We just bought a new travel trailer and it came with a dealer installed new 24MS battery, which does not have enough capacity for us as we do dry camping up to three days at a time. I am going to install a second battery and would like to get a 27DC battery, but am not sure if there would be problems having two 12V batteries of different capacities hooked in parallel. In theory hooking batteries in parallel, you should be able to add the amp hours together. So if the 24MS has a reserve capacity of 90 and the 27DC has a reserve capacity of 115, then the total reserve capacity would be 205. My question is will there be an issue with charging or will the battery life be affected since they are not the same capacity?

Isn't the MS a marine "starting" battery? You don't want to use that for your Rving, which calls for a "deep cycle" battery, but many have the hybrid deep cycle/starting type at least (such as that 27DC)If that is a starting battery, then your dealer cheated you on the battery.Anyway, going back to the question, yes you can parallel the same kind of batteries even if they have different capacities, such as a 24DC with a 27DC, as long as they are nearly the same age, and usage.

Well, the dealer threw the battery in at no charge, so I didn't get "cheated", but I don't believe it is the optimum type for an RV. However, I have noticed that some brands, such as Sears DieHard do not carry the DC, they only carry the MS, which is a "hybrid". Yes MS is a Marine Starting battery, which has a higher CCA rating than a true Deep Cycle, but is also rated for deep cycling. Anyway, I appreciate the response as I wanted to go with the 27Dc, and already have the box for it.

As BFL said, yes you can safely parallel those batteries. Not the best solution but for the price of one extra battery than must be 12 volts not a bad idea. Generically a Group24 Deep Cycle is 80 AH and the Group27 is 100 AH giving you 180 AH battery bank with 90 AH being useful (try real hard not to take those batteries below 50%). You should get a couple years of use out of themWhen they eventually go replace them with dual 6volts. I just picked up 2 new Trojan T105s for $90/each at a golf cart outlet. This would give you 225 AH which again shouldn't routinely go below 50% but with these batteries you can safely go down to 20% SOC occasional without significantly reducing life. Much more useful, robust and forgiving for boon docking purposes.

I would think that with different type batteries the charge rate by the onboard charger may be unequal...I suppose its possible for the smaller battery to receive an overcharge, but I haven't a need to try this...

Hi,It is not best practise.This is what is balanced and best for twin twelve volt batteries.As it often doesn't cost a dime more to do this, I think it is worth the trouble.If you wish to understand the "why" surf here:correctly interconnecting multiple twelve volt batteriesOthers may say it doesn't matter--but unless there is a compelling reason to not optimize charging and discharging why not do it the best possible way?

Is it okay to link two batteries of unequal size?

25 Replies

pianotuna
Nomad III
Mar 16, 2014
Hi,

You can do it while discharging at a high rate. Doing it while charging doesn't work too well as each chemistry type wants a different charge voltage. Connecting them in parallel while resting is a bad idea as one will act as a parasite on the other.

It is definitely not "best practise".

Lowsuv wrote:
Is it okay to parallel an AGM battery with a conventional battery ?
Bumpyroad
Explorer
Mar 16, 2014
Lowsuv wrote:
mexicowanderer has added some science to this which is rare on this forum with regards to batteries and tires .
I have a question for mexicowanderer .
Is it okay to parallel an AGM battery with a conventional battery ?

[COLOR=]FACTS, WE CAN't Handle FACTS.
bumpy
Lowsuv
Explorer
Mar 16, 2014
mexicowanderer has added some science to this which is rare on this forum with regards to batteries and tires .
I have a question for mexicowanderer .
Is it okay to parallel an AGM battery with a conventional battery ?
BFL13
Explorer II
Mar 16, 2014
Since the 24MS was free and it's only worth about $70 anyway, I'd ignore it and get the two 6s right now! :) The money not spent on the second 12 could help pay for the two 6s.
westend
Explorer
Mar 16, 2014
The footprint of a 6V GC2 battery is the same as a BCI Grp. 24, the 6V battery is taller. That may or may not influence your battery choice and box choice.

If it was me, I'd get a dual battery box that fit two 6V GC2's and use twin 12V 24's until they gave up the ghost. This would be in anticipation of using two 6V's in the future.

If you have a wish to go solar in the future, maybe even a bigger box would be the best choice. A lot of that depends on how much power you use in a day.
pianotuna
Nomad III
Mar 16, 2014
Hi Ken,

Add a modest solar system and it may be years before replacement batteries are needed.
KenNTennessee
Explorer
Mar 16, 2014
I appreciate most all of your responses, thank you! I didn't understand the tone of RJsfishin's response which seemed a bit hostile. Otherwise greatly appreciate the help. I found the connection information contributed by pianotuna interesting, as who would have thought there would be that much resistance in the interconnection, but a great idea and simple to do. I'm going to go ahead with the 24MS and 27DC for now only because I already have the new 24MS. When it's time to replace, I'll seriously look at Golf Cart batteries in series, as that seems to be the best bang for the buck in terms of capacity, number of cycles and cost.
BFL13
Explorer II
Mar 16, 2014
I remain dubious of the MS battery for any deep cycling. If the OP had suggested he has a Class C and would parallel both the engine and coach batteries for doing some off-grid 50-90s, everybody here would jump all over him.

I don't see how this is any different unless his particular 24MS is really a 24 "Marine/RV" type hybrid. The OP thinks it is, but is he right?
NinerBikes
Explorer
Mar 16, 2014
Run your group 24 MS, maybe pair it with a second group 24 MS. Run them until they both die. Replace with 6 volt golf cart batteries from Costco or Sam's Club, and call it good, or buy a pair of group 27DC's next time around. The 6V golf cart batteries will give 225 ah vs 180-200 AH for the 27DC's. They are also a better value, the 6V golf crt batteries, they are true Deep Cycle batteries, and 500 discharge cycles or more are possible, if maintained properly, watered and fully recharged at the end of the trip.
mena661
Explorer
Mar 16, 2014
Gau 8 wrote:

Faulty assumption on your part. You dont have a clue what I think.
I'm failing to see your point here nevermind your contribution? To the OP, pairing those two would be ok. Not the best as mentioned but ok.