I need to replace the fire alarm in a 2 bed flat. The alarm is in the hallway to the door. It is the only alarm in the property. The existing alarm is mains powered with rechargeable 9v NiCad. It has 3 wires: brown, blue and white. The brown is connected using a connector block to a red wire in the ceiling. The blue wire is connected to a black (I think) wire in the ceiling the white is not connected. Do I need to replace the alarm with a mains powered one, or can I use a battery powered instead? Will any mains powered alarm work with the two ceiling wires?