RE: Airphone intercom systems
We currently have the Airphone system that came with the boat. It has worked flawlessly for two years now so I assume that it does not have batteries. In fact it is connected to the electrical panel and can be shut off there.
We are upgrading our VHF system to an Icom 504 with a remote Command Mike which has intercom capabilities so will have a back-up system. Haven't installed (or even actually purchased) it yet so don't know how well it will work as an intercom, which is not its primary function anyway.