Does anyone have suggestions on how to evaluate the battery performance/capacity? I expect the capacity changes over time. When is it "bad"?

 

There are definite conditions that occur to tell when a battery is "used up".
The car will come out and tell you when it is time to be replaced.
There is a "battery had reached end of useful life" message in the software.

This is a case where no news, is good news.

It has a life expectancy of 15 years, and/or more than 100,000 charge discharge cycles. A charge / discharge cycle does not mean "all the way down". This pack is never discharged all the way. In fact, it never gets below 40% or higher than 60%. Being this gentle is what makes them last so long. Using your cell phone battery all the way down, even a few times, is what makes them go bad so quickly. ( 1 or 2 years )

-John

 

Something other than the "... end of useful life" message would be nice. These batteries loose capacity as they are used. How does this affect mileage? Is poor mileage an indicator of a low capacity battery pack? Does the "... end of useful life" message appear before the fuel economy drops?

Maybe a fully (60%) charged internal resistance indication would be useful. Or, some other means for the owners to evaluate battery condition

 

It might be... but they wouldn't give you it because it would open up the door for all kinds of customers insisting their battery capacity shouldn't be dropping "so much" and therefore insisting they needed a new $8000 battery pack when its all a normal matter of ageing. Class action lawsuits are the "american way" these days.