If you're using those lithium batteries, they tend to die out rather quickly (in a matter of an hour tell you the battery is empty).
If you use rechargeable batteries they run on a lower voltage, tend to run out of juice a bit sooner than lythium batteries, but their fall is not as sudden (the meter will tell you well before it's time to change them).
I have a 'similar' problem on my Jetbook mini, exchanging batteries resets the clock. I thought one user said (because the mini uses 4 AAA's) that one needs to exchange first one pair, and then the other, so to never run out of battery in the reader, but that seems incorrect too.
My only gripe about the mini (next to the slow FB2 loading time), is this; the exchange of batteries.