Amazon is offering the Aukey 20000mAh Power Bank with Lightning input for $17.09 when you use the coupon code C84K9OIR at checkout.
$17 (52% Off) Aukey 20000mAh Battery
One of the unique things about this power bank is that it includes a Lightning input, which means you can charge the battery itself using a Lightning cable instead of a micro USB. Pretty neat. At 20000mAh, you can recharge your iPhone 7 about seven times or an iPad Air about two times.
Reviewers also seem to like it with over 60 of them giving it a 4.5-star rating and it comes with a 24-month warranty which is nice.
Just be sure to use the coupon code C84K9OIR at checkout.