If you have a battery in good condition and intend to leave your car unused for a spell (at the long term carpark at the airport this summer?) and want to keep your battery topped up then Maplin are having a clearance sale of solar powered battery conditioners for £9.99. I used one for the last 3 months of 2010 while I was away and even with 3" of frozen snow on the car I was told the little blue LED was flashing away to indicate that charge was still going into the battery during the day.
http://www.maplin.co.uk/solar-powered-12v-...4qodnbyjdg 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

I'm not personly a fan of these. But if you use one fit a small inline fuse.

My mate has just had to put a claim in for his split window camper as the charger itself shorted, melted all the wire insulation and in turn caught fire to his camper. I trust his judgment as he is an auto electrition by trade making allsorts for campers and beetles.

so IMO i would stay well clear of these. 08MY HSE 3.6 TDV8 Stornoway Grey, Twisted performance cat back... and shed loads of extras!!!

I have been using the solar chargers for about four or five years now. I couldn't comment about the cheapo variants because I've never used one but my system has a proper regulator and also has the one way diode to prevent dis-charging etc.
My dual battery management system also has a dedicated connection for solar panel trickle charging... a feature that's probably not found on ten quid units.
These cigarette plug-in trickle chargers need the socket to be perminently live to work which is why a fuse is a good idea. Ra-Ro Sport Super Dooper Charged.
Ford F350 Super Duty Prototype.
Ni$$@n Patrol 4.8 Overlander.