Bests & Worsts Reviews from Amazon

I don't know if I received a defective product or whether this is how this item is actually supposed to work. When I first received the product, I tested it to make sure it functioned properly. I plugged it in to my iPhone 7 Plus, and it easily increased the charge by 10%. I thus assumed it was a fully functional product, so I disconnected it from the phone. I then plugged it into the wall and fully charged the product to 100%. After completing the charge, I placed it on a window sill that receives direct sunlight for a few hours per day. After four days, I connected the product to my iPhone 6S, and it only increased the charge by 20% before being completely out of power. Also, I had assumed the device would charge my phone as I cranked it. Even after cranking for five minutes, no charge was running to my phone. Again, I don't know if I got a defective product or not. But an emergency battery that either doesn't hold its charge or can only provide a 20% charge and that does not provide a charge while cranking the unit is almost useless to me.