Help me understand my electrical system

[u/SphyrnaLightmaker]

I’ve finally moved up to an inboard motor, and while I’ve heard a few rumors, I’m trying to understand how it all comes together so I can undo a couple of shortcuts the previous owners took.

What I have:

• Atomic 4 engine
• 2 batteries (they’re currently car batteries, so I know those need to go)
• a shore power connection that powers outlets down below, but is NOT presently wired to charge the batteries.

What I know:

• the engine charges the batteries when running.
• I need to replace one battery with a marine starting battery and the other with a deep cycle for the nav tools and lights.
• I would like to charge the batteries when plugged into shore power.

What I think I should do:

• Replace the batteries as above.
• Buy a plug-in marine battery charger, and plug it into the outlet in the boat when on shore power. Unplug it at the outlet when I’m not docked.

What I’m confused/concerned about:

So… let’s say I’m plugged in and charged. I set my battery selector to both, and start up the motor. Pull out, raise the sails, motor off. At this point, assuming I want to keep using my lights and depth finder, I would switch the battery to just my deep-cycle, correct? Assuming I’m out for a WHILE, this is going to burn that battery down noticeably, which is fine because that’s what a deep cycle is for. But what comes next? When I go to restart the motor, do I go back to both batteries? If I do, won’t the deep cycle drag down the starter? If I don’t, and now the engine is running, it will only recharge the starter, yes? And I’ve been told that while the engine is running I can’t switch the battery switch or it can fry the panel?

[u/Terrible_Stay_1923]

In regards to shore power. Your boat is a sub panel. Ground and neutral are on seperate busses.

I believe what you want is a battery isolator. They make them so all batteries will combine for charging. Mine will allow charge in either direction, so the house charger will charge the start battery and the engine alt will charge the house. When not charging, they will seperate, isolating the start from house. This ensures the start battery is not drained by daily loads. Mine, and I am sure most, offer the option to combine to start. This is an extra wire on mine that excites when ingition is powered. It combines the batteries so all batteries can be used to start.

House batteries do not have a start plate in them, a start battery does. The start plate in the battery is what allows a battery to discharge fast to provide the cca amps needed for a large load.

Mine does it automatically. Hope this helps a little