I bought a reciprocating saw with a rechargeable battery. The manual advised never storing or using the batteries in a room where there is a pilot light. (This means not in the same room as a gas furnace, stove or fireplace.) The battery, if damaged, could leak hydrogen gas, which could cause an explosion if the gas comes in contact with a flame.
If you store tools in your furnace room, find a safer place for the rechargeable batteries.