Peculiar charging fault.

[GWAYGWAY]

I accidentally left my headlamps on for a couple of hours then when I looked at the control board the volts on the engine battery had dropped to121 volts.
I had turned the lights off and started the engine ok. Drove off to somewhere to park for dinner, about 10 miles. Then checked the voltage on the board again expecting it to be charged up , but it had dropped to the bottom scale at 11.1 volts, I could not understand what was going on so went home 20 miles, we stopped for some fish andchips but not stopped the engine As the voltage was right down at the bottom . Wifey looked at the fridge as the red light was flashing and turned it off. I came back and she told me so I checked the controller and it was now charging at 13.8volts. The hab batteries were always fine at 12.8v.
I know the engin runs the fridge on the move but why did it drop right down when it should have gone up. I had thoughts my super expensive smart alternator had burnt out. But it seemed to not charge until the fridge went off. The fridge is a 3 way one but the gas was left off so was not going to gas. All seems to be fine and the run into home put the voltage right up to 13 again. Seems the fridge was holding the voltage right down and not allowing charging to the engine battery. I thought the hab batteries were the body electrical Supplier but it seems not.

 

[tonyidle]

Sounds like the usual problem of poor wiring practice. The alternator isn't the issue. If both fridge and leisure charge are taken from the same pair of not overly-thick wires then when the fridge is one it'll hold the voltage down below what is needed to charge the leisure battery. If by your last sentence you mean you thought the fridge ran of the leisure battery when the engine is running then no - that isn't the case. The fridge runs off the starter battery (which is being charged by the alternator) when you're travelling.

 

[Raul]

My opinion is the alternator is too smart. It detected the voltage of the fridge battery and throttled back. Thinking there is no need for charge. The starter voltage was just pulled up artificially by the hab voltage. When you stoped the fridge the hab battery has gone down and settled then the starter followed the same. Voltage is not a true indicator of state of charge. Under load, and wire loss, can result in diferent voltages that has nothing to do with SOC. Proper wiring and a decent isolated B2B will not get you into this and will charge the hab and starter batt on it’s true SOC. I never understood the laziness of using the body as a negative bus, just run two decent wires and keep it floating of the vehicle electrics. That way will never mess or interfere with its workings. Simples.