First live test of the car's dual battery switch! It works! The backup battery is getting charged when the engine is running as long as the primary is full.
Unanswered questions at this point are how best to switch between solar and alternator charging, and whether the battery box will ever leave the car.
For the first part, right now it's just going to be yanking the plug out of the battery feed and swapping in the panel. Simple and probably effective enough but I'd kinda like a big solar on/off switch somewhere. Maybe.
Everything is simpler if the battery stays in the car but maybe it's too inflexible. I'm about 50/50 on this issue.
Minor note about my wiring here - the most cost effective way I found to get all the cabling was to buy some discounted, shitty jumper cables and cut them up. I have hidden many sins under heat shrink wrap but hey, it works!
Final test for today, charging the laptop via an inverter off the battery, as 5v USB just doesn't cut it for recharging a Pinebook - which is the only general purpose computing device I'm planning to take with me.
The battery percentage is going up while I'm using it, so that's a pretty positive sign.
@mike That's a gorgeous laptop btw.