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Old 23-06-2012, 07:36   #1
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Battery inter-connect cable

I know this has been hashed out before, but just want to confirm. I am adding two grp. 31 AGMs but don't have room to put them adjacent to the bank. So the new batteries will be approx. 12 ft. from existing bank. So in your opinions what size cables will I need? The existing bank has two batteries and will be charged by solar or genset/battery charger
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Old 23-06-2012, 08:50   #2
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Re: Battery inter-connect cable

The + and - cables paralleling the batteries should be at least as heavy as your existing starter motor cables so you don't lose power or burn them out when drawing starting currents.
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Old 23-06-2012, 09:24   #3
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Re: Battery inter-connect cable

Do you have a separate engine starting battery? Do you have an inverter hooked up to the house batteries?

These are the only really high amp loads on a boat. If you have a separate starting battery and no inverter, then your highest load/supply device will be your alternator which is typically 80-100 amps.

#6 wire can safely carry 120 amps and will work as there is no supply or load that will exceed that. You will need a 100 amp fuse at each end of the 12' run to be safe.
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Old 23-06-2012, 09:53   #4
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Re: Battery inter-connect cable

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Originally Posted by djmarchand View Post
Do you have a separate engine starting battery? Do you have an inverter hooked up to the house batteries?

These are the only really high amp loads on a boat. If you have a separate starting battery and no inverter, then your highest load/supply device will be your alternator which is typically 80-100 amps.

#6 wire can safely carry 120 amps and will work as there is no supply or load that will exceed that. You will need a 100 amp fuse at each end of the 12' run to be safe.
Voltage drop is usually the deciding factor in boat systems.
Aswg 6 or 16mm squared is much too small for this application. The voltage drop at 100 A is 1.6 v. The current will not be 100A, but that voltage drop is way too high.

50 mm squared would be my choice, but you could get away with 35mm with some compromises.
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Old 23-06-2012, 10:23   #5
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Re: Battery inter-connect cable

With that length of run you would be better off setting the new batteries up as a second bank. You will need two switches. One to switch loads from one bank to the other and the other to switch the charging sources between the two banks.

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Old 23-06-2012, 15:16   #6
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Re: Battery inter-connect cable

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Originally Posted by noelex 77 View Post
Voltage drop is usually the deciding factor in boat systems.
Aswg 6 or 16mm squared is much too small for this application. The voltage drop at 100 A is 1.6 v. The current will not be 100A, but that voltage drop is way too high.

50 mm squared would be my choice, but you could get away with 35mm with some compromises.
I agree 50 sqmm at the minimum, thats a 8-10mm diameter cable
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Old 23-06-2012, 16:24   #7
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Re: Battery inter-connect cable

I currently have two banks, one bank has the refridge on it and the other does have an inverter. No designated start batteries, basically I isolate the banks to maintain one bank as starting bank for both engines by combining the two. And I do have solar wired into the system which charges both banks by combining them. Then when the sun goes down I isolate the two. The easiest would be to add one battery to each side or use two perko switches like the drawing #3 attached shows
Attached Files
File Type: pdf Fig%208501-8504%20Inst%20(8500INS1).pdf (48.0 KB, 46 views)
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Old 23-06-2012, 16:59   #8
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Re: Battery inter-connect cable

Configuration #3 looks good!

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Old 23-06-2012, 17:49   #9
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Re: Battery inter-connect cable

Maybe size for max apms you expect and the run. Then add 50% and you should be fine.

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