Battery devices are usually lithium ion...when these batteries get too low they cannot be charged...rather, the protections built into the charging board circuitry don't activate if they too low of a voltage. You can 'cheat' this by applying positive voltage to the + side and negative voltage to the - side. All you need to do is boost it above the cut off circuitry. If you have anything in the 3-5v range that can accomplish that, just a few seconds can give enough surface charge to cheat the board into starting to charge. The battery could be kaput in it though (of course). This is common with flashlights and other devices that suck voltage quickly...the batteries and/or boards are supposed to have low voltage shut off, but sometimes at the bottom the voltage creeps below due to sitting.