The rechargeable battery may be of NiMH, NiCD, Li-ion, or lead-acid type. The higher fully-charged voltages of lead-acid (2.0) and Li-ion (4.2) cells, if applied directly to a regular 1.5 volt glow plug, will cause it to burn out instantaneously, so either a resistor of the proper value and wattage, or a high-power germanium transistor's base/emitter junction (in a series connection with one of the plug's terminals) can be used to limit the current through the plug to an appropriate level. Even with an appropriate power input, glow plugs can burn out at any time, and hobbyists are encouraged to carry spares.