@OldGunny When wiring your Doorbell to an existing doorbell circuit to continuously charge the battery, the transformer must be 8-24 VAC, 40VA max, 50/60Hz. It is not compatible with DC power outside of the Plug-In Adapter as previously mentioned. You can view a list of compatible chime kits in our Help Center Article here as well, since intercom systems are not compatible. I hope that helps clear things up! 