There was a site posted here on HN that was something like: "All you wanted to know about batteries" or something like that. About Li-Ion they wrote, that they age faster if the charge falls under 20% (don't remember why) or gets charged over 80% (more voltage needed to push electrons in, battery heats up). GigaSet limits the charging to 90%. I use BatteryBot Pro from F-Droid (installed via Foxy Droid) to ring an alarm at 30%, 75% and 80%. [add] Depleting them completely is very bad for them.