I currently have a one-year old marine deep cycle 12 V ? 24 battery. I have just added the capability to add another battery to this battery. My question is can I add a 27 12 V battery to the size 24. 12 V battery? Would this cause any harm? Or, should I just add another 24?12 V battery to the existing one?
Battery question
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Telstar Ranger
Well-known member
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- Jul 31, 2015
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I am learning about this myself and take my answer with a little bit of caution. But yes, you can do it, but it is not advisable as it distorts the charging rates, or they don't charge at equal rates--something to that effect. They say it is best to add the same kind. I wanted to do the same thing, as I I purchased a used RV and the guy just threw in one 24 marine from Walmart.
John From Detroit
Well-known member
Two rules one for parallel one for series:
Batteries in parallel: What you purpose: you may mix and match size and age,, YES there is some "Danger" in mixing as opposed to matching however it is rather small.. So yes you can put a 24 and a 27 and a 29 and a 31 in parallel.
Rule #2, Batteries in series DOES NOT APPLY TO YOUR CASE YOU MAY STOP READING NOW
When you put two batteries in series, those two batteries essentally become "one" (Each one is half of a higher voltage battery) Thus they MUST be IDENTICAL.. Same make, Model, Size Age.. Once you attach that series jumper treat them exactly as you would a big 12 volt battery (example Two GC-2 Golf Car batteries, in series =about= a 4D)
So if you ever decide to upgrade to a pair of GC2 in serels (roughly 220 amp hours at 12 volt) think of the pair as one big 12 volt and many questions, Such as the one above, vanish (Woudl you want a 12 volt battery where 3 cells came from a 27 and 3 from a 24,, Clearly not,, Same applies to pairs of six volt)
Just remember, do not read past the shouted line till you go six volt
.
Batteries in parallel: What you purpose: you may mix and match size and age,, YES there is some "Danger" in mixing as opposed to matching however it is rather small.. So yes you can put a 24 and a 27 and a 29 and a 31 in parallel.
Rule #2, Batteries in series DOES NOT APPLY TO YOUR CASE YOU MAY STOP READING NOW
When you put two batteries in series, those two batteries essentally become "one" (Each one is half of a higher voltage battery) Thus they MUST be IDENTICAL.. Same make, Model, Size Age.. Once you attach that series jumper treat them exactly as you would a big 12 volt battery (example Two GC-2 Golf Car batteries, in series =about= a 4D)
So if you ever decide to upgrade to a pair of GC2 in serels (roughly 220 amp hours at 12 volt) think of the pair as one big 12 volt and many questions, Such as the one above, vanish (Woudl you want a 12 volt battery where 3 cells came from a 27 and 3 from a 24,, Clearly not,, Same applies to pairs of six volt)
Just remember, do not read past the shouted line till you go six volt
.
John From Detroit
Well-known member
Telstar.. You know what the Commune preaches right.. Everybody acording to their ability, everybody according to their needs (Except of course for the leader who gets more and does less but hey.. That's why they do not work).
Well batteries in parallel form the perfcet (And only working that I know of) Commune.
If you have a 75 amp hour battery in parallel with a 100 amp hour battery then let's say your charge rate is 17.5 amps .. The smaller one gets the 7.5 and the larger one the 10. Exactly in purportion to their size.
Put a 17.5 amp load on them, The larger one delivers 10 amps the smaller one 7.5
I hear how "one will charge faster and when it's full the converter will stop charging and the other will never reach full" This is true only for batteries in series,, It is FALSE for batteries in parallel.. Here is why
For any given condition of load and temperature the battery's state of charge determines the output voltage or the charging (input) voltage. The higher the state of charge the higher the voltage.
So if one battery is "Fuller' it will give more or take less till the levels are identical, and from then on they march in lock step. Can not do otherwise, Except possible in ultri high (load test level) discharge. Then the resistance of the connecting wires comes into play. But we do not go there in RV use save when load testing and that is done battery at a time.
Well batteries in parallel form the perfcet (And only working that I know of) Commune.
If you have a 75 amp hour battery in parallel with a 100 amp hour battery then let's say your charge rate is 17.5 amps .. The smaller one gets the 7.5 and the larger one the 10. Exactly in purportion to their size.
Put a 17.5 amp load on them, The larger one delivers 10 amps the smaller one 7.5
I hear how "one will charge faster and when it's full the converter will stop charging and the other will never reach full" This is true only for batteries in series,, It is FALSE for batteries in parallel.. Here is why
For any given condition of load and temperature the battery's state of charge determines the output voltage or the charging (input) voltage. The higher the state of charge the higher the voltage.
So if one battery is "Fuller' it will give more or take less till the levels are identical, and from then on they march in lock step. Can not do otherwise, Except possible in ultri high (load test level) discharge. Then the resistance of the connecting wires comes into play. But we do not go there in RV use save when load testing and that is done battery at a time.
Gary RV_Wizard
Site Team
As you can see, the technically correct answer is no, you should add the same size and type battery. But it also says it should be the same age battery, in other words, identical in every way. So the ideal solution is to replace both batteries with new ones of thesame type.
Now to the practical. The difference between new and one year isn't great, nor is the size difference between a 24 and a 27 all that great. Go ahead and do it - the benefits outweigh the drawbacks.
But if you really want good battery performance, junk the #24 marine battery and get two new #27 deep cycles (not marine). Either an AGM type or something like a Trojan 27TMH (12v) or any pair of GC2 golf cart batteries, wired in series to produce 12v. Anf of those solutiosn will give you excellent capacity and 7-10 years of useful battery life.
You might read this article from the forum Library: Choose battery
Now to the practical. The difference between new and one year isn't great, nor is the size difference between a 24 and a 27 all that great. Go ahead and do it - the benefits outweigh the drawbacks.
But if you really want good battery performance, junk the #24 marine battery and get two new #27 deep cycles (not marine). Either an AGM type or something like a Trojan 27TMH (12v) or any pair of GC2 golf cart batteries, wired in series to produce 12v. Anf of those solutiosn will give you excellent capacity and 7-10 years of useful battery life.
You might read this article from the forum Library: Choose battery
Thank you for your quick responses. I initially planned on putting two new size 24 batteries in the new battery rack that I installed on the front of my trailer. But, a friend who helped me install my inverter suggested that I just keep the one year old 24 battery that is in the compartment and add two new 27 12 V batteries to the new battery rack on the front of the trailer . Then, he suggested just try out adding just one new battery i.e. the 27. Again, I would appreciate your comments on a three battery system in parallel two new 27's and the 24. Thank you for your help on this decision.
Gary RV_Wizard
Site Team
No difference than the previous comments. "Best practice" is all batteries identical in age, size and type. Anything less is not quite optimal and a tradeoff of longevity and performance vs cost of one battery. Since the marine batteries you are talking about aren't the best longevity or performance anyway, a minor mix like you describe is likely to be of little consequence. That's assuming the group 24 battery is still in very good condition. If it has been run "dead" a few times, it probably is not and ought to be replaced.
