If you've been traveling recently, you may have seen a FuelRod kiosk at the airport. These dispense reusable, portable charging units that come fully charged. FuelRods can deliver up to 8 hours of talk time on a smartphone and 4 hours of internet browsing on a tablet on a single charge.
When your FuelRod runs out of battery life, you can just swap it for a new one at any FuelRod SwapBox location for free. FuelRod offers unlimited swaps, so you can get a fully charged unit any time you need it. You can also charge it yourself using any USB adapter and a standard wall outlet.