Dear Ralph
Northern Tools sell a 4W folding Solar panel for charging a 12 volt battery for £23.49 and is suitable for continuous connection provided the battery is larger than 35 Ah.
They also do a bigger more permnant one which can be used for vehicles like motorcycles for £ 70.49
The swedish company CTEK make some small and very good chargers but are likely to be about £70 each
I hope this has been of assistance
Chris Thomas