How to measure battery capacity accurately

Can we easily get a capacity measurement of the battery?

Short answer – no, you can’t.

Medium answer – Tesla is doing it right, because they constantly track and plot over time – this is what is necessary for us to reliably track capacity. I assume that they also account for the factors listed below.

Longer answer – the problem is that you cannot track capacity by a single measurement. The following factors can affect a capacity measurement / guess:

  • recent environmental temperature
  • recent pack temperature
  • how hard it has been driven lately
  • driving in L vs D mode
  • how far down the battery has been used
  • how far up the battery has been charged

While each of those may only impact by a percent or two, combining them all together means that you can get an annual swing of +/- 5% or more.

Considering that we are only seeing up to 5%-10% loss by 100,000 miles, that makes measuring capacity very difficult.

Furthermore, in order to measure degradation, you must have had a very accurate measurement when you first got the car. Maybe the car started with 57kWh or maybe 63 (we have seen cars anywhere between those two). So if I measure 55 now, does that mean 8 kWh loss or 2 kWh loss?

Lastly, even if you have a 2017/2018 and grab the value from OBD2 – that doesn’t really tell you much. You might see 55.8kWh in December, and in July see 58.2kWh. So what does that tell me?

Having said all that, if you want to get a present capacity measurement, charge to 100% and ensure that the “energy used since last full charge” has reset to 0. Drive to below 10%. Using OBD2 (or even the myChevy app) divide the kWh used by 1-(percent remaining). This will give you a present capacity measurement, but as I mentioned due to the factors above, you could immediately do this again, and have a 5% difference.

e.g. – 55.2kWh used with 4% remaining – 55.2/(1-0.04) = 55.2/0.96 = 57.5kWh available.

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