External battery packs provide backup power, but with the way some phones routinely die every evening, you may come to depend on one as a regular source of supplemental energy instead. In such an instance, it helps to pick up one of the smaller options available. It's also nice if you can get one for under $10.
Fortunately, we've come across a KMASHI 10000mAh power bank that scratches both itches. It's going for $14 on Amazon, but if you enter the coupon code GOQL7EVL at checkout, you can drop $4.50 off the price. That leaves you paying just $9.50 for a battery you can carry around in your pocket, backpack, or purse.
KMASHI's battery comes with two USB ports. This lets you charge a couple phones or tablets at once, but one will pull from an output of 5V/2.1A, while the other gets 5V/1A. There's no built-in flashlight or LCD screen that we've come to see regularly in other models, but again, this thing is tiny. At just under 5.5 inches long and 2.5 wide, there's a decent chance it takes up less space than your smartphone, even if it is twice as thick.