Talking about portable 5V "power bank" battery packs, an issue I have with them is that they are too "smart". They turn themselves off when they feel not enough of a load (e.g. the smartphone decides it's fully charged at the moment). And then don't turn themselves back on even if a load is there, until you manually press the "on" button. Not good when you're busy flying and don't notice. I've also had some of those packs stop working altogether until I took them apart and unsoldered the battery from the "smarts" board to reset the controller. I've looked long and hard for any with a simple on/off slide switch, they are rare. Did find this one, it doesn't have a huge capacity (probably not even the claimed one) but is useful nonetheless, also doubles as a flashlight:
http://www.ebay.com/itm/381654424368

BTW I love my Nook e-reader's reflective e-ink display, nothing is more readable in bright sunlight! (Runs TopHat which is a derivative of XCsoar.)