You could say external battery packs are the AA batteries of our times. They breathe life into your phone during an emergency, and some even come with a flashlight built-in. Such is the case with the RAVPower 16,750mAh portable charger that is currently Amazon's deal of the day.
This charger is going for $23.99, which Amazon lists as a discount of 79%. According to CamelCamelCamel, the battery usually sits at somewhere over $30. Regardless, you're saving money on a sizable battery pack that should be able to charge your device several times. With two USB ports, you can power multiples at once.
There are downsides to this product. It's not Quick Charge compatible, nor does it have USB C ports, but it does have 4.5A output altogether. It also has a snazzy LED light on top that shows you how much of a charge remains. That is not at all rare to see on a battery, but this one's blue and looks cool. For the next fourteen hours, you can buy the discounted battery in your choice of black or white. After that, the deal ends.