As szadzik said. The lithium series of batteries don't really like to be drained down to 100%. Normal cut off for these packs is around 3.3-3.4v to were it won't start heating up. Heavy cut offs 3.0-3.2v is where you can actually puff your battery if it isn't up to the task of feeding power, making it subject to breaking down, heating up to the point of smoking. Just be glad we don't have lipo packs! YouTube lipo fire to see just how volatile they are (when damaged or used without care) the advantage would be a battery that can hang with the phone all day (with proper c rating and mah)