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post #1 of 10 Old 05-04-2007 Thread Starter
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Positive ground or incompetent owner?

I just bought a 1975 Newport 28, which came with one battery in place, but two extras that the owner had never hooked up. The battery compartment is under the starboard settee with the AB switch mounted close by under the navigation table. I looked over the connections to see where everything went, and I had to look twice to make sure I was seeing right.

The AB switch has two red cables, at 5 and 7 o'clock, and a black cable at 6 o'clock. The black cable leads to the positive terminal on the battery, and the 7 o'clock red lead runs out of sight into the engine compartment. There is another red lead coming back from the engine compartment and is connected to the negative terminal of the battery. The battery provides power when the switch is set to B.

My first guess was that someone had misconfigured the AB switch, and used the wrong color wires to boot, as I'd figure the two red leads would go to battery positives, the middle 6 o'clock lead would be red as well, and lead to the bus, and a black cable would come back from the ground to the negative terminal.

However, a friend suggested that it might be a positive ground system, which seems odd for an American-built boat from 1975. I haven't had time to really look around yet, just sticking my head into an access panel, but it seems to me I've seen at least one other black lead going to ground.

Still, it all means I'm going to have to spend time tracking down those leads to see where they go, instead of just hooking up new batteries. Is there any real chance this might be a positive ground electrical system?
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post #2 of 10 Old 05-04-2007
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Sounds much more likely that the PO did a really pisspoor wiring job. I'd rip it out and do it right, with the proper colors for the wires too.

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post #3 of 10 Old 05-04-2007
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There are older motors with posative grounds but a 1975? I would highly doubt it, but what exactly is the make of the engine aboard? In a vessel 32 years young one never knows what might lurk down there. Does the engine run? Do the lights work? Have you seen sparks or smell smoke?
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post #4 of 10 Old 05-04-2007
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Positive ground EXTREMELY unlikely. Bea in mind that an AB switch only is connected to one polarity--normally positive, but in any event only ONE side of the battery. So if the PO used red and black cable both to it--he probably just bought some black welding cable way cheaper than marine battery cable and used that to replace one wire run.

Checking the cables from "here" to "there" is a good idea anyway. The only way to be sure there is no chafe, no bad splices, and the cable to properly secured to the boat every so often.

There's no real sin to using all black cables, but if that's what he did, get some RED gaffers or duct tape, or RED electrical tape, and put bands of it on the cable ends, marked "POS+" to help prevent future problems.

If the PO simply left those other two batteries unhooked, and didn't manually charge them every month, they are paperweights now. They will have no useful capacity if they sat for one year uncharged, and less than half their rated capacity if they sat for as little as six months--unless they are AGM batteries. AGM can sit without harm for a year.

There's a lot of folks going with one starting battery (SLI or starting type, small and cheap) and one separate large house battery, each charging separately and used in tandem only for an emergency start. If you have to throw out the two paperweights...think about that option instead.
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post #5 of 10 Old 05-05-2007
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If the PO did use welding cable, it is not a good idea, since it isn't as finely stranded or tinned like marine grade wiring will be and is subject to corrosion and then fatigue. The main battery cables are the last place you want to see any corrosion. The only thing that is good for is starting a boat fire... and I don't think you want that.

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post #6 of 10 Old 05-07-2007
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I just went through all this. Pull it out and re-wire. Consider upgrading your battery switch. I went with a Blusea e-switch panel that turns both the stating and house bank on at the same time, but keeps them isolated. They can be combined in an emergency. I also added an ACR charging relay between the banks and my batteries have never been happier. Either charging from the the AC charger or alternator. This is the slick system that I would rewcomend to everyone. Call Peter North at Jack Rabbit Marine. The prices were fair and he will answer all of your questions, even if they don't relate to what you are buying.
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post #7 of 10 Old 05-07-2007 Thread Starter
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So much info and help; thanks so much. The engine in the boat is an Atomic 4 which has a plate on it saying it was rebuilt at Moyer Marine in 1993. It runs fine. I'm pretty sure that the PO (maybe the owner before, even) just used whatever wire he had handy for the runs.

From what I can see of the switch, it looks to me like he's actually got the power lead on the B connection and the battery positive hooked to what should be the power lead. I'm guessing about this, because there are connections on the switch at 5, 6 and 7 o'clock and I'm presuming that A and B are at the 5 and 7 positions and the power lead is in the middle. I won't be able to tell for sure until I take it apart.

One more question: since it appears there is only one ground lead coming from the engine compartment, can the second battery be grounded to the neg pole of the first battery, and the one lead used for both? It would seem that electrically this would be the same as having them both separately grounded to the game place, but I'm no electrician.
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post #8 of 10 Old 05-07-2007
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Yes, one battery can be grounded to the other battery, since electrically that would be the same as grounding both separately to the same point. Depending on the switch, it may be that the post on the A-side of the battery is for the B-side, which was the case on the switch I have on my boat. On some other switches, the post on the A-Side of the switch is for the A-side.

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post #9 of 10 Old 05-08-2007
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There are probably some rare situations where connecting both battery negatives and then running a common cable to the engine (and using that engine point as the common ground for the boat) could be a problem, for oddball ground loops or radio interference, but I think the only real consideration is to make sure that last cable run (to the batteries) is heavy enough to take the power of both batteries, rather than just one. i.e. heavy enough for full starting loads and full charging loads.

In terms of ground loops, ideally you want one designated ground point in the system and everything making the home run directly back to that, but some splits or a "ground bus" aren't uncommon, and they usually are not problems.

Since you are going to be rewiring and exploring, I'd suggest either buying (for reference) or borrowing (most libraries can bring them in) a copy of "The 12 volt Doctor" or "The 12 volt Bible" both small books that go over a lot of the things you can do to make sure the system is set up right.

There are small points, like making sure the power cables are all crimped and sealed properly, that can pay off in the longer term.

It also helps if you make a rough sketch of the hull on a good sixed sheet of paper, and then draw in every wiring run so there's a schematic of it later. Folks may call you obsessive if you put numbers or labels on each wire and cable...but when you come back in five years to fix something and say "WTF is this?! Where does it go?!" those labels pay off big time, fast.
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post #10 of 10 Old 05-08-2007
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HS makes some very good points... and I'd second his recommendations for once... Also, make sure any heat shrink tubing you use to seal the connections are adhesive lined, not just heat shrink tubing... makes a much more secure and water/moisture proof seal.

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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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