Li-ion batteries have the second slowest self-discharge of any battery and the longest cycle life/easiest upkeep. An li-ion battery will do about 500 discharge

Li-ion batteries have the second slowest self-discharge of any battery and the longest cycle life/easiest upkeep. An li-ion battery will do about 500 discharge cycles from 0-100. They'll do several thousand from 20-80, and tens of thousands cycles from 30-70%. If you have a few days of storage, the battery as a whole will last an incredibly long time even when cycled every day. As for why: When the battery is kept at a high or low voltage, it puts an overpotential on one side or the other of the battery. There's more or less lithium on one side of the battery and it tries to diffuse across the battery very slowly. It never makes it to the other side and reacts with the electrolyte and is lost, reducing capacity. The closer it is to 50%, the less lithium is pulled/pushed out into the electrolyte.