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Old 01-26-2022, 02:55 PM   #1
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Battery question

Can I add a second 12v battery, in parallel, if the batts are not the same make, not the same AH.??
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Old 01-26-2022, 03:18 PM   #2
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Welcome to the forum!!

Suggest not doing this, if the batteries are not identical in age, capacity, type and brand. Different brands can have different charging and discharging characteristics, with some accepting a charge or delivering current faster than others.

https://www.nbccomedyplayground.com/...s-in-parallel/
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Old 01-26-2022, 03:30 PM   #3
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In general, you shouldn't do it, but if you gotta, you gotta and no harm will come (hopefully) to the batteries. There is always the chance that a bad battery will draw down a good battery and trash it too.

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Old 01-26-2022, 11:15 PM   #4
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While not ideal, I have been doing it for 3 years successfully. When I bought my trailer, it had a brand new group 24 FLA battery. I wanted more AH so I brought a cheap Group 29 FLA from walmart. Paralleled them and have been using it that way ever since.
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Old 01-27-2022, 01:14 AM   #5
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Yes, you can parallel them, but it's a bad idea because batteries of different types and different ages have different voltage profiles and different internal resistances.

But you an minimize one problem with proper wiring.
Look at the drawing below.

It's designed to explain PAC-N-SAV....
PAC: Parallel adds capacity (amp hours)
SAV - Series adds voltage (two 6 volt in series = 12 volts)

But the paragraph under the drawing makes a point about equal length cabling... there is a small voltage drop in cabling and this connection technique equalizes it and prevents one battery of a pair working more than the other.

Mike
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Old 01-27-2022, 05:51 AM   #6
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While not ideal, I have been doing it for 3 years successfully. When I bought my trailer, it had a brand new group 24 FLA battery. I wanted more AH so I brought a cheap Group 29 FLA from walmart. Paralleled them and have been using it that way ever since.
This is a good answer. Not ideal, but perfectly acceptable as long as the first battery is in decent condition.
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Old 01-27-2022, 06:00 AM   #7
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You'll be fine Dave.

The previous posts are correct, however, about amp load causing the old battery to try and keep up with the new battery.

But the load demands of a TT are hardly going to get anywhere near the kind of amp draw that would do damage to the old battery, as would be the case in say a diesel pick-up that has two batteries.

That said, I have been using mixed batteries in my F250 going on 4 years now with no issues. YMMV
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Old 01-27-2022, 06:06 AM   #8
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Different make and amp hours are less of an issue than different ages. If one battery is much older than the other it may draw down the newer battery simply by being the weaker of the two. If the batteries are of roughly the same age it's less of an issue, or will be less likely to become an issue in the future.
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Old 01-27-2022, 06:44 AM   #9
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Quote:
Originally Posted by AnotherMike View Post
Yes, you can parallel them, but it's a bad idea because batteries of different types and different ages have different voltage profiles and different internal resistances.

But you an minimize one problem with proper wiring.
Look at the drawing below.

It's designed to explain PAC-N-SAV....
PAC: Parallel adds capacity (amp hours)
SAV - Series adds voltage (two 6 volt in series = 12 volts)

But the paragraph under the drawing makes a point about equal length cabling... there is a small voltage drop in cabling and this connection technique equalizes it and prevents one battery of a pair working more than the other.

Mike
I'm a believer in balancing in spite of some claiming its a minor issue.
That said balancing "external" wiring with different battys... age and /or capacity can't compensate for the batty differences. My understanding is you will get the benefit of adding a second old used batty as the new batty only performs as good as the old one. I would opt to optimize with a new pair and save the old one for some stand aline use until it dies... or just advertise & sell it for something a little higher than the core value and move on.
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Old 01-27-2022, 06:48 AM   #10
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The mix of answers here demonstrates it's not a yes or no, but "depends". Two disparate batteries - age, cycles, capacity - can be connected in parallel without any sudden or dramatic problem. The two capacities will add. Where things can go south is depending on how far apart they are in age, cycles or capacity they will not charge and discharge identically and over enough time and cycles at least one and probably both will have reduced life vs those that are "matched". The question then becomes how they're used. If say this particular pair might only net half their otherwise useful life but that's still twice as much as you might need, then it doesn't matter at all. If their capacity is fully used and often, then that accelerated demise makes this a less optimum solution. Practically speaking the most cost effective battery you have is one that's paid for, so I usually go with mix and match of whatever I already have. Better to use up what I have and then buy the matched set after I got whatever service I could from ones I own. My position is don't let perfection be the enemy of good enough, unless these batteries are running your oxygen machine all night put 'em together, run 'em until they give up and then get your matched set. Even if all you got was one season out of them that's more than you get sending them to the recycler with life remaining.

Mark B.
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Old 01-27-2022, 06:59 AM   #11
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Quote:
Originally Posted by Mark_K5LXP View Post
The mix of answers here demonstrates it's not a yes or no, but "depends". Two disparate batteries - age, cycles, capacity - can be connected in parallel without any sudden or dramatic problem. The two capacities will add. Where things can go south is depending on how far apart they are in age, cycles or capacity they will not charge and discharge identically and over enough time and cycles at least one and probably both will have reduced life vs those that are "matched". The question then becomes how they're used. If say this particular pair might only net half their otherwise useful life but that's still twice as much as you might need, then it doesn't matter at all. If their capacity is fully used and often, then that accelerated demise makes this a less optimum solution. Practically speaking the most cost effective battery you have is one that's paid for, so I usually go with mix and match of whatever I already have because short of some critical long term application, the net result is I would have to buy new batteries. In my view, better to use up what I have and then buy the matched set after I got whatever service I could from ones I own. My position is don't let perfection be the enemy of good enough, unless these batteries are running your oxygen machine all night put 'em together, run 'em until they give up and then get your matched set. Even if all you got was one season out of them that's more than you get sending them to the recycler.



Mark B.

Albuquerque, NM
Generally I would be in this camp being frugal and not wanting to waste usable capacity. The problem with going this route is when the older batty finally dies you are back in the same situation... having a now used batty that's still OK? Do you repeat the less than optimum performance by mixing in a new batty?
I'm assuming that the reason for a second in parallel is you need more capacity... do you want the equivalent of 2 weak/ older battys or 2 new in optimum shape.
I realize answer depends how big the difference is!
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Old 01-27-2022, 10:01 AM   #12
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Lots of good advice above.

As many have pointed out above, everything will likely work fine.

Charge a little longer to equalize the batteries. Keep an eye on the older battery. It is most likely to fail first. It may use more water the the newer one.

Make sure the new battery chemistry is the same as the old. Most flooded cell batteries use the same chemistry. Avoid any of the more unusual battery brands.

Occasionally test each battery separately. If disconnected and resting battery terminal voltages are greatly different, that would be a bad sign for one of the batteries.

Hygrometer testing of each cell can also discover pending failure. (Did I spell that correctly?)
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