The car battery loses 16Ah every 2 months it isn't driven.  Letting it drop & recharging it every 2 months isn't a solution.  Borrowing $4 million for a house with a real garage isn't affordable, so something had to be done to top it off without manes voltage.

A $15 solar panel 

https://www.amazon.com/gp/product/B089793L93

didn't work so well, either. Wiring it as 5S2P, it gave 15V 75mA in direct sunlight. It gave 3.5V 3.5mA in the shade. If it was wired in 10S, it would give 7V 1.5mA in the shade. It needs to get above 15V to charge.

The car burns 270mAh every day, requiring 3.6 hours in direct sunlight to recharge. Being parked in direct sunlight would destroy the paint. Open circuit voltage goes to 30V, so it would need protection diodes to avoid blowing up the car if the battery was disconnected.

A more expensive solar panel, filling the entire windshield would probably do it.  It's a question of how much of a solar panel lions can afford before it becomes cheaper to let the battery degrade, how much of a solar panel can fit in the car.

Having a removable battery top off the car wouldn't work, either.  It would have to withstand high temperatures & be 16V 10Ah, very expensive.  It would be completely discharged every month, subject to the same degradation, so a car battery rated for 8 years would get charged by another expensive battery which degraded over 8 years.