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Old 07-23-2014, 09:33 AM   #1
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Wiring two 12 Volt Batteries

I had the dealer add a second 12 volt battery to my 2015 Shamrock 23IKSS Hybrid. Once I got it home I noticed that the negative wire on one of the batteries was hanging out of the battery box so I opened it up and the wire had come loose from the connector to the battery. It looked like they just failed to crimp the connector tight enough, but I also noticed they wired the batteries in series instead of in parallel. The battery disconnect switch I had on one battery wasn't working at all either. I've read many posts on this that says they should be wired in parallel, so I rewired the batteries in parallel with the battery disconnect on the negative post of the first battery. The battery disconnect still doesn't work so I'm figuring that the two negative wires going from battery 1 to battery 2 are still getting a connection even with the disconnect turned off. Two questions I have are: Is wiring two 12 volt batteries on a travel trailer the best way to wire them or is the series method better? Also, do I need two battery disconnect switches (one for each battery) for them to work? These are two brand new Interstate 12 volt batteries and I want the best configuration I can get to make them last without screwing something up.
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Old 07-23-2014, 09:39 AM   #2
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Negative to negative.
Positive to positive.

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Old 07-23-2014, 09:42 AM   #3
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You would only wire the batteries in series if you have 24VDC system. Series will double the voltage...Parallel will double the available current while keeping the voltage at 12VDC.

As far as the disconnect switches....It would depend on the location of the switch in circuit, but I would be willing to bet that the disconnect switch is not rated for the potential current draw you could get from 2 batteries in parallel. I would install a second switch or get a DPST switch rated for the current on each battery individually.

As a side note..If they hooked up the batteries in series and you have a 12VDC system...you may be in trouble and might want to check out the DC side of your system. At best you could have blown a fuse...At worse you could have fried all of your 12Volt appliances...
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Old 07-23-2014, 09:49 AM   #4
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I run two 12-volts parallel. First, if what you say about the dealer hooking them in series is true, it may have caused other problems since that would increase the voltage output to 24 volts and may have overloaded the system.

Just keep the coaches positive and negative wires connected to battery no. 1 and run one jumper wire from negative on battery no 1 to negative no 2 and one jumper wire from positive on battery no 1 to positive no 2.

Only one battery disconnect is necessary. It may have burned out or blown a fuse somewhere near it if the dealer hooked up in series. Worth checking.
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Old 07-23-2014, 09:56 AM   #5
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My mistake I said series but actually the dealer wired the batteries as shown in figure 2 of this link: Battery School | Batteriesnorthwest.com | Connect Your Batteries for Optimum Efficiency it seems like the best way is what is shown in figure 1 correct? I will go get a second disconnect switch and see it that works.
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Old 07-23-2014, 10:14 AM   #6
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Either way will work as depicted in the schematic and, although picture one is more efficient, I still like method #2 for simplicity. I know if I don't keep it simple, sooner or later I will mess something up.

Even though there is less efficiency it's not noticeable to me.
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Old 07-23-2014, 10:14 AM   #7
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The only electrical difference between Fig's 1 and 2 are that Fig.1 eliminates one small leg of wire to the total circuit. This "in THEORY" makes the circuit more efficient because any wire on the circuit is added resistance, thus you will lose a small amount of potential available current via heat dissipation through the resistance of the wire.. So the "MOST EFFICIENT" claim is technically true BUT..

In reality..it's really not going to be a noticeable difference unless the wires are undersized in the first place...
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Old 07-23-2014, 11:56 AM   #8
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Thanks, that helps a lot. I think I'll keep it the way the Dealer wired it then (figure 2) and just add the second battery disconnect switch. Seems like they would just have the factory installed disconnect take care of disconnecting everything but I'm sure there is a reason why they don't. For me I need to disconnect the batteries or some neighborhood kid will play with the buttons on that automatic jack for sure. I always have to make everything kid proof and that leads to me having to learn stuff that isn't fool proof.
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Old 07-24-2014, 06:16 AM   #9
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Originally Posted by TheBennetts View Post
Either way will work as depicted in the schematic and, although picture one is more efficient, I still like method #2 for simplicity. I know if I don't keep it simple, sooner or later I will mess something up.

Even though there is less efficiency it's not noticeable to me.
FYI, in this case "Less Efficient" and "simpler" is unhealthy for your battery bank.

The battery closer to the positive and negative leads from the camper will work harder due to its lower resistance and have more and deeper charge/discharge cycles than the one with the connecting wires in the circuit (whose resistance is added to the battery's internal resistance).

