I bought this Tapo D225 about 2 or 3 weeks age. It is hardwired to my doorbell power supply. The MAC address is 78-20-51-1E-3E-45.

For a while the battery percentage was going back and forth between about 60 - 80%. In the last few days it started going down, down. down and is now at 39%. I have rebooted it and also opened the button box and turned it off and back on. Voltage is 16 VAC, checked with a multimeter. 

The unit keeps turning off "Always on" mode. 

How do I stop the battery percentage from going down?