All you really need to prevent back-flow is a diode ... you don't really need a "controller".
Thank you. I have since purchased a solar panel that included a diode and a controller. It was ENORMOUS and provided very little recharge to the battery for its size, so I returned it.
... it could be soldered in-line with the panel's current wiring (snip one of the wires, insert diode (properly pointed), solder the leads and wrap it in tape).
Thank you. Gosh, seems so easy, but it’s unfortunately lost on me. I need to find a course to learn more.
To me that indicates that it is ok unless it fails. They don't say to unplug it during cloudy days or while shaded do they? If they don't mention that, a properly operating unit will not discharge your battery.
Very good points. Thank you.
This all began for me because I have a sudden battery drain of about 1v per day. Dealer says it’s the up-fitter’s work and I live quite far from the up-fitter.
After I posted my questions, I thought I would try my already existing 80w panel, prop it on the dash with a cigarette lighter plug into the hot socket under the cup holder. I can also now vouch that this works quite well actually. It’s a large panel and there’s no controller (I don’t know if it has a diode), so I top up the battery each morning while I have sun and unplug at noon before the battery might get over-charged and before the panel in darkness might drain. It’s high maintenance, but this short-term fix seems to work for now until I can get to the up-fitter, and I thank you guys for your wisdom.