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Is it possible to work out actual kWh battery capacity?

Idle curiosity, but I'm trying to find out if there is a way of evaluating what the real capacity of my car's batter pack is - to the nearest kWh.

Is this valid? I use Teslafi; it says the SoC increased, say, 50% and x kWh was added (the, usually, lower figure of kWh added, not consumed (often a larger figure).

If we say kWh added is y, then the capacity of the battery is another chunk of kWh equalling what you just added (because it was half)? So that would be:

100/y * x

Using a real world example with my own car, 59% added, 40.9kWh added, so 100/59 * 40.9 = 69.3kWh

Any caveats?

Are there any better suggestions?
 
@acoste Yeah, I'd seen that some time ago. It's not really practical for me to charge to 100 and drive until it's almost flat. The lowest I've taken to is 9% - not sure I'd trust the car (3.5 years old) to go much lower than that.

But on a recent drive I did 186 miles and according to Teslafi 80% used or 51.26 kWh. So add a quarter (a quarter of 80% = 20%) of 51.26 to 51.26 = 64.08. The trip was 186 miles, 276 Wh/Mile and 105.1% Efficiency. We didn't leave with 100% though, which complicates things slightly (left with 89% and arrived with 9%). In theory at that efficiency we could have driven from 100% to 10% and covered 208 miles. (We have a 'classic' 70D). I do wonder if the kWh added calculation is more precise though (because of vampire and other drain during the drive).
 

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
4,291
Buford, GA
It's not really possible to get the exact. That's a lot because the capacity changes based upon the current drawn from it. And to meaure it, you have to take it to 0, which isn't the best thing to do.

So, aside from it really not meaning much, and the fact that it probably won't ever be the same twice, and the fact the even temperature makes a difference, Just take a rough guess and stop worrying about it.
 
@acoste Yeah, I'd seen that some time ago. It's not really practical for me to charge to 100 and drive until it's almost flat. The lowest I've taken to is 9% - not sure I'd trust the car (3.5 years old) to go much lower than that.

But on a recent drive I did 186 miles and according to Teslafi 80% used or 51.26 kWh. So add a quarter (a quarter of 80% = 20%) of 51.26 to 51.26 = 64.08. The trip was 186 miles, 276 Wh/Mile and 105.1% Efficiency. We didn't leave with 100% though, which complicates things slightly (left with 89% and arrived with 9%). In theory at that efficiency we could have driven from 100% to 10% and covered 208 miles. (We have a 'classic' 70D). I do wonder if the kWh added calculation is more precise though (because of vampire and other drain during the drive).

I don't know if the remaining 9% is accurate. Some cars stopped with anywhere between 5-15% remaining charge.
 

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