The most annoying thing about modern smartphones these days is their battery life. Manufacturers are always trying to create thinner phones which means smaller batteries. Also with battery technology developing at a slow rate compared to the devices we use everyday, no one knows when they'll catch up.
If you're looking for a small portable charger to take with you because you're worried your phone won't last the day, the Pebble Smartstick may be a good option. It's a pretty small unit which has a USB socket, which means most modern devices should work with it. It has a capacity of 2200mAh. With the iPhone 5 having a 1440mAh the Pebble Smartstick should be able to recharge it from dead and then give you another 40%.