Originally Posted by CampCaudill
Ok - this is great...getting it all figured out. We have a 34J. I think I can add a total of four, but might have to stack them. Right now we have the US Standard 6 volts.
You should have room for 4 in that chassis. 6V vs. 12V is a personal choice... I would just add 2 more 6V (unless your current batteries are more than a year old, then swap them and get 4 similar batteries so they all charge and discharge at like rates.) 6V batteries tend to have more AmpHours in capacity than 12V but since the system is 12V, you will have two pairs that create two 12V units. Pretty typical.
If you need to stack batteries, then definitely get AGM batteries as they can be placed on their sides without any issues.