Is it normal for the battery to wear so much? At this rate the aircraft would only complete about 5 flights before needing new batteries. Not a gamebreaker, just curious.

I agree with your findings...  It seems battery usage is no more than a scripted counter event that decreases the status by 0.1% every 5 min 30 sec-ish.  According to Google the battery should last roughly 10,000 hours so I think INI opt'd more for a let's make this a feature that the simmer thinks is cool.   

I also noted that it makes no difference if there is power or no power.  It's just counts down.

For what it's worth as well, after around 40 hours of fly time in the same livery, batteries are 1/2 spent and my hydraulics are leaking like I had a night out at a spicy curry restaurant followed by some Taco Bell for dessert.