I’ve posted in other threads that I kept a 10W solar panel on the dashboard to keep the battery topped up.
No more, at least for the time being.
I first bought a 5W panel, thinking that would be adequate to make up for parasitic drain on my infrequently driven Tucson.
However, measurement revealed it was only delivering ~2W on my dashboard in the sun.
I chalked it up to windshield loss and bought a second identical panel, added in parallel.
I would periodically check battery voltage (hood up, accessories off) near the end of a sunny day and find the battery at ~13.5V.
I added a couple of 5W zener diodes to limit the panel output to 14.3V, to prevent over charging.
All was well for 2 years, then a sunny day check was under 13V.
I measured the panels output in direct sunlight and got ~1.5W (10W spec).
Did the heat in the closed car kill them?
I then purchased a 10W panel, which put out a measured 6W in direct sunlight.
Disappointing, but enough to get the job done.
Two months later, I started to notice the voltage display I keep in the 12V port on the dash started reading lower (12.4V vs ~12.8V) when I turned the key to ACC before starting the engine, even on sunny days.
I did the sunny day check and the battery was at 12.55V.
I measured the panel output, sitting on my dash in the sun: less than one watt!
I bypassed the little charge control circuit this panel came with and measured cell output in direct sun: 3 watts.
So that panel was $30 wasted.
I park on a city street, so I’m back to removing the battery periodically for an overnight charge in my basement.
Going forward I’m going to cobble together some 10AH LiFePO cells on hand, and an LDO voltage regulator to apply 14.2V through the under-dash connector I put in for the solar panel.
Overnight every couple months will keep the battery happy.