Someone (whose name I shall not sully) left my car ignition on for a couple of days recently, so when I came to use it yesterday morning; the battery was dead.
Luckily Wills had the above, and it was fully charged.
It's a small 'jump starter', which is charged via a USB socket. Turn on, attach the leads to your dead battery, and Bingo!
What a great idea. You can even keep the thing charged-up via your cigar lighter in the car (no-one smokes cigars in the car these days).
There's only one problem; it costs about €250. Ouch!