[Fulltiming]

The generator is typically powered by the house batteries (some manufacturers use a separate battery just for the generator) to provide a means of getting the engine started if the engine batteries are dead and merging them with the house batteries still won't start the large diesel engine. It is easier to start the generator than the main engine so it gives you a second chance. If the house batteries are down but the engine batteries are good, you can always start the main engine first without merging the batteries and let the alternator provide the power needed to start the generator without worrying about the house batteries pulling down the engine batteries.