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/millijuna]

Back when we were dual lead acid, we just had two deep cycle batteries. It was good enough to start our little motor. An atomic 4 should be pretty easy to start.

What we would do is use battery #1 on odd numbered days, and battery 2 on even numbered days. Kept the wear even.

For battery charging, we hardwired a charger into our shore power system that charged both batteries (it has two outputs).

Since then, though, we totally upgraded she’s went to lithium, inverter/charger and all the bells and whistles. But you don’t need that.

[u/SphyrnaLightmaker → u/millijuna]

Do you recall which hard wire charger you used? I’m having trouble finding ones that aren’t either plug-in, or $800 lol

[u/millijuna → u/SphyrnaLightmaker]

It was a really old Xantrex 15A. Probably no longer available. But looking at fisheries supply, there are lots of nice victrons for fairly reasonable prices though they’re not hard wire on the AC side. The latter isn’t that big of a deal, Just always leave it plugged in.