Forget the pissy $30 cheap ones. Get a 12v 5w panel, about 30-40cm square from someone like BCF or Supercheap autos. I use four, one on each car if it is standing while I am away, and 2 on my boat (1 for house batteries and 1 for the start battery). The won't keep up with normal usage with hjeavy drain (such as a 4WD-mounted fridge) but provide a trickle charge just to stop the batteries from dying and/or sulphating up. Used them for years and can be away for months and come back to a healthy start every time. The one I have cost between A$85 and 100 each and have lasted for over 5 years in the case of the oldest one.
Try ICP Solar Technologies Inc. [customers@icpsolar.com]
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18