As the battery begins to weaken, the transmitter would begin to send a low battery signal to the wireless receiver as part of its transmission for a trip or supervisory report, which in turn would notify the HAI controller - the HAI controller would report the low battery as a "HAD TROUBLE".
A weak battery will also result in a weaker signal, so it is also possible that the transmitter's supervisory signals may not be received by the wireless receiver, which would result in the receiver reporting a supervisory failure to the HAI controller - the HAI controller would report this as a TROUBLE NOW condition.
In either of the two cases above, this would not trigger an "alarm", although it would trigger console beepers and perform a dialout to a central station, if used.
