In my experience the batterymeter doesn't take battery aging into account at all. At some point it just realizes battery voltage is way too low compared to calculated remaining stored energy, and drops the battery bars rapidly.

As others mentioned previously, the voltage drop is even more significant with an aged battery, to the point where a really worn battery might cause the tablet to shutdown without warning under non-idle loads.

What's worse than keeping battery/tablet on charger constantly, is to run it down empty and the putting it in a drawer for a few weeks. Batteries hate heat and hate being left empty.