The battery power for fire needs be 3 volts or higher, with the positive and negative terminals must be close together. A 9-volt battery is perfect, but a 6-volt lantern battery (as seen in the picture) will work well, too. A 3-volt Surefire battery or a 3.7-volt cell phone battery can also do the trick. You can even use two batteries that are 1.5 volts each. Just align them side by side, with the positive and negative terminals head-to-tail. For this last configuration, you'll need to place a ball of steel wool at both ends of the battery stack to create the closed circuit that ignites the steel.