Over a year ago I bought a battery pack from Aldi for just that purpose and it seems to work ok.
It came with a slew of connectors for various devises and has 2 standard USB OUT ports.
According to the label it contains a 7400 mAh battery with an output of 5V at 2A.
It itself has a mini USB input allowing it to be charged either by using the supplied mains charger or ciggy lighter connector provided it is only a 5V supply.
Physicaly its about 100 mm (4 inches) square by 25 mm high, in a nice robust rounded edges shiny metal case.
I havent seen it since and I cant remember the price but I didnt think it was too expensive at the time for what it claimed to be able to do.
It has the Traveler brand, product number 36736, model number MPP 7400.
It may be out of date but Google it and see what turns up.
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