See the attached article for additional information.
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Old 07-24-2014, 07:46 AM   #10
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Make sure the switch is wired up in the right place. My switch is wired between the chassis ground and the battery that has the cable going to the chassis ground. With the switch in the open position there is no ground connected to both batteries thus no path for the current to flow (disconnected). With the switch in the closed position the chassis ground is connected to both batteries thus there is a path for the current to flow (connected). This setup works great for me, once hooked up never a problem. My batteries are wired like the top diagram in the above link with a switch between the negative terminal on the battery on the left and the chassis ground.
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Old 07-25-2014, 09:27 AM   #11
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That's a great point Herk7769. I may reconsider and rewire my batteries to avoid the downstream charging as you pointed out. Thanks for pointing that out.

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Old 07-27-2014, 11:30 PM   #12
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Make sure the switch is wired up in the right place. My switch is wired between the chassis ground and the battery that has the cable going to the chassis ground. With the switch in the open position there is no ground connected to both batteries thus no path for the current to flow (disconnected). With the switch in the closed position the chassis ground is connected to both batteries thus there is a path for the current to flow (connected). This setup works great for me, once hooked up never a problem. My batteries are wired like the top diagram in the above link with a switch between the negative terminal on the battery on the left and the chassis ground.

The current draw has NOTHING TO DO with the number of battery's, but rather what you are running. You only need ONE switch and it should go on the positive side of the battery bank in my view. Use Herc's drawing.

series for 6 volt bats parallel for 12 volts.

Be careful.


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Old 07-28-2014, 05:41 AM   #13
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In the big scheme of things, it matters not which side is switched (provided ONE side is!).

Switching the ground is completely acceptable.

In a DC circuit you can break either leg and there is no current path through the battery stack PROVIDED there is only one ground or hot to the camper (including an inverter if one is installed).
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Old 07-28-2014, 07:32 AM   #14
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Quote:
Originally Posted by garbonz View Post
The current draw has NOTHING TO DO with the number of battery's, but rather what you are running. You only need ONE switch and it should go on the positive side of the battery bank in my view. Use Herc's drawing.

series for 6 volt bats parallel for 12 volts.

Be careful.


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My two batteries are wired in parallel...I never said current draw has anything to do with the number of batteries? When I was talking about current flow I was assuming something in the trailer was on and with the switch either opened or closed current would flow or wouldn't flow from the batteries. I have been wiring the negative side going to chassis ground through the switch for many, many years....Herk is correct, it really doesn't matter which side you switch, I chose to switch the ground and it works fine. There is either a path or there isn't.....
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Old 07-28-2014, 08:45 AM   #15
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Originally Posted by Herk7769 View Post
PROVIDED there is only one ground or hot to the camper (including an inverter if one is installed).
I also do think it doesn't matter, except for the provided thing here. JUST make sure there is one and ONLY one lead from the battery to the shutoff switch. some find it useful to have a terminal post to attach all the grounds to after the switch, or the pos leads. Personal preference.
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Old 07-28-2014, 08:49 AM   #16
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Originally Posted by Bmbrady77 View Post
You would only wire the batteries in series if you have 24VDC system. Series will double the voltage...Parallel will double the available current while keeping the voltage at 12VDC.

As far as the disconnect switches....It would depend on the location of the switch in circuit, but I would be willing to bet that the disconnect switch is not rated for the potential current draw you could get from 2 batteries in parallel. I would install a second switch or get a DPST switch rated for the current on each battery individually.

As a side note..If they hooked up the batteries in series and you have a 12VDC system...you may be in trouble and might want to check out the DC side of your system. At best you could have blown a fuse...At worse you could have fried all of your 12Volt appliances...
This was the post I was referring to, not CAMPSTERS. Sorry for any confusion.

There is some interesting and confusing post on this thread and while 12 volts is not REAL dangerous, the way this was going could (or maybe already has) created problems that didn't exist before.

It seem that I am adding to the confusion myself LOL.
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Old 07-28-2014, 10:13 AM   #17
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This was the post I was referring to, not CAMPSTERS. Sorry for any confusion.

There is some interesting and confusing post on this thread and while 12 volts is not REAL dangerous, the way this was going could (or maybe already has) created problems that didn't exist before.

It seem that I am adding to the confusion myself LOL.

I re-read that and I did type that up in a rather confusing manner...That's what I get for trying to make a quick reply from a smartphone while doing other things!! LOL

What I was trying to get at, or the point of the post rather, was to point out that paralleling two batteries of the same class will keep the voltage the same but double the AVAILABLE current (in theory anyway). This is oppossed to wiring the two batteries in series and doubling the voltage.

There are two reasons for ever wanting to do this..

1. Increased load on the battery bank so that adding a second battery would make the available current enough to cover the new load...or

2. To increase the available run time on a single bank charge while keeping the same load. (ie..Increase the overall AH rating).

My comment about the switch current rating would only be applicable to reason No. 1 stated above..

Sorry for the confusion!!
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