Vigo (Forum Supporter) said:
Was just reminded of this thread by it being linked in another, and realized i never directly replied to Engiekev like a jerk, so...
It's my understanding that basically any battery can tolerate very low current 'overcharging' even when it is already full. IF i'm right about that i dont think one of these panels can damage an AGM. However, i'm not using a solar charger on any AGMs. I have a couple of AGMs but i dont use the solar panels on them because they don't really need it (lower self-discharge and i just don't short-trip those cars as much) and those cars are usually in shade/cover anyway so solar wouldn't work without some silly extension cords.
I think value for dollar if you want a slightly larger panel and a solar charge controller for some reason, this is the current hotness. I bought one a couple days ago for $20 but haven't gotten to mess with it yet. https://www.amazon.com/dp/B08HMMY1W5/ref=twister_B08HMNWPJB?_encoding=UTF8&psc=1
I also don't claim to fully understand this but it suggests that a typical car battery sized AGM could accept ~120ma 100% of the time no matter what without damaging it. https://www.cdtechno.com/pdf/ref/41_2128_0212.pdf
Good to know! That makes sense since its not much current, I wonder if anything in the electrical system of modern cars would encounter issues with higher than nominal voltage. I would guess not, since they likely engineer some safety factor in for higher voltages.
I have nothing else to add, other than I love the Jengrish (Chengrish?) ads for that solar setup. The "Giant of Sun" will "KEEP YOUR IDLE VEHICLE TO BE AT YOUR SERVICE" indeed.
Powar (Forum Supporter) said:
I have Rural King batteries in the FD, 900SE, and Mini. They've been fine so far and the price was right.
Aaaand the one in the FD failed already and was replaced under warranty. It had lived inside and on a trickle charger the entire time. Driven MAYBE 500 miles. Sad.
I wonder if anything in the electrical system of modern cars would encounter issues with higher than nominal voltage.
Well at very low currents the battery essentially limits the voltage the entire system sees. So even though the open circuit voltage of a tiny solar panel is 18+v, once you hook it to the battery that voltage is instantly pulled down to whatever the battery is at plus a tiny amount, basically. With a small charge current the voltage will never get too high, although as you increase the current at some point you start 'cooking' the battery. But it takes a pretty serious amount of current (like, one of the big wheeled-cart battery chargers left on the '200a jump start' setting for a long time) to make the entire system go up past like 16v. Which is why they say you shouldn't do that. 
Also, my past experience with alternator over-charging situations is that incandescent headlight bulbs start to pop at around 18v leaving you suddenly driving in the dark with no headlights (happened to me twice), but i've never actually broken an expensive module or computer with over-voltage. Times when i've majorly over-voltaged other small electronic devices and then taken it apart to see what the smoke came out of, it has usually been electrolytic capacitors and diodes that blew up. I am clearly not a fully trained electronics technician, just a mechanic who collects cheap crap and isnt afraid to abuse the hell out of it for 'science'.
