I've just done some online research on this issue and apparently you can use a solar-powered battery charger by plugging it into the "cigarette lighter" socket, if that socket stays "live" when the ignition is off. (We tested our's with the in-car mobile phone charger that we'd bought a while ago; the phone continued to charge.) Otherwise you'll need to use leads that clip onto the battery directly. Clearly the first option is easier - and also more secure - especially if your car isn't in a garage or somewhere secure.
warped cutting mat - any solutions?