What to do if an airsoft guns battery is overcharged?

I've asked my friend, a lecture in electronics engineering, he said that overcharging is the main cause that leads a battery to failure (neither it's NiMh, NiCd, or LiPo) or shorten its life, not the "memory effect" itself. Overcharging can cause crystal to be built in the battery cell/cells. The more we overcharging it (longer the time or do it several times) the more the cell/cells will be broken. If you only overcharged it once, its still be ok. If you have LiPo battery, I suggest you yo buy a balancer to be use with your charger. It will prolong battery service. There are several way to repair broken battery. I've read that we can short-circuiting Ni-Cd battery in order to get it back to work, but the chance is only 50%. So it's definitely NOT a choice. The latest news I heard is we can use a smart charger-balancer that can "fix" broken cells in LiPo battery.