ev range question..

[scrambler]

i did the number again with 9 miles left and came up with 18.09 usable capacity. its a 2014. is that good for a car with 4100 miles.

My 2015 that has smaller battery with 15K miles is showing 18.05 kWh, so 2014 that has bigger battery with only 4100 miles showing 18.09 seems to have degraded lot more.

No that is not accurate.
First the 2015 battery is only marginally smaller, but most importantly it is said to have the same usable capacity (estimated around 18.4 kWh).

So 18.09 kWh looks just fine , especially given the fact that the extrapolation from the battery energy screen numbers carry some imprecision of easily +/-0.5kWh

 

[CCIE]

There is also the issue that car doesn't actually know exactly how many KWH remain in the battery. The car can tell when the battery is full and when it's at a dangerously low state of charge. But, with Lithium batteries, it's very hard to tell anything in between those high and low values. So, it's making an estimate, which is what is shown on the energy screen and what your battery meter is based on. Some variation is to be expected even with identical conditions between discharges. I would ignore changes less than 0.5KWH since they're likely to be within the margin of error.

 

[evboy]

cc will warmer weather change the usable capacity calculation. 40 degrees vs 90 degrees.

 

[PhilPen]

Some battery management systems base calculations on previous coulomb counting versus voltage data so they may not be too bad in the middle range as long as there have been a few recent deep discharges to make the extrapolation more accurate.
On the temperature point most lithium ion battery chemistries show an increase in total capacity versus temperature (usually reaching a maximum at around 70F or a little hotter) so the battery could easily vary 3-5% over that temperature range.