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Author Topic: Parallel Deep Cycle Batteries  (Read 1773 times)

Rit

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Parallel Deep Cycle Batteries
« on: May 17, 2010, 04:35:08 PM »
I have been using one 12v deep cycle battery for the house and wonder what to look for when adding another 12v in parallel to have longer lasting charges.

seilerbird

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Re: Parallel Deep Cycle Batteries
« Reply #1 on: May 17, 2010, 04:39:36 PM »
One that is precisely the same as the one you already have. It is your only real choice.

vermilye

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Re: Parallel Deep Cycle Batteries
« Reply #2 on: May 17, 2010, 04:42:48 PM »
It is also a good idea to have them the same age, or, at least, the same condition.  If one is at a lower voltage it will draw down the other.

John From Detroit

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Re: Parallel Deep Cycle Batteries
« Reply #3 on: May 17, 2010, 08:27:24 PM »
I always see this "Keep them the same" Myth

Now if you are putting pairs of six volt batteries in series then each of the pair must be the same, same age, size, whatever.. However that is because they are two halves of the same 12 volt battery.


But when you put  them in parallel.. Unless you are putting like a 10 year old battery up alongside a brand new one there is not much danger (and that 10 year old is likely toast to begin with)

"If one is lower voltage it will take down the other"  Right.. Now tell me, how is one goign to be a lower voltage.? I mean they are PARALLEL. so while it is true that when you first hook them up one will indeed be lower than the other... That will last for one discharge cycle only.

UNLESS, of course, one battery has a shorted cell, in which case it's time to replace it.

The only thing is they should all be the same type

If the one you have is flooded wet where you can (And shoudl) add distilled water.. The other one should be too

If one is AGM. the other shoudl be too

Maintenance free (But not AGM) again stick with the same type

But as for size, age, capacity.. The batteries will work that out. I have no problems putting a Group 31 and a Group 27 beside each other.  Work well..

Just so long as you don't have a shorted cell (Which is one type of bad battery)
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Dave W

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Re: Parallel Deep Cycle Batteries
« Reply #4 on: May 17, 2010, 08:46:49 PM »
My 5er has parallel deep cycle size 27 batteries. One is a 2006 vintage Magna Charge and the other a 2009 Interstate. No problems so far. If one needs water, so does the other (so far minimal and only once/year). Tested exactly the same this spring after being left on a Battery Tender over the winter.

Gary RV Roamer

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Re: Parallel Deep Cycle Batteries
« Reply #5 on: May 18, 2010, 10:37:37 AM »
Quote
I always see this "Keep them the same" Myth
I'm not getting your point, John. You say it's a myth, but then caution to not have much difference in age and to make sure both are in good condition (no shorts, etc) and both the same type. Isn't that pretty much what "keep them the same" means, at least in a practical sense? They don't have to both be brand new, but they ought to be similar in condition and charge capability.

If you parallel a battery that can only sustain 12.3v with a battery that can do a full 12.6v, the system voltage will be the average - 12.45v. Isn't that dragging the voltage down"? And if one won't hold a charge, the other is supplying most of the electricity, which kind of defeats the purpose of having two in the first place.

It is always best to interconnect only batteries of similar operating condition. Since most of us don't know enough about batteries to evaluate that status, saying that age and type should be the same is a reasonable guyideline.
Gary
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Gary Brinck
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2007 GMC Acadia
Homebase: Ocala National Forest, FL

John From Detroit

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Re: Parallel Deep Cycle Batteries
« Reply #6 on: May 18, 2010, 01:41:53 PM »
No, I said it's a myth unless there is like a decade difference in age (10 year old batteries need to be replaced soon so you might as well do it now)

I will soon be putting new batteries alongside 5 year olds.  I anticipate no issues

And I've seen shorted cells in a battery 2 weeks old... Believe it or not (It was on my car, that battery had a lifetime warranty so it was replaced no charge)
Nothing adds excitement like something that is none of your business
My Home is where I park it.

vermilye

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Re: Parallel Deep Cycle Batteries
« Reply #7 on: May 18, 2010, 02:31:25 PM »
There is a fair amount of debate on the issue of paralleling batteries, but most seem to support not paralleling old vs new batteries.

