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Old 01-11-2007, 00:02   #1
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At what resistance do you change the battery wiring?

I'm about to install new AGM batteries to replace wet-cell batteries that were killed prematurely (opps, expensive lesson learned!).

My question is regarding the battery wiring and acceptable resistance/voltage drops. The existing wiring looks good with no visible corrosion. I'm going to replace the small (easily replaced) wiring that joins the six volt batteries into a 12V bank and do a nice job of heat shrinking, etc. But there are some other runs from the alternator to the batteries, batteries to the windlass, etc that are longer and not nearly as easily replaced. I'll put new crimped terminals on these as well but I'm wondering if the whole wire should be replaced.

Does anyone have an objective way to decide if the existing wiring is still serviceable or if it should be replaced. I could easily measure the overall resistance in each wiring run and/or put a known voltage on one end and measure the voltage on the other end to see what the drop is. But what sort of values would be "acceptable" and what would indicate too much internal corrosion, etc?

Thanks for your help,

Craig
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Old 01-11-2007, 00:38   #2
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My rule of thumb is the "flexibility" of the battery cable. Grab a piece of new stuff. It is very stiff. Old stuff gets like wet spaghetti.

Depending on length of runs, and you say it's somewhat difficult to get to, I'd replace it when I changed that batteries. It's not that expensive and is good insurance.
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Old 01-11-2007, 10:18   #3
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There is an electrical testing device called a Megger, it pumps a charge into a wire that is disconnected and measures the MegaOhms to ground, this establishes the wires insulation quality.

To test the conductance you need to allow a current to flow, near the maximum ampacity of the wire. Then get a IR heat gun (maybe $60) and look for hot spots.
IR camera is used industrially.

Most of the hot spots will be in the connections. If you don't have enough wire to redo the connection with the proper amount of slack then replace the wire.

Or you can just replace what you can get at.



A great wiring job will have a "pig tail" some where in the run to allow at least one replacement connection to happen.
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Old 01-11-2007, 12:32   #4
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Craig, look in the "Electrical Study Hall" and you will see a chart with guage and currrent and length. Use that for your guide.

If the cable you have looks OK, then likely it will be OK. As you have suggested, ensure the connections are clean and in good order. After connecting, apply CRC Softseal or similar, or Lanacote to all exposed connections to ensure they remain moisture tight and corroison resistant.
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Old 01-11-2007, 13:52   #5
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Craig, there are published tables on the web for the resistance of battery cables. Offhand, something like 0.01 ohms for one foot of four gauge AWG tined cable, and you are going to have some fun trying to find an ohmmeter that measures accurately that low.<G> So you'd wind up needing to pass a known voltage through the cable, measure the voltage drop in it, and run numbers.

A lot of work for a lot of short cables.<G>

So as Wheels says, if the cables LOOK good, look clean, have no corrosion or cracks in the insulation...they are probably OK to stay with. The bigger problem is usually that the end fittings haven't been properly installed, properly cleaned, etc. or that someone has cheated and used copper (not tinned copper) cables that have punked out. If your cables are not tinned copper, not professionally fitted at the ends, I'd say replace 'em in any case with a sufficiently heavy guage for the maximum load on your boat--which will be the starter motor, usually. If your starter pulls 2000 amps, you decide how many tenths of a volt drop you'll accept and size things up or down from there.
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Old 01-11-2007, 21:51   #6
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I wouldn't suggest changing wiring if the old wires have a good insulation. This is a lot of work and it has to be done PROPERLY. Also using Megger to check insulation is a risky business if you do know how to do that and what value you should expect.
Most of wires used in boating has very flexible tinned copper conductors. Old PVC insulation has tendency to become brittle over time. So, if the wire is flexible and its insulation does not crack it will work fine. However, what I would do is to check all high current connections. If the connections are not tinned they will get oxidized and these will be high resistance spots. Cleaning them would be an annual (or more often) event ( I assume that they were properly installed on the conductor). Their screws, bolts nuts, etc should be tighten and as AW mentioned, covered with moisture repellent dielectric lubricant.
Chris
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Old 01-11-2007, 22:18   #7
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Not to be argumentative but 3 boats here this year have replaced batteries only to subsequently replace old battery cable that couldn't carry the load.

