To save climbing in and out of the car through the boot, the boot light wire(live) can be used to charge battery as is permanent, as done this myself, as too much of a phat phuck to go climbing around cars, and doing a limbo dance under the rear strut!!
I would now use the tow bar lecks to recharge if needed, the ones I tow caravans with at 30mph everywhere ha ha!!
tabetha