As last option you can always do a real capacity measurement using Tesla Bjorn's method. For better accuracy try to get as low as possible on the trip.
I like the various various descriptions on
How to correctly measure battery degradation in Tesla from Bjorn's.
However, I don't have often the opportunity to
charge to 100%,
then immediatly
drive about 300 miles until getting 10% and report the
kWh consumption.
Note: This is exactly the way I test my smartphone portable batteries, by
decharging the battery into a load, and measure the total
Amp Hour.
Well, this was not recommended by Bjorn, however it is quite easy to
get back home with 10% and charge at low speed
until 100%.
I have a small
$20 Watt meter next to my UMC, so I can
measure the exact amount of energy used when charging.
This include both the
battery charging and the
various losses.
However, I can reproduce the same measurements overtime, and the lost are proportional to the energy used when charging.
I can estimate the
energy lost when charging by comparing the
total energy needed to get 100%
and
the energy that the car used when doing a seasonal road trip
until I reach 10%.
My point is that, instead of r
eading the total range given by the car,
value which can change just because of
new update range calculation algorithm,
I measure the
actual kWh capacity of the battery when charging it.
Note: I agree that discharging the battery provides a better measurement (see Bjorn method) than charging,
but I think that measuring the charing energy used is more consistent that just reading the given mileage.
In fact what would be more usefull, to
check if the battery was corectly balanced,
would be to access the individual cells or modules status like the
Leaf Spy App.
(I imagine that Tesla has this capabily to do so, but this is not available)