Is it okay to link two batteries of unequal size?

[KenNTennessee]

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?

[pianotuna]

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]

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]

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]

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]

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]

Hi Ken,

Add a modest solar system and it may be years before replacement batteries are needed.

[KenNTennessee]

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]

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]

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]

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.

[Gau_8]

Bumpyroad wrote:

Gau 8 wrote:

BFL13 wrote:
Isn't the MS a marine "starting" 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.

You "can" jump off of a bridge but it is not a good idea.

so you think he should throw away a brand new battery since using it will be the same as suicide?
bumpy

Faulty assumption on your part. You dont have a clue what I think.

[MEXICOWANDERER]

Pardon me but the entire subject can be confusing as hell. Let me explain why.

Let's exaggerate for perspective...

A 2.75% antimony battery from a manufacturer and another 2.75% antimony battery from the same manufacturer. One has plates the surface area size of a quarter and are 1/8" thick.

The second has identically -constructed- plates, but their size is that of a billboard and are a foot thick.

If the same plate construction AND the same plate paste are used in both batteries, guess what, the batteries are going to get along fine.

Some OEM's use thicker plates on some models. And can cram FEWER stacks in each cell. Fewer stacks = less surface area and I'll leave it to others to state the performance reaction. Let's leave cycle life count out of this for now.

A consumer reacts to R/C A/H and CCA without regard to the relationship of weight of an accumulator and stack thickness.

Job one is to find out for sure the antimony alloy percentage. Plate and pasting techniques should be limited to a single OEM, got it? No Johnson Controls mixed with US battery if you want a reasonable match.

But antimonial percentage content differences are the kiss of death as far as "battery matching is concerned". Most new breed batteries seem to use 1.280 weight acid so this is less of a concern than it used to be. Acid densities must match.

I do not know of any OEM that plays musical chairs with plate and paste composition in their run-of-the-mill various car jar cyclable batteries.

So determining the manufacturing origin of that OEM battery would be high on the list. Matching it to a sister battery regardless of capacity would serve it's purpose. Only OEM knows for sure construction details. But by factoring BCI group size, cell PLATE COUNT, CCA, and weight, there is a very high chance of finding a match to the OEM battery.

Since the existing battery is almost "new".

A telephone call to the manufacturer and not to anyone else would be required. In this case an APPLICATIONS ENGINEER would suffice to provide information accurate enough to rely on.

Then determine which re-brands are the same battery.

15 minutes on the telephone should do it.

Hope this helps.

[BFL13]

Need more info on that particular 24MS. How much of a hybrid is it? Is it like a "marine/RV" battery or a 27DC "starting-deepcycle" or is it just a starting battery?

Perhaps different brands use that MS label differently where some are starting and some are hybrid? Been Googling on this a bit and get different answers for what MS means.

[Lowsuv]

Ideal is nice .
My Dad said " There is no substitute for cubic inches "
He was talking about dual 430 cubic inch Lincoln engines in his Chris Craft Roamer .
It applies to batteries .
Against advice I have given on this forum I have used an AGM battery in parallel with a conventional 12 volt .
I had a battery fail and I purchased just one .
A year later I bought another AGM so that I now had two .
I would use the group 24 with a group 27 , 29 , 49 or whatever fits .
I am 100 % AGM in everything now .
Exide Edge AGM is $ 140 at BiMart .
Wikipedia : AGM battery for the reasons .