Most people forget that on a light airoplane there are two methods of power generation used. one is the alternator which only supplies power for the ancillary equipment and the other is the Magneto which provides the spark to the engine.. .. .The magneto is a good bit different to an alternator and the only link I know of that would give an explaination of how they work is <a href="http://old-engine.com/maghma.htm" target="_blank">here</a> .. .. .The magneto can work without any power being present in the battery which is why you can hand start engines on planes with flat batteries and why if the alternator fails you can turn everything but the magnetos off and still fly to your destination preserving your battery for when you need to make radio calls and such.