If you're worried about no USB ports in some rental car, you don't need to get battery banks, you just need to get a "cigarette lighter" to USB port adapter. The "cigarette lighter" connector (for 12VDC power) seem standard on all cars (even old cars from independent rental places that rent old cars). Gosh, you can even get "cigarette lighter" to 120V indoor-type power converters too. Not all cars include the cigarette lighter itself any more, but the connector still always seems to be there somewhere, because it's used for so many powered car accessories (including compressors to inflate your tires!).