Cool idea for battery and spare tire rack that goes underneath truck!

[graynomad]

And what if those batteries were submerged while being hooked up to 500 to 750 watts of solar panels during the day?

It's voltage that's a concern not the "watts" available and even if nearly charged the voltage is still only about 14v (assuming a 12v system) as opposed to say 12v if discharged. So bugger-all difference.

That said I don't like the idea of submerging batteries, it just seems wrong, but I can't think of a reason it would do them harm in small doses.

One thing that would be a problem in Oz (or anywhere with a lot of red dirt) is the dirt buildup on the terminals, red dirt has a high iron content which will cause a parasitic load on the batteries and when under the vehicle like this they will get pretty dirty unless you frequently immerse them in water

It's true that most (all?) commercial RV builder ignore the under-body area and it's a great place for storage, expecially of heavy stuff like batteries and water, really helps keep the CoG low.

 

[magentawave]

Since the batteries are totally exposed to road grime, do you need to clean the terminals often?

 

[graynomad]

I would assume so but although I've hung batteries low before it's always been on a truck and well off the ground, at least 2 feet, so there would be less crap at that height. Your rig appears to be very low so the batteries will be much more prone to collecting gunge, I guess you just have to clean (or at least check) them more often if you go this route.

As for venting, if you use AGMs or even Lithium that's not really an issue. AGMs hardly vent at all (in fact I'm not even sure they do vent under normal circumstances), either way as long as there's a path for the gas to the outside world you don't have to worry about a good seal to the inside.

 

[SoCalMonty]

Since the batteries are totally exposed to road grime, do you need to clean the terminals often?

I'm sure it wouldn't be too hard to put either a rubber cap or coating of some sort. Smear them with dielectric grease and put a rubber boot over them, they'll probably stay clean for quite a while.