all real world variables, especially if conducted by a biased party. Check some independent tests on real cars here https://avt.inl.gov/vehicle-button/2014-BMW-i3. All three vehicles show about 8% degradation at 12K miles. That’s too steep, in my view, the degradation should slow down later down the road for these very EVs.