Lithium batteries don't ever want to be totally charged or discharged. The effort involved in either is futile. On the charging side, picture a wall of small boxes about 20 inches cubed. Now randomly throw a bunch of balls (just under 10" diameter) at the wall and try to fill all the cubes up perfectly. That's how a lithium battery charges... Getting it up to 75% is easy, beyond that takes a lot of extra throwing, which builds up heat and wastes energy. On the other side, discharging it is like pumping water out of a well with a submersible pump. You can get all but that last couple percent out pretty easy. But below that, your pump has to work extra hard, the flow is irregular, and it quickly builds up heat and eventually the pump will fail. Most devices never tell you a percent. They show you "4 bars" or some such. In that case the break down is usually about 17% per bar, with bars representing >75%, 74%-57%, 56%-39%, 40-23%, and 22-5% being NO bars. If the battery only charges to 92%, the first bar lasts just as long as the others. And as long as its over 75% the user thinks it's "at max".