Li-ion batteries can hold a charge for many months. It is best to store a lithium-ion battery with a partial or full charge. Occasionally, a lithium-ion battery with a very low charge is stored for a long period of time (many months) and its voltage slowly drops to below the level at which its built in safety mechanism allows it to be charged again. If the battery is going to be stored for several months it's a good idea to take it out and recharge it after a few months. Better yet would be to actually use the battery every few months and then leave it partially or fully charged.