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Old 02-08-2013, 11:57 PM   #1
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Wiring two batteries together question.

Ok, I know there have been postings on this elsewhere. I tried responding to the specific thread, but it was too old. Herk posted a picture showing two 12V RV Batteries wired together in parallel. The picture showed running a wire neg to neg and pos to pos. from battery 1 to 2. It then showed the 12V power running from battery one to the TT and the ground running from Battery 2 to the TT. And I cannot figure out how to copy and paste the picture. Now, the thread also referenced "The 12 Volt side of Life", which I read thoroughly. No where on the site does it say to do this, it says to run the 12V power and Ground from Battery 1 to the TT while having the two wired together as above (Neg to Neg, Pos to Pos). My online research also comes up with most people saying to do it this way. Hence, my confusion, I know Herk is very knowledgeable from reading his numerous and detailed posts. Anyway, I have two 12V batteries and will be wiring them together for this season. My questions are:

1) Which way is correct?? Or does it even matter?
2) Will the batteries charge together, either way, when either hooked to electricity through the TT being plugged in or the vehicle when towing?

I realize you don't need the batteries when hooked to electricity, but we sometimes camp without being hooked in.

Thanks for helping clear up this fellows muddled confusion.

Mike LeBlanc
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Old 02-09-2013, 01:17 AM   #2
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"...neg to neg and pos to pos. from battery 1 to 2. It then showed the 12V power running from battery one to the TT and the ground running from Battery 2 to the TT"
That is how I hooked up my 2 batteries.
After reading quite a lot about wiring bats in parallel, I decided to just hook them up in a proven way and use them
The bats will charge either way. Ideally, they should be charged separately. Ever see a bank of bateries in a forklift or RV? The owner or user never separates the batteries and charges them independently.
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Old 02-09-2013, 06:20 AM   #3
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I wired pos to pos and neg to neg. Increase the wire size do to a voltage drop, I think I used #4 copper. I bought the wire at NAPA and they even soldered on the ends. Your 2nd battery will charge the same as the org. Either from your converter or your TV. You can also install a selector switch with bat #1 Bat #2 or all. I did not think I needed that my disconnect switch is connected to the #1 battery and will control both. With the selector switch you would shorten charge time by doing one at a time but not really needed.
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Old 02-09-2013, 07:34 AM   #4
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Mine is wired parallel and has the + side from one and the - side from the other hooked up to the electrical panel. Have been doing it that way for a few decades now. Works for me.
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Old 02-09-2013, 07:53 AM   #5
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Mikeg,
Not knowing precisely the setup Herk was describing in the post you mention, I can't be 100% sure, but I think his method was describing a "balanced" approach to wiring 12-volts in parallel.

The 12-Volt Side of Life offers a safe but unbalanced approach.

It is best to have the pos on one battery going to the trailer and the negative on the other battery connected to ground. This balances out the batteries.

Wiring both the positive and negative to the same battery in a parallel grouping will put more of the the load on the first battery in the string.

See this for reference to different methods of connecting parallel.

I'd attach a diagram but iPad,internet and weather are not playing together well right now.
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Old 02-09-2013, 08:00 AM   #6
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This is a balanced approach as I was trying to describe ...
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Old 02-09-2013, 08:04 AM   #7
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The proper way to wire a pair (or more) 12 volt batteries is to use exactly the same length wires between batteries (to keep the internal resistance of that battery equal to all the others) and attach the positive of the closest battery to the charge/discharge source and the ground for the system to the negative terminal of the farthest battery from the charge/discharge source.

Otherwise, if both the ground and the charge/discharge wires are attached to the closest battery, and all the other batteries are piggybacked onto the battery with the load on it, each successive battery will have the resistance of the wires connecting them added to the battery's internal resistance.

This means that each battery with the higher resistance will "work" less hard than the battery in front of it and the last battery will work the least hard of them all. That means the BANK will have less total capacity than the sum of each battery because once the lead battery goes totally dead it suck power from the higher voltage batteries to try to charge itself up until they are all dead.

Here is some reading to better explain the concept. I have also included the proper way to wire 6 volt golf cart batteries.
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Old 02-09-2013, 04:48 PM   #8
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Wiring two batteries together question.

Thanks for the answers.
This is the way I will be doing it.
Come on spring!!
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Old 02-13-2013, 04:37 PM   #9
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Quote:
Originally Posted by herk7769 View Post
The proper way to wire a pair (or more) 12 volt batteries is to use exactly the same length wires between batteries (to keep the internal resistance of that battery equal to all the others) and attach the positive of the closest battery to the charge/discharge source and the ground for the system to the negative terminal of the farthest battery from the charge/discharge source.

Otherwise, if both the ground and the charge/discharge wires are attached to the closest battery, and all the other batteries are piggybacked onto the battery with the load on it, each successive battery will have the resistance of the wires connecting them added to the battery's internal resistance.

This means that each battery with the higher resistance will "work" less hard than the battery in front of it and the last battery will work the least hard of them all. That means the BANK will have less total capacity than the sum of each battery because once the lead battery goes totally dead it suck power from the higher voltage batteries to try to charge itself up until they are all dead.

Here is some reading to better explain the concept. I have also included the proper way to wire 6 volt golf cart batteries.
Thanks herk, I did not know of the balance system. I rewired my battery's to this approach. The only problem is that I can not make the plus and the minus the same length. will cause a problem? both are the same interstate battery's. How important is that?.........
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Old 02-13-2013, 04:41 PM   #10
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Quote:
Originally Posted by gljurczyk View Post
Thanks herk, I did not know of the balance system. I rewired my battery's to this approach. The only problem is that I can not make the plus and the minus the same length. will cause a problem? both are the same interstate battery's. How important is that?.........
The critical wires are the "inter battery" ones since they effect the "effective" internal resistance of each battery. As long as the red wire goes to the positive terminal of the closest battery and the Black wire goes to the farthest Negative terminal the batteries will charge/discharge evenly.
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