My car is presently at 38,000 miles so it's probably one of the higher mile units on this forum. It's been in service about 20 months.
I have believed that the usable capacity of a new battery is approximately 18.5 kWh (somewhere in the mid-18's). My reasons for believing this were based largely upon my acquisition of an OBD scanner about a year ago.
1. The OBD scanner showed ETE of generally 18.5 kWh after charge (varies slightly between 18 and 19)
2. The OBD scanner showed the range of SOC to go between 90 and 8% approximately (i.e., 82% usable)
3. The battery is rated at 23 kWh; 23 x 0.82 = 18.8
So everything looked generally consistent
I am now changing my view. More forum members are using OBD scanners, and they are frequently reporting ETE values in the mid-19's. Also, I found Idaho National Laboratory test reports
http://energy.gov/sites/prod/files/2015 ... dfocus.pdf
which show approximately 19.5 kWh for a new car.
So now I'm thinking 19.5 kWh might be the right number. It's also possible, I suppose, that 18.5 was the correct number for my specific FFE and Ford gave INL primo ones. The INL reports show tests of four separate FFEs, and the measured battery energy was between 24.0 and 24.8 kWh, well above the rated 23 kWh. Who knows?
While the better test is the one that INL has promised to perform but has not yet (retesting the same cars periodically) we can make some estimates. I believe, based upon the trip-meter (both driving and with a heater rundown test), that my current battery capacity is somewhere between 17 and 17.5 kWH. Let's call it 17.2.
So if 19.5 was the original number, it's dropped about 12% over 38,000 miles
If it was originally 18.5, then it's dropped about 7% over that distance.
I'd say these numbers help give some idea of the actual degradation with use. My car has been through two summers, one before I got it, one in my possession. It was built 24 months ago.
To put this number in perspective, a Leaf loses its first battery bar at 15% reduction. If we accept the generally but not universally accepted definition of "useful battery life" as the point where it has 70-80% of its original capacity, this suggests the FFE battery is good for somewhere between 80 and 120K miles, at least when used as I use it. There is a time factor also involved (separate from miles) but I am burning through the miles so fast this probably hasn't been much of a factor for me.
Hope this is helpful and I would value other data forum members may have.