Update on my battery + cap combo:
I haven't had any more mornings where the car didn't start, so perhaps I left a light on or didn't close a door completely. Had an experience I'd like to share though.
I had been using a cheap 100A relay from eBay between my DC-DC converter and the caps, to separate the two when parked and cut down on parasitic drain. The build quality was shoddy but it just needed to last long enough for my engine swap. Very occasionally the relay failed to engage and needed to be jarred before it would *click* and the car would start charging.
I left one morning for work when it was around -5F (about a week and a half ago). Around half a mile from my house my rear-view screens began to cut out and I got an EPS light, indicating to me the 12v system wasn't charging. I pulled over in a residential area, tilted the driver seat forward (hoping to get the charging system going again before the engine quit firing) and DISASTER.
The 8 gauge wire going between the capacitors and DC-DC converter was the only line I hadn't placed a fuse or breaker on, in part because it was so short - maybe 24 inches and with the relay in the middle. When I moved the relay to try and get it to engage, the wire leading from the caps came loose and touched... something, I'm not sure what, which was grounded. Instantly the 8 gauge braided copper lit up like a tungsten filament in a light bulb and vaporized. The insulation from the wire burst into flame. I beat out the flames with my bare hands to keep my car from burning to the ground. Here are the largest bits I was able to find of the 8 gauge braided copper:
Moral of the story, shorting out very large capacitors is not remotely like shorting out a large battery. Treat them with respect.
I managed to limp the car back home just using the lithium battery, and called a coworker to pick me up. That evening I added a 100 amp breaker a few inches from the capacitors, and manually turn it off whenever I park the car.
~
To protect my LiFePO4 battery from further damage from the cold I ordered a pair of 5w heating pads. I attached these in parallel to a thermostat, then wrapped the battery in closed-cell foam and duct tape. I planned to have the battery heat itself, but for now I'm just running it from the DC-DC converter when the car is on, and relying on the insulation to keep the battery warmer longer. Fuses on everything, of course.
