Connecting batteries in parallel

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Connecting batteries in parallel

Postby adow » Thu Sep 13, 2007 8:30 pm

I have a 115 AH lead acid battery in my camper charged from the car alternator via an Anderson plug and rearc battery isolator. I want to add another battery to the camper trailer. The plan is to connect a 100 AH gel battery in parallel to the other battery - simply by connecting the positive terminal of the 115 AH to the positive terminal of the 100 AH and the negative terminal of the 115 AH to the negative terminal of the 100 AH (using very thick wires and good quality connectors). Thus, the charge from the car will charge the two batteries simultaneously. Can anyone think of any problems with this setup - is there a danger of either overcharging or undercharging either battery (because they are not identical) or any other issues? Are there any improvements that could be worth considering?
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Postby double_d » Fri Sep 14, 2007 10:42 am

every battery has an internal resistance
so check to see if you can see this
the two batteries will get different current suplied to each if different resistnace
so one will charge quicker than the other
thats all i know
dont know if it is dangerous though

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Postby Komodo » Fri Sep 14, 2007 11:29 am

Dont have the time to go in depth - but I would advise against it.

If you did ignore my advice and you still want to go this route then use a 1 / 2 / 1+2 style manual isolator. switch it to 1+2 when driving to charge then when stopped choose one of the batteries then when that runs low change over to the other.

Best to get 2 identical batteries.
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Postby adow » Fri Sep 14, 2007 7:42 pm

That sounds like good advice - I intend to find a way of completely isolating each battery when charging and discharging, so that they are never linked. One of those switches should do that.
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Postby jasonb » Fri Sep 14, 2007 8:20 pm

ide say youll have to get a new alternater {bigger} cause it will burn it out in virtually no time ,,,,,,
are we there yet ?
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Postby double_d » Fri Sep 14, 2007 8:34 pm

im no expert
but wouldnt you only need a bigger altenator when you draw more current
like stereos and such
charging the batteries wont take more current it should just take what is left
shouldnt it ???

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Postby Derek Bullock » Fri Sep 14, 2007 8:53 pm

This is a diagram Derek Bester from Sidewinder sent me when I queried setting up my battery system. As you can see this system allows for three batteries and only one isolator.

My inquiries from the techs is that you CAN set different ah batteries in parallel. This is also copnfirmed in an article from Caravan and Motorhome
You can readily parallel a 100-amp/hour battery with an otherwise similar 200-amp/hour battery to obtain 300 amp/hours.
http://www.caravanandmotorhomebooks.com ... teries.htm.

I am sure though if you follow Derek's diagram you could do it easily. Simply move the aux battery to the trailer and run the cables.

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Postby adow » Tue Sep 25, 2007 10:54 pm

Thanks Derek. The plan now is to isolate the two batteries for charging and discharging purposes with a Hella 1, 2, both switch, partly to ease the drain on the alternator if both batteries are low, partly because gel and lead acid batteries have different charging requirements, and partly because it will mean that I can't flatten both without being aware of it. When I have completed it and it's working I'll post the diagram.
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Postby Komodo » Wed Sep 26, 2007 10:50 am

double_d wrote:im no expert
but wouldnt you only need a bigger altenator when you draw more current
like stereos and such
charging the batteries wont take more current it should just take what is left
shouldnt it ???

daniel d


Wont need a bigger alternator - most modern alternators are designed to make in excess of 65amps @ 2200 RPM engine speed. Alternators are designed to charge yuour battery as quick as possible - due to the fact that so many trips are under 10 minutes they need to put back the energy taken to start the vehicle asap. But even if your alternator is capable of 65amp (or more) you will find that it only makes 25amps or so most of the time - because thats all the draw. IE a straw only allows flow in accordance to how much you are sucking on the other end of it - if your not sucking, it doesnt flow - no matter what the diameter (ie ability for it to flow)
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