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Old 09-04-2015, 01:21 PM   #1
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parallel battery disconnect switch

I have 2 12 volt batteries in parallel, with POS-POS and NEG-NEG. Standard set up.

I have the trailer negative attached to the NEG on one of the batteries and the trailer positives connected to the POS on the other battery. This is to draw them down equally. ( I know this is another separate debatable subject).

Am I correct that just having ONE disconnect switch is not enough?

Apologies for the bad drawing.

I'm not electrician either so if there's a way to do this with one switch I'd love to hear it.

Thank you!
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Old 09-04-2015, 01:34 PM   #2
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Dave, if you put the disconnect switch between the batteries and the trailer (instead of between the two batteries as in your drawing) it'll work just fine with one switch. The batteries will still be connected together but as long as they are of the same age, size, and health that should be no problem.

However, if it were me I'd go with a marine battery selector switch that would allow you to select battery 1, battery 2, both batteries, or disconnected. This way you will avoid the potential issues of one battery draining down the other. You would still have the option of running off two batteries at the same time but you'd also be able to fully deplete one and know there is a charged one ready to go with the turn of a knob. Check Amazon for battery selector switch, they're about $20 to $40.
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Old 09-04-2015, 01:38 PM   #3
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I'm not an electrician but I think you would be OK with a single disconnect between the trailer and the battery negative. Any spark chasers out there with words of wisdom?

Howlin' beat me to it!
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Old 09-04-2015, 02:05 PM   #4
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Quote:
Originally Posted by dangerdave View Post
I have 2 12 volt batteries in parallel, with POS-POS and NEG-NEG. Standard set up.
Quote:
Originally Posted by dangerdave View Post
I have the trailer negative attached to the NEG on one of the batteries and the trailer positives connected to the POS on the other battery. This is to draw them down equally. ( I know this is another separate debatable subject).
Am I correct that just having ONE disconnect switch is not enough?
Thank you!
The BEST hookup for (2) 12Volt batteries in parallel is to wire them as you show them. As for the Disconnect Switch, it should be between the batteries and the TT, and not between batteries.

With 2 12VDC batteries, you have 2x the Ah of a single battery. If you have an 85Ah 12Volt battery, you have 170Ah when connected in parallel, of which you can only use 85Ah (50% rule, batteries power at 12.0VDC). You are better off leaving the 2 batteries connected in parallel, this way they will discharge at the same rate and charge at the same rate. If you put a switch and run off one battery and then the other, your charging will be affected because the odds discharging the 2 batteries to the same point is next to impossible, thereby causing an unequal charge (one will require more charging power than the other).

So if you decide to use the batteries one at a time, you need to MONITOR the battery status and switch to battery #2 when battery #1 drops to 12VDC (43Ah), and when battery #2 drops to 12VDC you need to shut down your batteries, or charge them.

You will get no additional power or time out of using the batteries one at a time or both in parallel, 85Ah is 85Ah, whether it is 85Ah in parallel or 43Ah x 2 batteries, the total power available is the same.

Leave them in parallel.. Less work… discharge/charge more evenly..
your batteries will love you!!
Don
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Old 09-04-2015, 02:43 PM   #5
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Ahhh. Yes I see that. Duh. I'm not putting the switch in the correct place.


As always, thank you guys!
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Old 09-05-2015, 07:54 AM   #6
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Move the disconnect to between the battery and the trailer otherwise one battery will always be powering the trailer.
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