Resound Phone Clip Rechargeable Replacement Battery

Does anyone have experience with replacing the rechargeable battery in a Resound Phone Clip?
After an all night charge it only lasts 4 or 5 hours. When new would go 12-15 hours.
The folks at my Costco, where I bought it, didn’t know anything about it and seemed disinterested. There is a small torx screw at the lower rear which I removed but the case doesn’t just pop open. I think it must be pried off much like on a laptop but I’m afraid of breaking it.