HV battery capacity decreasing over time?

Are you speaking of actual range (miles actually driven in EV) or the range estimate as given on the MFD? The range estimate is based on recent driving history and the battery's state of charge, temperature etc. Actual range is highest at about 80f and falls off with lower temperatures.

 

the estimated range is almost useless. you would have had to do a defined route, at a defined speed, at a defined temp/humidity when your pip was new, and then repeat each time you want to measure it. that's the only way to get a handle on it. you can start now, if you want to measure future degradation.

fwiw, my estimate has dropped about 1 to 1/2 miles at peak time of year, and my actual is probably down a half mile or so, but i'm not absolutely positive. i do have 17,000 miles of ev.

 

I wouldn't look at the number of miles anything. Just keep track of how much energy it takes to fully charge the battery from fully empty and what the temperature is during those charges. I.e. if it is too hot the fan will come on and drain power which makes it look like the battery pulled more power.

In the EV world, some estimates say you could lose 10% capacity in the first year and 1%-2% each additional year but that is for a full EV which may use 100% of its battery capacity and thus stress the battery much harder than a PHEV like the Prius and Volt which use only a portion of the battery to extend longevity. However, ambient temperatures play a big role in battery degradation and lack of thermal management systems and high average temps can cause a battery to degrade faster. Being in SF you shouldn't have much of an issue with this though as your average temps are ideal for EV batteries.

Battery Capacity Loss - Electric Vehicle Wiki