The panel claims to ouptut 2W and was a tenner when Maplin were closing, so not the biggest expense. It is about A4 size when open, but the panels are probably half that size in the frame. 2W at 14 volts (roughly what a car charger can run at) means about 150mA max output.
With a bit of playing with panel position and a clamp meter, I found in bright direct sunlight the panel could actually push 150mA or so into the battery, largely unchanged by being behind glass, so the claims on the packet are correct. However without being in direct bright light, this rapidly drops to 50mA which is about as low as my meter is accurate for but is about the normal drain for the car.
So, after a few days, the battery was again down to 11.7V so no real benefit from the panel. I charged the car again, and locked it with the key (hoping to reduce drain) and again after 4 days it was 11.7V. However I couldn't detect any drain on the battery and after another few days without the panel it was still 11.7V so what it looks like is the the BMW detects when the voltage is dropping and shuts down other features of the car to protect the battery. This sort of thing is not uncommon, Mclarens drop the window an inch when the battery gets to this level - not only a visual clue the battery is going down but it also means you can open the door to get to the bonnet release to get to the battery.
IF the panel was in direct bright light then in summer I expect it might be able to keep up with the drain, but obviously things like cloud and darkness limit the output so really it is more ornamental. If I had a spare battery I might try it and see if t charged it over time, as I suspect for powering a few LED lights off a leisure battery then it might be OK, but really for keeping a modern car topped up you need more output.
I would try it on the old Toyota which has no remote or electronics, but as we use that once or twice a week it isn't feasible.
Paul
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