I've gone through a lot of portable USB charging batteries. My current favorite is this model by RAVPower.
I love these batteries for keeping my iPhone going in both music, and GPS mode, over long motorcycle rides. By GPS I mean Waze, and Waze is a pig when it comes to battery power! Even with the screen off, and giving only audio cues via bluetooth, that app drains power. I'll also charge the helmet audio system up whenever I stop for coffee,a meal or to stretch my legs.
Two 2.4A smart sockets can full speed charge two tablets at once. The 16750 mAh capacity will charge an iPhone back up 6 or so times, or keep just it running for 2-3 days of heavy use while riding. It does take a good 3-5 hrs to charge this brick back up, so make sure to plug it in before you go to sleep.
The flashlight on these batteries is just a farkle, but one that can come in handy.