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Old 07-13-2012, 09:39 AM   #15
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Originally Posted by Okotoks View Post
Lol! while ohms law and series-parallel circuits are great to know, if it isint used frequently then it's almost forgotten, as I have just shown!
yes series connection doubles voltage?, parallel doubles amperage?.
So while I didn't actually look at the golf cart battery specs I just assumed.In actuality the 6v golf cart batteries are probably 200-220Ah? If I used the 2 6v in series and paralleled them to the 12v I should have around 300Ah, this week of 30+ temperature must have fried my brain. My math and logic on the last 2 posts makes me feel like a noob.

If I am now correct then back to the possibility of making this work.

Will a setup like this work long term? I am crazy enough to try!
Comments, Suggestions and or ridicule ? I'm all ears(eyes?)
Your trying to cross the street by going around the block.

Series paralleling batteries is a very bad idea.

Using 2 six volt batteries in series as opposed to 2 twelve volt batteries in parallel, has a HUGE down side.

Example # 1. One of the 6 volt batteries gets weak. Your done. Nothing will work correctly.

Example # 2. When using 2 twelve volt batteries in parallel and one gets weak, simply disconnect the weak one, and your still in business.

Also, NEVER, EVER, parallel or series connect batteries that are not exactly the same. The charging of them will be very different, resulting in more trouble.

Ohms law has not changed. Following Ohms law rules, is the only safe way to go.


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Old 07-13-2012, 10:02 AM   #16
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Of course if one of your six volt batteries in series fails you still have the option of disconnecting them and running off the tow vehicle until you can get to the battery store.

While my experience is strictly subjective, I am now starting year 10 on 2 Exide golf cart batteries. Never made it past 4 years on any 12 volt deep cycle I had.

If I had both the 6 volts to put in series, and a 12 volt to parallel with it on hand I would do it. If I had to go buy the batteries I would not.

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Old 07-13-2012, 10:13 AM   #17
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Originally Posted by Jammer View Post
That is the received wisdom. ... there isn't any science to support the assertion that that will somehow affect the life of either battery string,...
I started not to post this here, but it does seem in the general vein of the OP, so here goes...

Whats your thoughts on Science verses and Imperfect world when it comes to battery failures? I have gone through literally dozens of batteries in RV's and Trailers with many being parallel failures. I'm brutal on them...

My old RV had dual house batteries, most of the time I had two different types or ages and it seemed as soon as one got some age, I let it get low on water or ran it down on a trolling motor (the 'Brutal' part comes to play here...) I would end up with with one boiled and the other dead or injuried. (2 stage charger/inverter)

One outing, my 29' airstream killed the Group 24 that I had attached to the umbillical while the GRP 27 in the box survived. (Univolt)

My latest Galvani homicide involved a 3 battery set, the 1 year old boiled dry while the 2 newer matching ones survived. (3 stage charger/minder)

With my 'lack of attention' method of maintance, I just 'seem' to have better luck if all the cells in parallel are of the same type and manufacture. Thoughts?
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Old 07-13-2012, 10:29 AM   #18
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The problem with having batteries in parallel is that if you have a shorted cell you'll end up discharging the good batteries into the one with the shorted cell.

You can get a shorted cell from charging a battery that's low on electrolyte, or from lots of cycles, especially if you deep cycle a battery that isn't meant for deep cycling.

For that reason you don't want to leave junk batteries in parallel with good ones, but there's no actual problem mixing two serviceable batteries of different ages, or two batteries of different shapes or sizes. To some extent the newer/larger batteries will have lower internal resistance and will charge and discharge somewhat faster at the beginning of the charge or discharge cycle, but that doesn't matter much.
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Old 07-13-2012, 12:05 PM   #19
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Hate to keep repeating the same suggestion, but install a marine battery isolator switch; and use and/or charge them one at a time.

When one gets discharged, switch to the other one. Same with charging, which will take half the time compared to charging two batteries connected in parallel.

Also, if one battery goes bad, switch to the other one until you can buy a replacement. By isolating them, differences in battery size and age do not matter (though both should be 12 volts).

Additional advantage: Switching to OFF completely disconnects both batteries, so they hold a charge while Airstream is in storage.


Link to battery isolator switch below is provided for reference only. Other brands and sources may be cheaper and/or better: Perko 8501DP Marine Battery Selector Switch: Automotive
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Old 07-13-2012, 12:51 PM   #20
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Pheonix a battery isolator switch is a really good idea with the setup the OP is talking about and also minimizes the problem of someone forgetting to turn off some light while boondocking since only 1 battery or the 2 6V in series would go dead and you could switch to the other battery.
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Old 07-13-2012, 03:33 PM   #21
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I used this same switch in my previous SOB. In that case I had two 12 volt banks, each made of 2 6 volt golf cart batteries. If I was connected to Shore power or to the TV, I put the switch in the BOTH position to keep both batteries charged. When not on shore power or TV, I would run off one bank at a time, so I would have a good idea how much useful time was left after the first one discharged. I also had solar panels on the roof and another smaller switch to independently switch them from one bank or the other or OFF.

It was an interesting and expensive experiment, that I would not do again. It is a lot simpler to stick with as many 12 volts batteries as practical in parallel and keep track of their state of charge by monitoring their voltage. If you suspect one is going bad, just disconnect it until you can replace it.

I got lost a bit in the calculations in the preceding posts. Just to clarify: two 100AH 6 volt batteries in series = one 12 volt 100AH battery.

Also, contrary to stern warnings previously posted, connecting different capacity but same type and voltage batteries in parallel is not the most efficient way to operate, but it will work fine and will not ruin either of the batteries.


P.S. Keep in mind this is hopefully just camping, where you can pick up and leave if something stops working and not life and death survival. In that case all the rules are much more strict.

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