Ring Doorbell 2 battery issue

My Ring Doorbell 2 is wired into the transformer for the doorbell. For 2 years it worked without issue. A month ago I noticed the low battery indicator. I didn't even relize there was a battery in it with it being on AC. I replaced the battery and it was working again. I charged the dead battery and it works fine. Now a month later the battery is dead again. What is going on with the doorbell?