lmgtfy: Its the extremes (0% and 100%) that degrades LiIon the quickest, if you want the longest life ideally you'd keep it 30-70% the whole time, ymmv (and also thresholds).

And it is a pity manufacturers don't support you in doing stop-charging-at-x if you wanted, e.g. a simple thing for software to do, but impossible without rooting on Android.