It is well known that overcharging lithium batteries can seriously affect their service life, and as a direct result: the actual usable time of a fully charged battery is getting shorter and shorter. We all know this, but it's very hard to unplug the power cord when a battery is just fully charged. Because we tend to forget that our phones are charging, and we overcharge them almost every time (even put them in front of our eyes). A regular alarm clock is totally useless for this, because you can't be sure when your phone will be fully charged exactly.