Hi Woodlands,
I haven't tried the solar charger you have identified, but I do have a few concerns about its specification. 1200mAH or 12Ah! I know battery technology is improving and we can now get substantially higher power densities than we used to, but considering the quoted dimensions I do have some nagging doubts about the product.
Secondly the cost of the the item at only £12.99 makes you wonder what quality the components will be. Especially the solar panel, how efficient is it? Just think 10 of them and you could replace the caravan battery - Ok I know the voltage would be wrong, but just think how much smaller and lighter it would be!
Thirdly, The available area for the solar panel is qute small compared to the claimed capacity of the battery. If the battery is fully depleted, I think to recharge it solely by solar power will take a few days.
Do check the dimensions, as that might determine how you use it, will it fit into a pocket? or do you need to have a separate bag? And of course it won't recharge when it can't see daylight.
If you're thinking of carrying outside your pockets, consider how annoying it will when it constantly flaps against you if its dangling on its carabiner, and how many times will you need to turn it over to let it see daylight.
I suppose it's not massively expensive, and it might manage what you want, and I expect you can recharge in the car, which I am certain will do a more efficient job than the solar panel.