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Has anyone found a good way to determine usable battery capacity without doing a full discharge? I know the leaf has a battery life graph.

I think I am now around 50kw total capacity. This is based on usage vs % of battery. Seems a bit early to be down that much as I am only at 34k miles and just over 1.5 years in service. Charged almost exclusively to 85% (nightly) and normally only discharged to about 50%. Never run out, but have been down in the bottom 1-2 bars 3-4 times.
Seems to me that if you take a long drive, then divide the kWh Used for the drive by the change in battery raw state of charge for the drive (using an OBD reader, perhaps?), you would have the kWh contained in each 1% of soc. Multiplying that by 100 would give you the full battery capacity. That won’t give you the usable window, but it will give you a general idea about whether or not the amount of power contained in the "usable window" has diminished because the full capacity of the battery has diminished.

Full charge = 85% raw SOC
End of trip = 15% raw SOC
kWh Used for the trip = 56.9 kWh
56.9 kWh / (85-15)% SOC = 0.813 kWh/1% SOC
100% SOC battery full capacity = ~81.3 kWh

An example from the numbers you posted in comment #4, where you normally charge to 85%, and used 46 kWh as you drove from an 85% SOC to an "almost empty" SOC (perhaps 15%?), then:

46 kWh / (85-15%) = 0.657, full battery cap = 65.7 kWh.

Think of the results of this method as an approximation of the current full battery capacity. The precision of the measurements you record is limited (displays show what... a single decimal place?), so you’re dividing a rounded off number by a rounded off number. Also, the accuracy of the calculation improves as the size of the change in SOC approaches 100% of full capacity, so a short drive produces a rougher guess than a longer drive. If for you, "full" is 85% and end of trip is only ~50%, that’s not a large sample.

For Volt owners trying this, keep in mind that once the car "switches to gas," the generator kicks in and normal operations will see a raw SOC reading fluctuate around the "switch to gas" number (i.e., kWh Used won’t change while driving in Extended Range Mode, but take the reading at the moment of switch to gas because it might be different later).

Perhaps it would also be wise to obtain multiple sets of data (i.e., several trips) to see if the calculations produce relatively consistent results.
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