Suggestions on battery/solar upgrade due to space issues

[restlessways]

I am helping a friend upgrade her Class C motorhome battery bank in order to install solar. The RV is horrific in terms of usable space. Here is what I am dealing with:

The current house batteries are located under the curbside steps into the coach. They are Group 24 12 volt and there is zero room for larger batteries like a pair of 6 volts, which I would have preferred. In fact, the Group 24s are tight. The only way to mitigate that is to start cutting and welding, and I do not have the time, place nor inclination right now. So, I will leave the two group 24s there.

There is an external storage cubby next to the entry door that is roughly 24" inside width, by 14" tall, by 18" deep. It is not rated to hold any weight, but I figure I can fabricate a tray to support 3 batteries underneath the plastic box. There are anchor points on the frame and some bolts coming through the coach floor that are also used to hold the current battery tray.

Here's where I'm trying to think a little outside the box. Since I can't do 4 6 volt batteries in series/parallel for a total of 460 amp hours, I could get 3 Trojan 12 volt deep cycle batteries at 130 amps each for a total of 540 when you include the 150 for the pair of group 24 batteries. This would provide 270 usable amp hours. This isn't the way I'd prefer, but it's the only way I see with the storage that's available. 3 6 volt batteries wouldn't help me in the cubby, and a pair of 6 volts wired in series then paralleled with the current 12 volts would only provide 380 amp hours, which isn't enough.

There is literally nowhere else for the batteries, save for a rear compartment that is 20 feet away from the current location. I'd have to move all of the electrical and I don't want to do that. The way this coach is configured is ridiculous. They have the batteries and associated stuff up front, then the converter, inverter, transfer switch, panel, etc. all the way at the rear under the bed.

I do know that using different brand/amp hour batteries in parallel isn't ideal, but I don't have a lot of options. All the batteries are brand new, and I could also keep the two banks separate from one another with a switch, though would prefer not to. Anybody run into these space issues before?

[DarthMuffin]

Not sure if it's within your comfort level, but my solution to the space problem was to build my own lithium pack from lifepo4 prismatic cells. I fit 200Ah into about the space of a group-24. Bet you could get 300Ah plus the BMS into that space.

[MrWizard]

You did say "INVERTER"
So I assume that is the major reason behind all of this, I've done mixed batteries, is not ideal, but it is better than have too little battery capacity, I would think about a pair of trolling motor batteries in GRP 24 size, for under the step, they are true deep cycle
Are the Trojans flooded or AGM, If wet cells, will she be able to check the electrolyte levels ?

[valhalla360]

Just a couple thoughts:
- Make sure they are legitimate 12V deep cycle batteries. Most of the ones you find at big box stores are not really deep cycle. The real ones are expensive.
- For house loads, putting a secondary battery bank 20ft away isn't a big issue. Look up a voltage drop table with an estimate of peak amp draws but house loads are generally relatively small, so you don't need monster cables.

[theoldwizard1]

Nothing much to add.

T1275 are very expensive.

When stored, I would disconnect the two sets of batteries and charge them separately.