iPhone 7 Smart Battery Case Charge Rate

[imagineadam]

Hey I recently picked up an iPhone 7 smart battery case off of eBay. It was one of those open box ones that supposed to be new. I couldn't justify buying one brand new for full price so I got this one for about half. Anyways, I'm wondering about how fast yours charges from 0 to full? I used mine today heavily from 100% down to about 5% and then I put it on the 12w charger and its up to 55% after an hour and the iPhone battery itself has been full the whole time. Does this sound about right? Is the smart battery case suppposed to "fast charge" to about 80% then taper off similar to the iphones own battery? Anybody's charge faster or slower? Just trying to get a feel for this case and make sure mine isn't a defective/super used one. Thanks!