You can never have too much battery power. Actually that's not true - strapping a Duralast to your back via some kind of shoulder rig so that you can play Crossy Road for 200 hours straight might be considered overkill. But any external battery that you can shove in your pocket for a long day of travel is a good one. If you're in the market, an Amazon vendor is offering a 16000mAh model (big enough for 4-5 charges of a current flagship) for just $23.99.
You'll need to use a coupon to get this discount (which also means that it's probably limited to the US version of Amazon). Head to the listing for the EC Technology® 3.Gen 16000 mAh listing, add it to your cart, start the checkout process, and enter the code "ZKJPRRJH" before paying. That will change the $32 retail price, which the seller rather dubiously claims is already "discounted" from $80, by a further $8. This battery alone isn't enough to get you free shipping on Amazon, but Prime members can take advantage of free two-day shipping.
This battery doesn't have anything particularly mind-blowing in the features department. It's about the size you'd expect at this capacity, it's got three USB-out chargers that can operate at 5 volts, and it has some basic LED capacity indicators and an LED flashlight. Pretty standard stuff. If you need some extra juice for a trip or your old battery pack is getting long in the tooth, check it out. Remember, that coupon code is ZKJPRRJH.