Spencer the Great;4300193Reliability is paramount in an aviation engine and this is why they use magnetos. Magnetos are heavy and not very adjustable and they have been replaced in cars today by a battery and alternator. If you disconnect the battery of a modern car the engine will not run.
Actually the battery is only used to start the car. If you disconnect the battery whilst the engine is running it will still run using the alternator. Of course if power consumption exceeds the output capacity of the alternator then problems will arise eg lights, indicators, cd etc.
Military Aircraft like the Spitfire, Bf109 etc don't have internal batteries, they use one of three methods to start, external power (power cart), hand crank or starter cartridge. Batteries in that sense are dead weight in military aircraft, extra power can be gained via generators etc.