Update 11/14/14: The previous coupon expired but a new one popped up.
Even the best of us have miscalculated and run low on battery before heading to an outlet, but that's what external battery packs are for. These devices pop up on sale from time to time, and today there's a good deal on the Poweradd Pilot X5 with a 16,000mAh capacity. The price usually hovers around $40, but you can score one for $24.99 with Prime shipping using the coupon code below.
Drop the Pilot X5 in your cart and use coupon code
CD9DWIF3 F7RLZ3BU at checkout. That'll knock another $7 off the already reduced price to get you to $24.99. This device has two 2.1A USB ports to juice up any device you've got around. There's also an LED flashlight on the end next to the USBs. That's kind of fun.
You tend to lose a little power when charging and discharging an external battery, but there should still be enough capacity to fill most tablets a couple times. 16,000mAh is a lot, after all.