[bostonnerd]

One of the battery life metrics is number of charge cycles. A charge cycle is NOT every time you plug the device in. One charge cycle equals going from 0% to 100% charge capacity.

So....charging a battery that is at 50% is half a charge cycle. Do that twice and it is one charge cycle.

Waiting for the battery to go to 20% eats up almost one charge cycle - doing that repeatedly will shorten the lifespan of the battery by prematurely reaching the charge cycle limit.