On my progress to my camper van conversion I needed to add a 2nd battery for accessories so that my primary battery would avoid a drain. After many measurements and checking I confirmed a 34/78 battery would likely fit and then found a battery box that would fit under the front seat and avoid any interference. I decided on the optima yellow top because it sealed ($220), Taylor box which was the only one I could find that was small enough $100, and Battery Doctor $70 that I used before that provided simple "smart" isolation as well as the ability to combine or isolate the secondary battery. Wasn't the cheapest set up but it's reliable. Because the battery is so close to the primary you can use aftermarket battery cables from any auto store to make connections and simply just have to add a fuse to the + cable from primary to secondary.
Triple checking the mount location, I decided to use simple straps which will secure the battery at the end. Makes install easy, and removal of battery easy if I need to remove it.
You will have to remove the seat (6 bolts) and disconnect the airbag and seat sensors so you can remove the entire seat off. This is the picture of the two electronic connections you have to disconnect to completely remove the seat.
Make sure you fuse the + line from primary to secondary. Safety is key. I purchase a 150 amp fuse on ebay and some ends I spliced to connect to the fuse box.
Here you can see all the connections. The Battery Doctor is super easy wiring. + line from primary to Battery Doctor, + fused line from Battery Doctor to secondary, - ground line from secondary to chassis, and a - ground wire from battery doctor. Super simple diagram available on their website. I used 4 gauge battery cables from pep boys-colored tape was used to identify positive and ground, ran out of red tape so I used white, just wrapped for identification.
The secondary battery vent hose can be plumbed to the stock location with the other battery vent.
Picture of the battery doctor, I mounted this just front of the battery under the seat so I can easily control it and push a button to jump or see the monitoring.
You can see battery doctor and wiring here to battery
Picture before seats and straps go on
I used simple straps that I ran through the "seat frame" similar to how you would mount a boat battery. No way this is going anywhere and it allows easy removal of battery. Nothing interferes with the position of battery and it isn't gong anywhere. You cannot remove battery w/o removing top of seat, so its super secure. You can drill and mount into floor or frame...gas tank is directly below the seat so really this is the best option. The box is vented and the use of a sealed battery makes this a safest set up w/o putting battery on outside of van which there is no place to do anyways.