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Testing your traction battery charge capacity?

I’m looking to try and check the capacity of my traction battery. Any ideas of how this can be done without a rather expensive trip to the dealer.
I had thought of running the battery down on my road outside the house until the car stops, rolling it up to the plug and then using a battery monitor (like a utilities company smart meter) that clips around the battery charge less and measures the current.
Does anyone have a more accurate way without removing the pack?
I’m pretty convinced I can only get about 20 mile range and I’m now getting a (!) light and ‘STOP’ battery symbol all intermittently.
I believe the (!) means something wrong with the traction battery.

I use to keep the battery to zero nearby a public charging station. At the end of recharging the display on the charging station shows how many kWh are gone in the battery. Later an invoice is sent to my, showing how many kWh I have bought. The two numbers are, obviously, the same.