Running two similar but different size batteries in parallel

[RichP]

I've done some research and found conflicting information so I'm here to ask the experts. Our trailer came with a group 24 deep cycle battery which seems too small to me. I already have a 6 month old group 29 deep cycle battery that I used on my old trailer literally once. I went out and bought it when the battery on it died in the middle of our last trip so it is practically brand new. What do you guys think about running the two of them together on our new trailer? Or am I asking for trouble. It looks to me like they will fit on the battery rack on the tongue but I haven't tried to mount it yet. If I were to buy two new ones I would obviously get two of the same but I already have this one and would like to use it as long as it would work.
Thanks guys

[Vicr]

All the battery experts (and I might be one of them) recommend not doing this as your older group 24 battery will just draw down your newer group 27. You can install a battery selector switch and use the 24 until voltage drops and then switch to the 27.

[RichP]

I like that idea. Did you set yours up this was or did it come like that?

[Mustang65]

Originally Posted by Vicr

All the battery experts (and I might be one of them) recommend not doing this as your older group 24 battery will just draw down your newer group 27. You can install a battery selector switch and use the 24 until voltage drops and then switch to the 27.

x2

With one additional bit of info. Some disconnect/selector switches have an option to connect battery bank 1&2 together. When charging the batteries, even though they are all 12VDC, you only want to charge batteries with the "SAME" Ah/mfr rating together, as batteries with different Ah's will not charge equally. They need to be charged separately.

Below is a comparison of hooking up your batteries (same Mfr/Ah rating) in different configurations to get the most out of your batteries.

Don
My Registry

RVing with SOLAR

[RichP]

another question then. If I was only going to run one battery which one would be a better choice?
The one in the trailer now is an Interstate 24DP (dual purpose I assume) specs are 405 cca 505 ca and 100 reserve capacity at 25 amps

Option B which is sitting in my basement is an everstart maxx 29DC -Walmart brand. specs are 845cca and a reserve of 85 - the amp draw is not listed

That being said, my gut says that the bigger battery is better but with the group 24 having a higher reserve would that make it the better choice?

We do usually boondock camp so maximizing run time is the priority. Also by the looks of the specs it almost seems like the everstart is just a re stickered starting battery

[Mustang65]

Any battery that has a rating of CCA or MCA is a dual purpose (hybrid) battery and is not a "TRUE" deep cycle battery. So, if you are interested in the max output for dry-camping you should look at batteries that are "TRUE" deep cycle batteries.

For dry camping you want the batteries with the biggest Amp Hour rating. You could use your existing size 24 batteries, not sure if they are any good, as I do not know their history (let the voltage drop below 12VDC... below 10.5VDC (actual dead battery). how many times... Age of the batteries... maintained proper water levels... kept at full charge.... sitting at less than full charge... The list of things goes on.

How long do you dry camp?
Do you monitor your battery(s) voltage?
Do you shut them down if you drop below 12VDC? (battery 50% rule)
Ever run the battery(s) down to a point where things did not work normal?
Do you use an inverter? For what?
Battery connections tight?

Don
My Registry

RVing with SOLAR

[RichP]

Both batteries are basically new. There is only one of each. The 24 came with the trailer it hasn't been used yet and it's been plugged in since we brought it home a month ago.

The 29 was used for one night and day running two lights and the furnace in our old pop up. It came home and has been on the work bench since on a 2 amp maintainer.

We dry camp for a week or less. Usually just long weekends I do monitor the voltage passively but I'm not going to stare at it the whole time I'm camping either.

I don't use an inverter for anything. All of our powered camping stuff is 12 volt.

I understand neither of these batteries are the best of the best I'm just trying to make sure our trips will be as trouble free as possible

[Mustang65]

You don't have to stare at the meter 24/7. Just as long as you check it. My 12VDC accessory outlet happens to be next to the door, out of habit, I look at the Digital Voltage display, coming and going.

Get that a battery selector switch that allows you to switch between battery banks, like the one below. Mount the batteries permanently on the tongue and switch between batteries when one goes low. When both batteries hit 12VDC then you need to figure a way to charge them, or live with batteries that will have a short life. At 12VDC (50% battery rule), you need to shut down your system. If you constantly are running out of battery power, you will need to invest in a small generator to run 3 hours a day, or install SOLAR on the roof, in your case 200+ watts.

Based on your comment about checking the battery voltage levels, I would not recommend investing ($$$) in deep cycle batteries, because you will be replacing them in a few years.

Don
My Registry

RVing with SOLAR

[Vicr]

Don, just an FYI in response to the sailing community several battery manufacturers are now putting a CCA rating on thier true deep cycle batteries as sometimes their battery bank is needed to start the engine if the starting battery dies or has to low a voltage to start the engine. My J185's are batteries with CCA rating.