MUST be the panel at fault. Take it back, change it, see what happens. I've used them before with no problem, they have been very useful for cars used less frequently.

The panel could be defective, but I'm not sure it it will charge correctly without the little circuit in the lighter adapter.

It sounds like your battery is bad. Unhook the solar charger and see if the battery still discharges.

try to check the panels

I recently bought a 5W solar panel to keep my car charged when I don't drive it. Since my cigarette lighters turn off with my key, I just attached the solar panel to my car battery under the hood. I attached the panel + to the battery + and panel - to the battery -. However, my battery is not charging. It even seems like my panel is discharging it. Over a course of few days, my battery dropped from 12.5V to 9.5V.

Am I doing something wrong?

Thank you!

Extra info:

I've checked the solar panel voltage. It's consistently > 20V unless it's connected to the battery. Then it hits the battery voltage when connected.

I've measured the current going into the positive terminal of the battery and it's >100ma. I've also measured the current leaving the negative terminal of the battery. Naturally it's the same value.

There is a blocking diode within the solar panel. (current =0 when panel V < battery V)

Car battery never had such a drastic voltage drop (>3V in a couple of days) without the panel, so it's definitely panel related.