I've tried a number of power banks, both solar powered and not. This is BY FAR the best one I have used...and also the CHEAPEST! (currently $18.99) I have paid close to $100 for one power bank.
https://www.amazon.com/gp/product/B012CX3Z2C/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B012CX3Z2C/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1