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Old 08-11-2017, 08:24 PM   #1
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Using different lengths of cable in battery bank:

So I have four 6v batteries on the right side of my garage. I want to add 4 more on the left side. To accomplish this I'll have to run two large gauge wires(+, -) from one side of the bus to the other. I have always kept all of my cables the same size, whether for an actual reason or because I am superstitious. The run of cable will be at least 6 or 7 feet.

What gauge cable should I be running? I believe I'm running 2/0 gauge between all of my batteries as of now. I don't think I can find anything higher than 4/0 ...
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Old 08-12-2017, 12:41 PM   #2
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Figure out your loads and then you can use a calculator such as this: DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk

You don't want to go any lower than 5% acceptable loss. 3% is a nice compromise. 1% is ideal, if you can afford the copper.

From what I understand, if you have a wire with a 5% loss rate at a given amount of current that means that 5% of your electricity is being converted to heat in the wire.
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Old 08-12-2017, 01:47 PM   #3
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here is the best information on battery wiring i know:

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank


it may not answer your question directly, but the last method uses different cable lengths and keeps them balanced on either side. maybe that helps.
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Old 08-12-2017, 02:15 PM   #4
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Figure out your loads and then you can use a calculator such as this: DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk

You don't want to go any lower than 5% acceptable loss. 3% is a nice compromise. 1% is ideal, if you can afford the copper.

From what I understand, if you have a wire with a 5% loss rate at a given amount of current that means that 5% of your electricity is being converted to heat in the wire.
Just as I expected that calculator pretty much tells me 4/0 might not even cut it pulling 170 amps at a length over 6 feet. This was why I opened the thread about a 24v system -- it would cut the amps in half at the same wattage. I think I've outgrown my 12v components...

Additionally I have seen these calculators and charts before but I wanted to ensure each cell in my batteries is charging at the same rate, or as close as I can get. These are expensive batteries. I wasn't sure if a long run splitting the bank was acceptable or not.
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Old 08-12-2017, 04:12 PM   #5
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???

6ft = 1.82 meters. The calculator indicates that 4/0awg would work fine at 170 amps with 1% loss. With an acceptable loss of 5% you could even be using as small as 4awg (though that's not recommend )
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Old 08-12-2017, 04:17 PM   #6
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You will have equalized batteries as long as your load/charging run attaches to opposite sides of your battery bank, something like this for 6x 6v batteries:
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Old 08-12-2017, 05:39 PM   #7
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Wow, don't know how I screwed that up. I think i input 2.5m. Thanks jaz.
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Old 08-12-2017, 07:41 PM   #8
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Depending on battery / inverter location and other considerations like battery capacity i would run separate wire from each battery string to the inverter.
That would equalize the current draw from each battery string.

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Old 08-12-2017, 08:01 PM   #9
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For the past 51 years I have just used heavy duty welding cable. Never had any problem, and was easily obtainable. Not jumper cable, but cut up pieces of cable from welding on the docks at the mill. Worked on my first drag car with battery in trunk, and works great on all my house and bus batteries now.
Second.
For the past 15 yrs I have bought heavy copper bars from the scrap yard to make up isolated connectors for all batteries, not to mention inverter and other 12v house batt necessities. I place one on each side of my battery box and make up ends to bolt cables as necessary. Is easy and any battery can be r/r without disturbing the elect connection of any of the others.
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Old 08-12-2017, 09:32 PM   #10
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Quote:
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Depending on battery / inverter location and other considerations like battery capacity i would run separate wire from each battery string to the inverter.
That would equalize the current draw from each battery string.

later J
If I'm understanding you correctly, this would not equalize the current draw since the wires would all be different lengths. Wires have resistance. Longer wires have more resistance than shorter wires; meaning that the strings would not pull (or push) equal current.
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