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Old 04-11-2016, 01:46 PM   #1
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Battery Cable Size?

Just started replacing all the batteries for my inverter. I had 6 gp31 AGM, changing to 8 6v trojans T-605. The two main feeder cables should stay the same; 2/0. But I need to recalculate the inner connect cables to series the two 6v together and the cable size for paralleling all the pairs together. Each battery has a 6v 210Ah rating. So the entire bank should equal 12v 840 Ah. Is there a formula for this? Thanks
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Old 04-11-2016, 02:04 PM   #2
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Well, there isn't a formula, but there is a process for figuring the current load and wire size.


Depending on the wattage rating of your inverter, the maximum current that you can pull from the batteries is about 200 amps for a typical 2,000 watt inverter. That amperage is divided among 4 pairs of batteries or 50 amps each. It could be more, even 200 amps, depending on how you wire them in parallel: radial or daisy chain.


The series jumpers will all see a maximum of 50 amps. And both the parallel and series jumpers are short, maybe a foot or two each, so voltage drop is inconsequential. But I would use big enough wire so that any of the pairs can carry the full 200 amps, in case some of the batteries go bad.


Bottom line: I would use #2 for all jumpers. It can safely carry 200 amps. But if the batteries are in the hot engine room space, go with #1.


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Old 04-11-2016, 02:05 PM   #3
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2/0 sounds small for a bank that size. What's the maximum rated continuous current for your inverter (the inverter's maximum current at lowest input voltage).The installation manual for the inverter should have that info and recommended cable sizes.
Anyway, the best practice is to use the same size throughout the system.
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Old 04-11-2016, 02:17 PM   #4
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For the interconnects on the batteries we used 2/0 and from the batteries to the invertor we used 4/0 as per the manufacturer. Since we were using a lot of cable plus all the lugs, it was cheaper to buy everything in bulk when we made up the cables.

Calder's book, Boatowner's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems, is good reference.
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Old 04-11-2016, 02:30 PM   #5
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Hopefully one of the marine EEs will chime in.

When I did this, I ended up using 1/0 cables for the interconnects. I would have preferred to use 2/0, the same as the feeds to the panel, but really couldn't get 2/0 to actually fit.

To do it right, you need to calculate the voltage drop that you get from the cables from each of the connections between each 12v pair of batteries. You will have 3 12v pairs so add up the total length of the positive and negative cables between those pairs and plug that into a wiring calculator to figure out voltage drop. Your total voltage drop will be the total from your series connects and your run to your panel plus the run to ground. You want your total drop to be under 3%.

Plenty of calculators on the web. They will need total length of the circuit (some calculators take one way run and double it, others you need to provide the total circuit length), voltage (12v DC in your case), and the max amp draw.

What I did was to first calculate the voltage drop from my house bank to the panel with the 2/0 cables that I had. I then calculated the voltage drop from the series cables that were used in my bank (4 x 6v golf carts). In my case, the difference between using 2/0 interconnects and 1/0 interconnects was about .14%.

BTW, I think that the parallel interconnects in your bank (with 3 pairs) is going to be 1/3 of the total load. Again the series connection will carry the full load.

In short, I would first make sure that your existing 2/0 is adequate to give you under a 3% drop to your panel. Just because that is what is there doesn't mean that it is what it should be. Then figure out what will be necessary for your interconnects. The best way to do it is to make the series connections the same size and then downsize the parallel connectors. As I mentioned before, I didn't do this because I couldn't get the 2/0 to make the necessary connection in the tight space I had.

Finally, while you are doing this make sure that you have a fuse in the positive line within 7" of the battery bank terminal that is size appropriately to protect your wiring. I sized mine to protect the 1/0 interconnects, as that was the lightest gauge in the system.
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Old 04-11-2016, 02:33 PM   #6
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Quote:
Originally Posted by Larry M View Post
For the interconnects on the batteries we used 2/0 and from the batteries to the invertor we used 4/0 as per the manufacturer. Since we were using a lot of cable plus all the lugs, it was cheaper to buy everything in bulk when we made up the cables.

Calder's book, Boatowner's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems, is good reference.
That is a very clean install Larry. The only thing I would wonder about is that you are pulling the positive and negative lead from the same end of your bank. I was always told that the positive and negative should come from opposite ends of the bank.
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Old 04-11-2016, 02:45 PM   #7
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The AH rating of batteries in series is not additive, watts are but AH are additiv only in parallel.
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Old 04-11-2016, 03:15 PM   #8
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Voltage is additive in series, amps in parallel. Dhays, you're right the feeders should be connected on opposite ends. But Larry has really got on my mad side; where did you get those white battery boxes. I've got the same boxes in black; didn't know there was a white opposition. LOL.
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Old 04-11-2016, 03:18 PM   #9
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Larry, it looks like you have T605s; did you have any trouble getting the lugs to fit the stud height?
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Old 04-11-2016, 03:18 PM   #10
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That is a very clean install Larry. The only thing I would wonder about is that you are pulling the positive and negative lead from the same end of your bank. I was always told that the positive and negative should come from opposite ends of the bank.
Thanks.

