So these are a thing - portable battery packs containing 12amps or so at 12V, designed to jump start a car with a dead battery and also charge phones etc.
See AE review of some here:
http://www.autoexpress.co.uk/accesso...rter-pack-2015
But would anyone actually recommend them? They seem to be largely rebranded imports, the insides of which from some you tube videos look quite scary. I'm not sure I trust a lithium-polymer pack to be anywhere near the alternator output of a running engine without some very reliable cut offs.