Of course, it immediately occurred to me that simply
putting a battery there isn't really useful if you don't actually
connect it to something, so I quickly planned out the wiring for this. I decided to use one of those cheapie battery switches that flips to charge the secondary at 13.3V and otherwise keeps the primary charged; once again, I know Slee has some really cool kits for several hundred dollars that can mesh the secondary battery into the system for things like starting if the primary is dead and so on, but considering how much money I've sunk into the car by now with no sign of it ending, I figured I'll stay on the cheap and just carry some boost cables around if I need them (to just connect the secondary to primary). So I went and ordered some 0 gauge cabling and some terminals. Although many threads I've seen seem to somehow omit discussions of fuses, I worry about hundreds of amps of electricity running around unfused, so I also got what probably is a little bit too many fuses, but hell. Better safe than fire, right.
Thick, thick wiring. However, I'm well aware that this is the "cheaper" kind of 0ga - copper plated aluminium or whatever they call it, rather than pure copper, but in fairness, I needed to route it through some convoluted spots, so pure copper wouldn't have worked, and it would be overkill anyway. Or so I figure.
This is the battery isolator I'm using:
I ran the ground from the new battery to the frame down around the suspension bolts, and the red positive in front of the radiator cavity to the battery isolator.
I also had previously connected everything to the main battery (the amp, etc) but now I undid the fuse->battery cable and replaced it with a longer run over to the secondary battery, so I can run my fridge / lights / whatever without worrying about the main battery dying (blue wire).
And it all worked! Amazingly, without setting anything on fire once again.
