I agree with at least 98% of what these humans have advised (didn't read all because everything was so good). I have nothing to add.
Also, stickers improve performance? Noted. @vidicon
I love my Ravpower bank that is only 20k and I recently got a 30k AuKey that is nice for the less mobile charging due to its weight being a bit to large for pocket use tho if in a backpack it works nicely.
Most of ingress players will say Anker, because years ago there wasn't really a reliable and cheap powerbank except for Anker. Now a days you have other good choices, but I stay with Anker. @jplolo is Right you have to sticker up yours to have 100% charge power.
I have an Iphone SE, and now I'm using a 13000 mAh. I was thinking about buying one above 20000, but even if I play very intensely, I'll never reach out all power bank capacity in a weekend. Also, I suggest you to buy two. Because it'll take you at least 12 hours to recharge completely.
Also, since no one mentioned it to you (or at least I haven't seen it) I suggest you to buy anker cables. More reliable, faster, and they are really indestructible.
Yet another vote for Anker. I have a few. My favorite for Ingress is the Anker PowerCore Speed 10000 QC that I bought years ago. Quick Charge, small enough to easily carry in a pocket, built like a little brick. Has survived international travel and full days of missioning.
Anker are best!
IMHO Anker Is Always the best. Best product, best support, best style! :)