I put a new (3rd) battery in my, admittedly small, boat and the cable and connectors cost less than $30 bucks.

Very cheap insurance in my book.

BTW - Every worn out battery cable I have seen in the last 20 years was like a wet noodle. When cut open they are all but corroded through at the low point.
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Old 02-11-2007, 00:46   #8
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Thanks!

Thanks guys.

I agree with the statement that it's cheap insurance to replace the cables. After dropping $1500 on batteries I'm not worried about the cost of the wires. That's why I'm replacing all the short and readily accessible wires. The problem with the long wires (going to the alternator and windlass) is that it'd be A LOT of work, not money, to replace them.

I think all the wires are the same vintage, and they do visually all look good. An added bit of info is that the only wet part of my boat's bilge isn't anywhere near where the wires run, so I can reasonably confident none of them are sitting in water.

Here's my plan: replace the easy stuff and give them the wiggle test. I'll dissect a couple of the short ones to look for internal corrosion. If there's not thing scary in there, I'll apply a known voltage at the far end of the long runs and see what voltage drop I get. That combined with a thorough look at the accessible portions of the long wires will help me decide if I can live the existing long wires.

Thanks for all the input!

Craig
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Old 02-11-2007, 05:51   #9
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Watch for the fire

-which will be the starter motor, usually. If your starter pulls 2000 amps, you decide how many tenths of a volt drop you'll accept and size things up or down from there.[/quote]

I assume that 2000 amps was a typo because if it wasn't you have a much bigger problem than the wiring. 200 amps for a fair size starter.
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Old 02-11-2007, 08:18   #10
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"The problem with the long wires (going to the alternator and windlass) is that it'd be A LOT of work, not money, to replace them." And oddly enough, you have to pull them, measure them, and then send out for fabrication unless you can find a shop that properly lugs cables. DIY or home cable lug setting tools do't seem to exist--at least, not reasonable ones for battery cables.

Lancerbye-
Yes, 2000 watts would have been more appropriate. But, watts, amps, hundred, thousand...who knows what kind of starter you have, as long as you run the numbers and the math looks good when you're done.<G>
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Old 03-11-2007, 22:56   #11
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BTW - Every worn out battery cable I have seen in the last 20 years was like a wet noodle. When cut open they are all but corroded through at the low point.
If that happens, and it can, then it means moisture or even worse, acid is getting into the cable and migrating along the wire. It means the ends have not been sealed effectivley. A good joint well sealed should last for ever. Well, close to forever. Well, maybe a very very long time.
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Old 04-11-2007, 02:15   #12
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If that happens, and it can, then it means moisture or even worse, acid is getting into the cable and migrating along the wire. It means the ends have not been sealed effectivley. A good joint well sealed should last for ever. Well, close to forever. Well, maybe a very very long time.
I agree so long as it hasn't been overheated by too much current. Then you have another problem. Let's face it electricity and salt water are not a good mix.
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Old 04-11-2007, 11:39   #13
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I would think if it was over heated thaqt much, the inside would be the least of your concenr. I would be worried about the dripping and chared insulation. It takes a lot of ehat to turn copper like described and the insulation would be nothing but toast.
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Old 04-11-2007, 18:08   #14
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If that happens, and it can, then it means moisture or even worse, acid is getting into the cable and migrating along the wire. It means the ends have not been sealed effectivley. A good joint well sealed should last for ever. Well, close to forever. Well, maybe a very very long time.

Absolutely correct. A properly done installation should last and last and last...

And on our "personal" boats we probably have good installs. It's everyone else I'm worried about - LOL.

Seriously, with the huge investment in batteries being talked about in the other thread, and if your cable runs are years old, it's good insurance that reduces risk of problems in remote locations and potentially lowers resistance on starts and that's good mojo for the batteries.
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