Average SOC affects longevity?

[insaneoctane]

I think I read that the ideal SOC to maintain your li-ion battery at for maximum life is 50%. That's not usually practical because you're not using half of your battery's capability (half you range)

I usually charge to 80%, and my daily commute uses approximately 20% each way. I plug in each night. I just wanted to see what my use case results in for an average daily SOC for my battery....

Turns out I'm doing pretty good. I was thinking of maybe charging to 90% and skipping charging every other night, that's what really got me thinking. That resulted in 41% average and the inconvenience of having the end of day 2 at 10% SOC. I'm happy with my charging schedule and happy with my average SOC!

 

[timk225]

I wonder if Tesla knows the lifetime average SOC for our batteries. They track lifetime KW used and Wh/mile already, so it should be easily done.

 

[jerry33]

I think I read that the ideal SOC to maintain your li-ion battery at for maximum life is 50%.

That's true for storage. The beetles in the pudding are that for a given amount of power output, more heat is generated at a lower SOC (basically what you want to do for long battery life is to avoid heat). So it's best to drive above 50% rather than below 50%. Also there's no real data showing that there is a measurable difference in battery life in a Tesla that's charged to 80% or 90% as opposed to some other lower number. Most of the articles on battery life use RC car batteries as a reference.

 

[insaneoctane]

Also there's no real data showing that there is a measurable difference in battery life in a Tesla that's charged to 80% or 90%

Tesla batteries are fairly new in the industry, so I will conceded your point that no Tesla-specific data exists from 80-90%, it does seem reasonable that Tesla batteries, which have generally common chemistry, will follow general li-ion characteristics (like charging to 100% isn't good, or very low charges aren't good, etc). It's fairly well documented that keeping li-ion at 50% SOC results in the least degredation and maximum life.

 

[bedoig]

It would be cool in some ways if Tesla released detailed data, but obviously they have an incentive to just say charge it to 90% and don't worry about it. My guess is the differences are close enough to negligible to justify their stance. My guess is that over an 8-10 year period the difference between a "set it at 90% and don't worry" battery vs a "micromanage for optimal lithium ion life" battery are less than 5-10%. That's purely speculative based on reading for a number of years, but that's the thing, we don't really have fleet level hard data.

I'm a set it at 80% and don't worry guy...

 

[SSonnentag]

I don't think it's the average state of charge being at 50% that benefits the battery, but the lowest standard deviation from 50%. Being at 90% is no better or worse for the battery than being at 10%, both are 40% off from the ideal battery state of charge.

Charge to 51% and drive only to 49% and then plug in again and your battery should outlive us all.

 

[darth_vad3r]

I like the average for the day by hourly breakdown.

Would be nifty to have an automatic calculation by giving a few inputs: charge target, charge rate, charge time, leg A duration, leg A consumption, leg B duration, leg B consumption.

I guess that’s 7 inputs. Hmm, that’s missing stay duration. So 8? Oh I’m still missing the delay to scheduled charge. 9

So a weighted average of 5 stages, 6 I guess with a post-charge/pre-drive stage:

1. pre-charge aka ante-charge (ac)
2. charging (c)
3. post-charge (pc)
4. drive leg A (a)
5. stay (s)
6. drive leg B (b)

SoC Average (Savg) = (1/24) * (Dac*Sac + Dc*Sc + Dpc*St + Da*Sa + Ds*Ss + Db*Sb)

Where Dx = Duration of stage x in hours, Sx = SoC % (or average) over duration Dx

It’s nice to keep this lower rather than higher (closer to 50%), but as someone else mentioned there are also other considerations like the fact more current is required at lower SoC to drive so that can generate more heat during use.