Charge whilst driving or park on a hill (manual transmission only)!
Better to get a spare lead/acid battery (sealed or gel electrolyte types are best for portable use) with a FUSED socket fitted. Make sure the terminals are well covered!!
Golf trolley batteries are OK and you can find plastic moulded enclosures for them to fit your socket onto. They are rated at about 25Ah which is 300Wh. You need about 70Wh to re-charge so you could expect 3 charges from it.
If you also get a universal charger (computer type) as used by most RC fliers you can charge just about anything else from it, as they also use a 12Vdc power input. You have to set all the parameters yourselft though, charge current, cut-offvoltage and so on. Just fit the same type of plug to it.
Hope that helps.