with the ni-cad battery (that is what most rechargeable battery are) if left
drained for a extending period of time it will shorten the life of the
battery or have the battery fail all together. the best way to store any
battery is fully charged. That is just the way batteries are. it is best to
follow the instruction that came with the unit to get the longest battery
life, not doing so will result in a shorter battery life.