It's very easy to "slag off" these battery packs. True, they contain a small (around 12AH battery) and you do need to keep them charged if they are to be of any use! They are sufficient, however, to start the car if the battery is down, or failing, (the usual scenario) but you nay not be so lucky if the battery is as flat the witch's left one!
Maplin's had an offer on not so long ago for a booster battery charger system IIRC about £10. This allowed a booster pack to be charged through its "cigar lighter" socket, powered from the host vehicle's cigar lighter. It (I would suggest) uses blocking diodes to prevent one battery discharging the other, and for people like me who run automatics (can't push them down the road and drop it in second!) a battery box and charger are a necessity.