I find balloons to be a useful analogy when describing charge limits to users (and when configuring ThinkPads I limit the max charges). If you have a balloon an

I find balloons to be a useful analogy when describing charge limits to users (and when configuring ThinkPads I limit the max charges). If you have a balloon and repeatedly inflate it to 100% then deflate it, at some point it's going to pop (or you're going to have to start putting less and less air in) due to the repeated stress and stretching. If instead you inflate it only to 80-90% you're avoiding that last extra bit of stretch and will likely get far more inflation cycles.