You need the diode to charge a battery. NO battery, then you don't need the diode... All it does, is change the AC to 1/2 wave DC, and allow the current to go FROM the engine, to the battery. These engines don't put out enough current to even need a regulator. If you don't have a diode, the battery would self-discharge through the alternator coil when the engine wasn't running. Then, your battery would be dead all the time... You can use a Radio Shack silicon diode, and you don't need a very big or expensive one either.