I agree that the positive and negatives should be at opposing ends of the bank so the voltage flows through the bank.

That picture wasn't very clear. I'll try this one. The upper right negative (yellow) cable is 4/0 and goes to the inverter. The lower right positive, (2/0) goes to the additional 4 T-105's on the other side of the engine room. The upper left negative (yellow) goes to the negative of the other battery bank on the other side. On the other bank, there is a positive 4/0 that goes to the inverter.

It's not an ideal set up but we have 10 batteries in our house bank. Trying to keep the cable runs short as possible was the goal.
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Old 04-11-2016, 03:29 PM   #11
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Well, I thought my system was a little overkill; Mastervolt 200/4000 charger/inverter with a 840aH battery bank. But now all I can say is, Larry, you got it going on!
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Old 04-11-2016, 03:31 PM   #12
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Larry, it looks like you have T605s; did you have any trouble getting the lugs to fit the stud height?
They are T-105's and yes I had problems with the lugs and the stud height.

If you go with Trojans and want bolt on terminals, I would suggest you go with the EHPT (embedded high profile terminal) post. The difference is the ability to stack more than 2-4/0 or 2/0 battery cables per post. With the standard post, the ELPT (embedded low profile terminal) you are limited to ~2. If you look at the lower right terminal, I have 1-4/0 and 2-2/0 lugs on the post. I would like to have a little more post height but it is what it is till I replace it.

And the battery boxes were made by Blue Sea but are discontinued. We bought them 9 years ago from Fisheries Supply. There may still be a few out there.
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Old 04-11-2016, 03:42 PM   #13
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This is a once in your ownership upgrade.

Since there is no downside , besides co$t the real fat 0-4 4-0 stuff whichever,, thick as your thumb is the best choice.

Yes its costly , but a whole boat load custom made becomes cheaper.

Sometimes bigger IS better.
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Old 04-11-2016, 05:45 PM   #14
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The title of this thread really caught my eye since I just ran my first few feet of what seems like a mile of battery cable yesterday. The 1,240-AH house bank--8 6-volt AGMs--is just under the plywood. 4/0 isn't the easiest stuff to work with is it?

(Fred, your rationale is exactly what I've been telling myself.)

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Old 04-11-2016, 06:02 PM   #15
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With a group of parallel banks (3 banks of 2 6V batteries for example), shouldn't each positive leg from each bank be sized to handle the entire load. That way bad banks can be removed from the parallel arrangement until replaced. I had to do that last year when one 6v in one bank went south. I took that bank out of the parallel arrangement until I could replace both batteries.

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Old 04-15-2016, 08:15 AM   #16
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Quote:
Originally Posted by Larry M View Post
For the interconnects on the batteries we used 2/0 and from the batteries to the invertor we used 4/0 as per the manufacturer. Since we were using a lot of cable plus all the lugs, it was cheaper to buy everything in bulk when we made up the cables.

Calder's book, Boatowner's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems, is good reference.

Where did you find those battery boxes?
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Old 04-15-2016, 10:18 AM   #17
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Where did you find those battery boxes?
The battery boxes were sold by Blue Sea but are discontinued. We bought them 9 years ago from Fisheries Supply. It looks like they are still made by Bonar Plastics. The 6 volt boxes I think are part#: 4021

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http://www.snyderindustriestanks.com/PDF/D001838.PDF
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Old 04-15-2016, 04:12 PM   #18
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Quote:
Originally Posted by Larry M View Post
The battery boxes were sold by Blue Sea but are discontinued. We bought them 9 years ago from Fisheries Supply. It looks like they are still made by Bonar Plastics. The 6 volt boxes I think are part#: 4021

Battery Boxes

http://www.snyderindustriestanks.com/PDF/D001838.PDF

Thanks. Just the information that I was looking for.
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Old 04-19-2016, 08:11 PM   #19
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as a rough estimate...

the inverter has a >= 90% efficiency, so 4000/.9 = 4444.5 watts input at full load.

4444.5/12.6 = 352.7 Amps @ 12.6 volts
4444.5/12.2 = 364.3 Amps @ 12.2 volts
That is the current you need to support to run the inverter at full power with ZERO voltage drop.

4444.5/12.2 = 364.3 Amps @ 12.2 volts
4444.5/11.8 = 376.6 Amps @ 11.8 volts
That is the current you need to support to run the inverter at full power with 3% voltage drop.

It would be a real feat to lose only 3% at that current level. The main culprits are the wire size, the quality of the crimp and it's connection to the wire, and the quality of the connection to the battery posts.
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Old 04-20-2016, 07:15 AM   #20
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It would be a real feat to lose only 3% at that current level. The main culprits are the wire size, the quality of the crimp and it's connection to the wire, and the quality of the connection to the battery posts.


And dont forget the class T fuse that should be in the circuit.


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