Yeah, the short circuit current of this one is listed as 610ma... I've just held it right under some marine T5 lighting and managed to get it to top out at 90ma.

Cheap rubbish perhaps.

I'm pretty sure (but I don't have sources) that PV cells do better under proper daylight because of the broad spectrum. I may be wrong.. it has been known.
Anyway, 90ma should at least reduce the net rate of discharge by a decent amount. During the day at least.

I'd be happy with 90ma, but on a day like today I was getting 15 tops, which isn't worth the hassle.

And that 90 was under really intense lighting - though I know bugger all beyond that.

I'm thinking of just sitting a spare battery in the boot and plugging it into my constant 12v socket. Put a freshly charged battery in there and that should extend the length of time I can leave it. If the car is parked up for quite a while, I'll just swap the battery with another charged one. I'll put a 10amp fuse on it just in case I forget to unplug it when cranking, to stop the starting power being pulled through its relatively puny wiring, or equally a lot of charge current being dumped through it.

It sounds like you're getting quite close to dual batteries and a proper split charging system ;)

I've thought about it... but I don't want a battery permanently in the load space, and I don't think I can quite squeeze one in the space next to the LPG tank :(

I looked at PV myself for a while until I realised I don't live in S. Europe.... but went with a secondary battery in the end, too - which I just remove and (fully) charge up occasionally; It's 'The Sun in A Box' (so to speak..) and is not so affected by clouds, winter etc... and is simply mounted in the passenger footwell (No, not very convenient but better than finding the volts have dropped too far and my rig is 'upset'..)