Adding a new battery...questions....(venting/gassing)

[SJ-Chris]

I've got two 2015 Thor Majestic 28a Class C RVs. One of them now has two AGM Group 29 batteries (I believe they are Group 29s) for the coach. The other RV has two lead acid Group 29s. All the batteries are ~1 year old. I'd like to add a 3rd battery to each RV (an AGM to the RV with the AGMs, and a lead acid to the RV with the lead acid batteries).

The easiest place for me to put this 3rd battery (perhaps the only convenient place) is in the passenger side front compartment. This compartment is a decent size (~1 foot tall, 4 feet long, and 2 feet deep). I currently store the welcome mat, a 9'x16' mat, some campfire supplies (lighter, etc) and a pop-up garbage can for the campsite.

Note: The other two batteries are on the driver side.

I checked the compartment regarding adding the extra weight of a battery. I'm confident it could hold it without any problem, but I like to go overboard on safety so I will be adding some extra metal strapping underneath.

My question is regarding venting the compartment and battery gassing. I'd like to put this battery in a battery box like this, and then secure it in the back of this compartment:
https://www.amazon.com/attwood-9084-1-Large-Battery-Box/dp/B0000AY9DQ/ref=sr_1_2?crid=3ETMNZURVN47I&dchild=1&keywords=battery+box+group+31&qid=1614843096&sprefix=battery+box%2Caps%2C281&sr=8-2
That way, there would never be any risk of anyone shorting any metal across the top of the battery (yikes!). My plan is to mount it near the rear of the compartment so it will mostly be out of the way. I will drill some holes in the side of this battery box for the battery cables and then the battery cables will route out the back of the compartment and over to the other two batteries on the driver side.

What sort of venting do you think I need in this compartment (not the battery box, but the actual side compartment where the battery (in its battery box) will live))? Will 3-4 half inch holes near the top and bottom of both ends of the compartment do the trick? Do lead acid and AGM batteries gas a lot? If so, will they cause any damage to any items that are stored in this compartment along with the battery (like the items I mentioned above)? Should I also drill some vent holes in the battery box? Should I worry that drilling some holes in this compartment make it no longer waterproof?

Not sure if I'm overthinking this or not. Figured I could learn from those much wiser here 🙂

Let me know what you think.

-Chris

[CA_Traveler]

The battery box should be sealed with 2 vent hoses one high and one low.

Seperating parallel batteries (if that is your plan) by 8' means unbalanced batteries ie load/charge usage distribution will be unequal. You might want to research this.

Have you considered switching to 2x GC batteries instead?

[jdc1]

pianotuna wrote:
Unless the batteries are almost unused, adding a 3rd battery may not be a great idea. If you can afford it, replace all three.

Or, put the 4 older batteries on one bank, and the 2 new batteries on a separate bank.

[jdc1]

A Prius has a 1/4" hose for their battery vent. That battery is in the passenger compartment. This may tell you something. I believe Toyota might have better engineers than trailer manufactures, so I would think a 1/2' or 3/4" hose attached to your battery box and plumbed to the exterior would be sufficient.

[pianotuna]

Unless the batteries are almost unused, adding a 3rd battery may not be a great idea. If you can afford it, replace all three.

[drgslayr]

We had a Majestic 24a. When I added an extra battery we didnt have the storage space to loose so I added one of these.

We went with AGM so we didn't have to worry about adding water or anything else.

https://www.torklift.com/rv/hiddenpower.

[2112]

Use a vented box and hose. THIS May provide some ideas. See the 2 white boxes with the black vent hose.

HERE is a wall mount vent. See the Frequently Bought Together section