Phone batteries keep getting smaller and smaller, but if you need a little more juice in day-to-day usage, an extended battery case is one option. ZeroLemon is known for making great battery cases, and the company's Pixel and Pixel XL cases are at an all-time low on Amazon. The Pixel case is now $27.99, and the Pixel XL case is $41.99 (once you factor in the 30% coupon for each). That's a $13 and $16 reduction in price respectively from the usual cost.
The Pixel case has an extra 6,500mAh of battery capacity, but is rather thick at 0.88 inches. If you don't mind having a literal brick in your pocket, you'll likely get at least twice the Pixel's normal battery life. The Pixel XL case has an even bigger 8,500mAh battery, and comes in at 0.9 inches thick.
Both cases have a soft TPU covering, and both have USB Type-C ports for charging. That's always nice to see, especially considering many battery cases (even iPhone ones) use microUSB. The only major downside to these cases is the lack of quick charging, so topping off the massive batteries will take a while.
You can buy both cases from the source links below. Don't forget to apply the 30% coupon before buying it (click the 'Applicable Promotions' link on the product page), so you get the full discount.