Just had a look at the mymemory power pack bungle talked about, and on the basis it’s roughly half the price of the Anker one I bought last week, and you can never have enough back-up power, I’ve just ordered one!
Thing is, if I go away camping for nine days, like I did last year, while I can use the car battery via a USB adaptor in the ciggy socket, or 12v aux, as it’s now known, a couple of those big battery packs are better than using the car when the engine’s not running; had to call the AA out last year after the battery died! It was thirteen years old, mind… 😳