Rob on this forum and the Facebook user group did a second battery modification that I thought was genius. Second battery is mounted to the rack. He purchased the plug to connect direct to the controller bypassing the main batter. The second battery does need a key switch to turn it off and on. Switch the main batter off, switch the secondary battery on. No rewiring, no chance of shorting anything out. I think the mod was explained on the FB users group. Maybe he'll see this post and share the plug and procedure he used. Or try a search on the FB group.
Here is the copy and paste of his post. Once I understood what he was doing, it made a lot of sense:
(Link Removed - No Longer Exists)
Robert Kobylarz Chuck Fedora
Y cable with dean plugs.
Dean plug is used inside XP frame to connect battery to controller.
Y cable allows to connect 2 batteries at the same time.
You just have to make sure you dont run both at the same time so 2nd battery needs key switch too like original.