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Old 08-07-2015, 11:51 AM   #1
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Adding additional batteries (Advice)

I am in the process of adding two additional 12 Volt batteries to my RV. (NOTE: looked into Trojans T 105 but they won't fit into existing battery rack). The two additional batteries have been mounted on the drivers side. The length from the new 2 batteries to the existing battery bank is 7'.

My question are:

1. What size cable should I order. The converter has a max amp output of 90 AMPS. I will only be running LED lights TV, Dish receiver, radio etc., no large appliances are run of the small 1,500 WATT modified sine converter.

2. In order to have equal draw am I correct that the POS from one of the new batteries will run to the existing POS terminal of the one of the batteries and the NEG from the other new battery to the other existing NEG terminal of the other existing battery

Thanks for your input!
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Old 08-07-2015, 12:11 PM   #2
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Quote:
Originally Posted by SmokyRV View Post
My question are:

1. What size cable should I order. The converter has a max amp output of 90 AMPS. I will only be running LED lights TV, Dish receiver, radio etc., no large appliances are run of the small 1,500 WATT modified sine converter.

2. In order to have equal draw am I correct that the POS from one of the new batteries will run to the existing POS terminal of the one of the batteries and the NEG from the other new battery to the other existing NEG terminal of the other existing battery
That's a pretty long distance but few things in life are ideal so ya got whatcha got!

#1: Using this chart, it appears you can get by with #2 cable (assuming worst case of 120 amps for your inverter). If size and cost aren't a big difference, I'd go one step larger - particularly for those times that you might use the starter interconnect. However; buying copper can be expensive!

#2: I'm not sure about "equal draw" but to keep 12 volts (not increase to 24 volts) and assuming all your batteries are 12 volt batteries (not 6 volts wired in series) you must wire the batteries in parallel. See this page for a description and diagram. In short, yes, your description is correct.

I'm not an electrical expert but I've heard that the only way to optimize the draw from these batteries is to run the two leads to opposite ends of the battery bank. So, positive to house is connected to battery one, negative to house is connected to battery four.
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Old 08-07-2015, 12:11 PM   #3
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I think you can get away with 2 gauge wire between the banks.


I would rotate the batteries every year. The batteries closer to the inverter are going to discharge a bit more, during heavy inverter draw. Once the load is off or charging, they will balance out.


May not make a difference but something I would do.


In a perfect setup all cables would be as short as possible, the same length and size and meet on the Inverter.
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Old 08-07-2015, 12:12 PM   #4
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I'd suggest gauge 2 wires to be sure the batteries are connected with the minimum of current drop. You are correct, (+) to (+) and (-) to (-), a parallel set up, it the proper configuration. How old are the present batteries? Your system will only be as good as the weakest battery in the group.
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Old 08-07-2015, 12:22 PM   #5
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The existing batteries were have a manufactures date of March 2015. Not to old that's why I decided to do it now.
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Old 08-08-2015, 10:21 AM   #6
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Under the conditions you cite I would use standard starter cable, This is 4GA usually, get it at any Auto Parts place, comes with lugs attached, Pre cut, factory assembled (Usually in CHINA however) and thus lower cost.

I use heavier cable, US assembled, but that is because I had it in stock.
(Actually battery to battery I use standard starter.. I use the big stuff Battery bank to inverter.. All batteries connect to the same terminal the big wire comes off of)
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Old 08-08-2015, 10:39 AM   #7
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The 4 GA sounds like a good idea as long as what is desired is longer time on battery.
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Old 08-08-2015, 03:33 PM   #8
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You need to forget about AC power at this point. Your inverter is good to 1500W. That converts to a 125 Amp draw at 12Volts DC. #2 cable is rated at 120 amps @12 Volts DC. #1 is rated at 150 Amps. Both will see about a 3% voltage drop at 6 feet. Personally I would feel fine at #2 cable since you will probably never use your 1500 Watts and you are already buffered at the original battery bank. You also have breakers on your inverter for protection. For safety you could install a 120 Amp DC Breaker on the extra batteries.
As for emergency starting, a typical V8 gas engine draws 250 Amps @ 12 VDC. Since starting time is short undersize wire is always used. If you have ever had trouble starting an engine you will notice the battery wires can get very hot.
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