Video Doorbell (2nd Gen) I cannot see any video on computer browser or Android app

Does this Gen 2 doorbell battery always get power from the a/c power? My Video Doorbell (2nd Generation) battery is discharging even with my 12v ac supply is connected and powered up. I have had the doorbell installed for over one year now and the battery condition has always shown as full until yesterday when I got the critical low battery alarm. I would have thought that the a/c power that is used by the doorbell push would trick charge the installed battery all the time. It seems that this is not the case and my battery is now fully discharged. I have removed the doorbell and am recharging it via the USB socket. Is there a fault with this battery if it is not being trickle charged via the a/c power? Can anyone help to explain?