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Old 30-06-2011, 20:18   #1
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Battery Cable Interconnections

im wiring 4 6volt batteries for my house bank and was wondering what guage wire most people use to interconnect the 6volt batteries together is 2 awg cable suffecient?
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Old 30-06-2011, 20:25   #2
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Depends on your maximum expected current. Maybe the windlass or charging. And the size of the fuse. Keep them short.
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Old 30-06-2011, 20:26   #3
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Re: batttery cable interconnections

Battery cable sizing
This might answer your question.
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Old 30-06-2011, 20:37   #4
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Re: batttery cable interconnections

no windlass or large inverters, just a 55amp iota charger and 100amp alternator for charging.
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Old 01-07-2011, 04:42   #5
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Re: batttery cable interconnections

There are two criteria for wire sizing: acceptable voltage drop and ampacity, the amp carrying capacity of the wire before it gets too hot.

#2 wire in engine room temperatures can carry 180 amps. You probably have a fuse or breaker (if not you need one) on the main DC supply wire to your electrical panel that is smaller than that. So you will never see more than that current and your #2 interconnections will work fine.

The 6-12" length of these jumpers will contribute negligible voltage drop.
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Old 01-07-2011, 05:53   #6
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Re: batttery cable interconnections

AWG2 is not sufficient IMHO and experience.

You must use 1/0 or larger in order to keep the total resistance very low (it matters, as measured by conductance/resistance battery testers such as the Midtronics MDX-650).

Keep the cables as short as possible....arrange the batteries so that this is possible. It requires a very short cable to interconnect 6V batteries in series, and a slightly longer one to interconnect the series-batteries in parallel.

Be sure the lugs are installed correctly with a high quality crimper and, preferably, covered with heavy-wall adhesive heat-shrink tubing. West Marine generally has crimpers you can use, and they sell the heat-shrink tubing as well.

Alternatively, buy the cabling from genuinedealz.com and they'll make them up for you to your specifications.

Spend some time and do it right the first time...you'll not have to worry about them for several decades :-)

Bill
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Old 01-07-2011, 06:50   #7
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Re: batttery cable interconnections

thanks for the input, wow that 2/0 is expensive. i guess 2 awg would be too thin for my run from the batteries to the selector switch an panel 10' ??
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Old 01-07-2011, 07:01   #8
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Re: batttery cable interconnections

Quote:
Originally Posted by rappitysnap@yah View Post
thanks for the input, wow that 2/0 is expensive. i guess 2 awg would be too thin for my run from the batteries to the selector switch an panel 10' ??
Yeah, but you only need a short length for the interconnections. And, 1/0 will do it.

For the run to the switch and on to the panel, it depends on what kind of loads you expect. If you're going to provide for engine starting thru that switch, then you'd need heavy-ish cable. But if you're just running onboard loads (lights, instruments, etc.) then smaller cable would do fine.

Best to draw out the circuits, estimate the anticipated loads, and decide upon wire size (and fusing) based on that information.

Normally, fuses are intended to protect the WIRE, but it's sometimes OK to use smaller fuses than the wire's ampacity dictates...though not larger ones.

By the way, you need fuses in the large cables very near the batteries, and they may be of only three types to be compliant with ABYC recommendations: ANL, MRBF, or Class-T.

Bill
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Old 01-07-2011, 10:51   #9
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Re: batttery cable interconnections

A 1' length of #2 used as a jumper with a current of 50 amps has a voltage drop of 0.01V, which is probably much less than the connections.

Your boat probably has a 1/2/all switch with the 1 and 2 connections wired to the starting battery and the house battery respectively. The common terminal is wired to the starter and to the house DC panel.

That house DC panel is probably fed with #6 or so wire which is plenty big enough for the maximum current expected (maybe 30 amps) and will have a voltage drop of about 2% for a 20' round trip run.

So you can wire the house bank with #2 or even smaller if you protect the wire with a fuse- 100 amps or so and never try to start the engine on the house bank. If you do you will blow the fuse.

But most boats wire each bank to the 1,2,all switch with 2/0 so that there is no fuse in the starting circuit, and so if you wanted you could start the engine from either bank. That is probably your best bet. Then put a 50 amp fuse in the circuit between the switch and the house DC panel.

Having wired the boat this way, then if you use #2 for jumpers there is a slim chance that you will exceed its ampacity if you ever try to start the engine on the house bank. I wouldn't worry about it though. The starting time is very short and while #2 may heat up, it won't be long enough to damage the insulation. And you only do this by mistake, right?

David
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Old 01-07-2011, 11:01   #10
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Re: batttery cable interconnections

David,

A couple of points:

First of all, we don't know how the OP's boat is wired, so it's hard to give specific recommendations.

Next, the size of wire for interconnecting batteries has nothing to do with the rated voltage drop. Rather, as I indicated, it has to do with keeping the total resistance in that battery bank as low as possible to ensure even charging and discharging amongst the cells. Remember, what you're doing with the jumpers is not feeding a load; rather, you're creating a BIG BATTERY, so you want absolutely low resistance amongst the cells.

Third, as Nigel Calder says, there's absolutely no reason NOT to fuse the starting circuit on most small boats-- regardless of ABYC recommendations. In fact, it's downright dangerous not to. A short in the solenoid or a stuck start switch could cause a FIRE (as it did to one of my clients when 100 miles offshore). I've fused mine (4-108) and have done so on client's boats. Never had a problem with it and I sleep better.

I agree with you that AWG #6 may be large enough to feed the panel, but we can't be sure until we know what the maximum expected loads will be and the type of load.

Bill
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Old 01-07-2011, 11:34   #11
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It's popular to use very heavy wire in an attempt to limit resistive losses. But it's a fool's errand. The internal resistance of batteries is so big that imagined savings using huge wire is just a waste of money and weight. Use the wire that is safe and no more.
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Old 02-07-2011, 04:36   #12
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Re: Battery Cable Interconnections

Both of btrayfors posts: +1

After meeting the basic ampacity and voltage drop limits for the circuit, the reason I install heavier gauge wire to the client's panelboard is that I have never seen the electrical requirements on a vessel diminish over time.

Charlie
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