Store at about 50-70% charge, in a metal box, in the refrigerator. That will allow the cells to last the maximum amount of time. You might need to periodicall

Store at about 50-70% charge, in a metal box, in the refrigerator. That will allow the cells to last the maximum amount of time. You might need to periodically recharge them, but this could be months at least for electronics that can be turned off. If the device doesn't have a charge meter, draining it and then charging for 30 minutes will often work, since many devices are designed to charge at 1C, i.e. 0->100% in one hour.