DDK - Die Deutschen Klassiker

Recently I had an issue on my 6v 356 that the generator light was coming on but I have amp meter on the car which was showing that the generator was producing about 30 amps most of the time. This is not normal as normally the amp go up and down as rev the car, turn on lights etc.
Next I measure the voltage across the battery when at 3000rpm with the lights on and saw about 5.95 (it went up from about 5.9 at idle). My understand is that normally there should be 7.5 volts to recharge the batter. Someone on the 356 registry advised me that normally this is a sign of a battery with a bad cell so I replaced the battery and all returned to normal. Although I must admit I haven't yet tested the voltage across the battery but a quick test drive produced no generator light and amp produced dropping by the generator as the revs reduced.

The bit I don't understand is why when the battery was at fault did I see < 6v delivered to the battery by the generator? My knowledge of car electronics is small but I would of expected to have seen the 7.5 volts across the battery then the car was reved but the battery not hold the charge? Can somebody explain this to me?

The generator is regulated to produce a particular output voltage say 7.5V, high enough to fully charge the battery. As the battery charges, it's terminal voltage rises and the current delivered into it falls.

Your battery comprises three series connected 2V nominal cells. If one of those cells is short circuit, the battery voltage will be 33% down even when fully charged. With low battery voltage the current deliverd into it from the generator will be high. The current delivered is governed by the voltage difference and the resistance of the circuit (including battery and generator internal resistances).

Hope that helps,
Chris

Thank you

Useful point.

The geni light is connected between the battery and the regulator's reference output. If the regulator output (volts) is the same as the battery volts then no current flows through the bulb and it doesn't light. If the regulator output and the battery don't match, current flows from high to low and the light comes on.

The useful point is it can flow either way. So if the fan belt snaps it flows from battery (6v) to regulator (0v). If the regulator is knackered and pushing out 12v then the current will flow the other way. The first question is which way is the current flowing.

Although you don't need a volt meter to diagnose a snapped fan belt