Doorbell battery issue

The battery on my ring doorbell keeps discharging even though it is hardwired. The lines have power, I've checked that. I've removed the device a few times and charged it with a USB cable to 100%, but every time it starts a gradual discharge until it becomes unuseable. I don't live in a cold climate, so the cold is not a factor in Phoenix. It was good until a few months ago. The firmware states "Up To Date", and the wi-fi signal strength is strong.