The batteries are fairly new, and this is the first time they've been run all the way down. They came right back once the engine had had a chance to recharge.
Define "came right back?" Batteries that have been completely or nearly completely discharged take quite some time to be fully recharged.
Also, wouldn't the depth sounder, cabin lights etc. work once the engine was running?
Yes. But I'm a bit curious about this:
but I finally got a battery in there, got the engine started, switched the batteries out, and recharged my dead batteries.
did you effect the battery switcheroo? I'm hoping
you either used the temporary battery with jumper cables to one of your regular batteries. Or, if you temporarily replaced one of yours with the temporary battery, I'm hoping you did something like:
- Install temporary battery in "1"
- Start engine
- Switch to "2" (this should be safe with modern, make-before-break battery switches that are in good working order)
- Put regular battery back in "1"
- Switch to "both"
that at no time was the engine running with anything in the 12V electrical system energized and there being no battery at all connected. Besides the possibility of toasting the alternator, doing that, you can toast anything that was energized at the time. Don't ask me how I know that.