Another fan of this project!
Couldn’t you possibly add a small solar panel to trickle-charge your car battery? Or even have a separate small 12v battery from a Uninterruptible Power Supply. I bought a 12v 7Amp battery that was meant for security surveillance systems and a small 12v solar panel the size of an average tablet, and both weren’t very expensive. You could even have a relay circuit that’ll charge both batteries when the car’s alternator is running, but keep the car batt separate from the second batt. I’ve seen this used in cars with high powered sound systems that run off of secondary car batteries isolated from the main car battery (with some people even adding a second alternator to their engine belt drive).