Yah, i was talking about the alternator. And that was my point.. you won't be charging up your batteries when running unless the switch is on to a particular battery then that will charge. I know about not using "both" batterys at the same time. I get that, but then why do we have the "both 1&2" on the selector switch.

Usually i start the boat and use the radio on battery 1 and keep battery 2 in case i run down battery one. I use it as a spare.

My main point is, if battery 1 is down on power, you can't really charge it by running the boat. The only way is to use the onboard when you get back to electricity. Can't switch the battery during operation from what i was told.