For a hardwired Tapo doorbell in Always-On mode, it’s normal for the battery to maintain a level between 70-80% rather than charging to 100%. This is a design feature to protect and extend the battery’s overall lifespan.

As for another situation -- if your hardwired doorbell frequently exits Always-On mode by itself, please refer to this troubleshooting article for guidance. Why does the Tapo Doorbell still discharge when powered through hardwiring?