Another question, unrelated to the AFCS, what exactly the function of the hot battery bus? I'm seeing this bus in the CQT questions, but the manuals don't really say much about it, nor do the electrical schematics I have show anything. I read something about it being for a dome light and the fire bottles. Is it just a direct connection from the battery to these items? Or is it from the right essential? Also, is the APU fire bottle then connected to the hot battery bus?

PS : Am I right in saying the Lav does have fire protection in the form of a fire bottle in the waste container plugged with wax that melts to activate? No flight deck control other than a fire detect light?