Some quotes from battery manufacturers & Suppliers:

US Battery - See Item #12  http://www.usbattery.com/usb_care_maint_2.html
Crown Battery - "A weak battery will cause premature failure of companion batteries." - http://www.crownbattery.com/?page=support_cm_deepcycle
Ample Power Company - A article that supports paralleling 12 v sealed batteries - http://www.amplepower.com/pwrnews/parallel/index.html
Oasis Montana Inc. - "Where multiple batteries are in series, parallel, or series/parallel, do not mix batteries differing in age, size or usage level.  Your best batteries will operate only at the capacity of your poorest or oldest cells.  When purchasing batteries, plan on their longevity, and upgrade as needed at the end of their expected life span.  If you add new batteries to old, expect them to perform at the capacity of your poorest or oldest cells." http://www.oasismontana.com/batteries.html
Optima - "When hooking batteries together in parallel they should be of identical make, model and of similar age." http://www.optimabatteries.com/product_support/faqs.php

I have no problem with paralleling similar batteries, however I'd still suggest that when adding a new battery in parallel they both are in the same condition. If not, your charger/converter may either overcharge the one producing the lower voltage or go into the float stage prematurely.



carson

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Re: Parallel Deep Cycle Batteries
« Reply #8 on: May 18, 2010, 02:46:11 PM »
Vermilye, it seems that you went to substantial amount of work to provide all that info. I personally want to thank you for all that effort in providing links

  I have no dog in this fight, I only have 2 -27 batteries in parallel and they have been going well for 6 years now.

  I hope that other readers will appreciate all your links. Seems like this problem arises over and over again.

Thank you,  carson FL

Carson, 
 West Central Florida
Ex RV'er. (1995 Winnebago Adventurer)
...Logic works like a charm...

seilerbird

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Re: Parallel Deep Cycle Batteries
« Reply #9 on: May 18, 2010, 02:52:56 PM »
I thank you for the links. I have always heard it was bad to mix battery types. As an electrician I know you are going to drag the best battery down to the lowest if you mix types. But this debate has me rethinking my position. Great thread.

Lou Schneider

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Re: Parallel Deep Cycle Batteries
« Reply #10 on: May 18, 2010, 06:09:46 PM »
The problem with paralleling batteries is keeping the currents equal between them.

With a pair of 6 volt golf cart batteries in series, there's one and only one path through them.   All of the cells get precisely the same amount of charging current so all cells charge at an equal rate.  When it's time to power a load, all of the cells contribute an equal amount of current.  With all the cells working evenly, you're less likely to develop a weak cell.

In a parallel connection, the charging current divides between the multiple batteries.   If there's any difference between them, one battery will receive more charging current than the other, and when it's time to power the load that battery will supply more current than it's mate.  In other words, one battery is working harder than the other.

Even something as simple as a partially corroded connection can unbalance parallel-connected batteries, making one work harder than the other.
« Last Edit: May 18, 2010, 06:13:26 PM by Lou Schneider »

Gary RV Roamer

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Re: Parallel Deep Cycle Batteries
« Reply #11 on: May 19, 2010, 09:17:51 AM »
I'm pretty flexible on mixing batteries in parallel. I've put a new AGM 12v with a year-old 6V pair, and matched up various new and older 12v's as well in both my RVs and boats. I agree with John that age per se is not a good indicator and as long as the condition is good, you can get away with quite a bit.  But matching condition takes a bit more discernment than just saying "it's been working ok". I mean measuring voltages and taking specific gravity readings (where possible) and doing load tests on older batteries. Those techniques give you a fair idea of actual condition.
Gary
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Rit

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Re: Parallel Deep Cycle Batteries
« Reply #12 on: June 01, 2010, 09:25:48 AM »
Quality answers in this thread and I will do the testing on my present battery to match it up!

Thanks.

John From Detroit

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Re: Parallel Deep Cycle Batteries
« Reply #13 on: June 01, 2010, 02:52:26 PM »
I like the conflicting posts post.

One note I will re-iterate.

If you parallel brand new batteries, consecutive serial number batteries.

Remember I said I had one that failed in under 2 weeks?  It was replaced under warranty.

So haveing all batteries identical does not guarantee that one will not fail quickly and "Wear down" the others.

Page 2........

That thing about a weak battery causing the companions to wear out sooner is true.. I mean it's like this.

Let's say you have 400 amp hours of battery.. You often draw 100 amps off them that is perhaps 25 amps per battery

Drop one due to old age and it's 33.3 amps per battery

The faster you draw (or charge) the faster the battery goes bad.

But it's not the bad battery "Drawing down" (less it is a shorted cell) it's the additional load that is wearing them out faster and when you replace the bad battery that is corrected.
Nothing adds excitement like something that is none of your business
My Home is where I park it.