Group 34 vs Group 31 dual battery opinions

[texascrane]

I'm getting close to finally installing a dual-battery system in my 100-series and I could use some input. I currently have a group 27 optima yellow-top installed. Because I have (and want to keep) my ARB twin air compressor, which takes up the space where Slee's dual battery tray would go, installing a second battery in my vehicle is a bit more complicated than normal.

I'm considering 2 options. The first is to have a bracket fabricated that will hold both a second battery as well as my air compressor. I think that a group 31 battery will fit horizontally in my application along with the compressor if I relocate my power steering fluid reservoir to in front of the air box.

The second option is to get Torfab's dual battery bracket, which puts 2 group 34's in the stock battery location. This approach does require cutting some of the fuse box bracket out of the way, but nothing major.

The advantage I see of the first approach is having a nice big group 31 battery to run my accessories. The primary accessory that I'll have on the second battery is my ARB 50qt fridge. I may hook up some additional LED lights as well. I'm not going to be running the Odyssey battery so I won't have some of the charging issues that battery has, but I am worried about keeping a battery that large charged enough to prevent sulfation.

The benefits of the second approach as I see it, is that it will be cheaper due to not having to custom fab anything and it will require far less battery cable to hook everything up. I'm not sure if there's any benefit to having both batteries be 34 vs having a group 27F and a group 31. I suspect my charging system will have far less trouble keeping up with two group 34's. However, I'm not sure how long a group 34 can reasonably be expected to run an ARB fridge.

Regardless of the approach, I'll be installing Interstate MT7 AGM batteries. Their group 31 is a 96 AH battery and the group 34 is a 62 AH battery. I might eventually add a solar panel to supplement this setup and provide additional power for other accessories/gadgets when camped for longer periods of time.

I'm hoping to learn from the collective wisdom here. Is one approach preferable to another? I'm leaning towards Torfab's solution right now because I think it may be easier to get set up. Thanks for your input.

Here's some pics of my engine bay as-is:

Here's an example from another member here of a battery oriented like I'm picturing. My application would add the compressor mounted to the side of the battery on a bracket facing the firewall. I obviously don't have the AHC reservoir in the way:

Here's a picture of Torfab's dual battery layout:

It does require removing some of the bracket so that the batteries actually fit:

 

[Ducky's Dad]

The second option is to get Torfab's dual battery bracket, which puts 2 group 34's in the stock battery location. This approach does require cutting some of the fuse box bracket out of the way, but nothing major.

That's probably what I would do, especially if you will be adding solar. In TX summer conditions, a single 34 won't run the fridge as long as you would think, so plan to isolate the batteries and add the solar. Size of fridge makes a difference, but as a point of reference, my Indel-B 50 discharged a fresh and fully charged pair of Optima 34 Blue DPs in about 38 hours in So Cal summer temps around 100-105F. This was at the end of a trip, and everything in the fridge had been mostly frozen for a week. I parked the truck when I got home and left the fridge running just to see how long it would run, set on max cold, no solar.

 

[AndrewP]

The old saying in drag racing is that there is no replacement for displacement. That kind of applies here. You are going to want the bigger battery. One, because it will provide more power for longer. And, two, because each "cycle" won't be as deep and thus the battery will last longer. The dual group 34 would be great for a back up starting battery but not one where you also want to run real "house" loads. With that approach you'll only have 30 ah of energy to use. With the bigger battery, you'll have 50. A fridge can easily use 30ah on a hot day. Once you add on some battery charging, some radio use and some night time camp lighting, you can easily get to 50ah.

How are you going to isolate the batteries from each other?

My view-you need to do what it takes to get the 100 AH battery in there. If that means moving your compressor, then move it. In the end, it will be the better solution.

 

[texascrane]

These are AGMs so really you're looking at 80% discharge, instead of 50%, but I see your point.

How are you going to isolate the batteries from each other?

I have a Blue Sea ML-ACR.

 

[Ducky's Dad]

If you are willing to relocate the compressor, consider three 34s instead of a 31 and a 34. If the 31 will fit where the compressor is, then consider a 31 and a pair of 34s. That is the setup I have, and it works for my purposes. An advantage of three identical 34s is that you can rotate the batteries every year or so, so that the starting load is sort of equalized and then you are more likely to get similar life from all three batteries. My experience with dual battery systems and identical batteries has been that the starting battery fails before the secondary battery.

 

[thethePete]

What about 3 34s where the compressor is now, with your relocation of the PS resivoir you have planned already, and put the compressor where the stock battery was? Or could you fit your compressor set up by the stock battery?

Has anyone considered a slightly less conventional set up, and using a small racecar sized battery for starting duties (they make them in quite small form factors that will start a large displacement engine, and using that strictly for starting/stock battery duties, and then jamming as much house battery as they can fit under the hood? I think you could get a starting battery that would fit quite out of the way and all or some of the stock tray available. Also have to figure at some point you're approaching the weight of another offroad bumper on the front of your truck. Batteries aren't light. What about your house power in a battery box up the bed-side, on the frame? Or behind the transfer case, between it and the gas tank? If you use a proper enclosure I don't see anything wrong with sticking it under the vehicle somewhere.

 

[Ducky's Dad]

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