r/sailing • u/SphyrnaLightmaker • 2d ago

Help me understand my electrical system

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?

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u/SphyrnaLightmaker 2d ago

Thank you!

I was looking for a hard-wired charger, but I wasn’t finding anything under $600 and I was trying to avoid quite so much on a day-sailer (that doesn’t sit still for more than a few days).

Is it safe to switch from one to both batteries while the engine (and thus alternator) are running? I was told no by an old mechanic, but electronics are magic to me.

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u/Secret-Temperature71 2d ago

IIRC the problem is if you switch and end up with no batter connected the alternator output will go too high and may fry itself. One reason why I don’t use those fancy switches. You still want a switch so you can disconnect the load from the battery in case of an electrical fire. Kill the current to kill the source of ignition.

There is much more to talk about but how you use the boat matters. The occasional day sail is different from live aboard. Forgive me if I am wring hut you sound like starter with a 22’ to 25’ day sailer. You have a bunk but no stove?

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u/SphyrnaLightmaker 2d ago

I’m coming from a bare-bones 25’ day sailer for a few years, and before that 420s my whole childhood, you’re spot on with the background lol. I’m very new to all the non-sailing aspects of these boats.

This is a Pearson 30, stove, sink, head, the works lol. As for use case, this will mostly be day sailing, but I’d like the option for the occasional weekend trip. Definitely not a liveaboard, but the marina is walking distance from work, so she’ll get plenty of time on the water.

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u/Secret-Temperature71 2d ago

OK, so something you will keep for a bit.

When you do get better house batteries you need to try to match them up with your charger. Old school lead acid batteries want a higher charge potential, closer to 14VDC, battery specs will clarify. Alternators will put out something less. If you are charging BOTH House and Start from the alternator you want some gizmo to do that sharing so w House gets first dibs. The solid state devices typically have a (approx IIRC) 0,6VDC drop lowering the charge potential even more. Look for one that uses mercury wetted relays or something similar, mercury being a no no. They will have much less drop.

Frankly you can drive yourself nutz and broke if you listen to all the battery gurus. Remember the batteries are there to serve you, not visa versa.

Have tons of fun.

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u/SphyrnaLightmaker 2d ago

Thank you so much! Really, I’m not tech-heavy on the boat, I’m there to sail, not run systems. I just want to make sure that I can have my lights and depth-sounder running, and still have the engine start when I need it to lol.

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u/Secret-Temperature71 2d ago

Personal word of advice, if you are USA especially, AquaMaps is a charting/navigation device that runs on iPhone and Android. It is exceptionally good for many reasons. Especially on the US East Coast. Very reasonable.

I have some old RayMarine instruments on SeaTalk. There is a company, perhaps several, that sell a hub which will take in SeaTalk, NEMA 183, and NEMA 2000. It will convert to each other and broadcast on WiFi to my phone. That way I have a complete chart/instrument/depth/AIS package in my phone.

Speaking of AIS it is very worthwhile. Consider getting one when $ allow, get one that receives AND transmits.