To monitor a battery correctly for the necessity of charging operation by detecting the ambient temperature of a backup battery and setting the threshold level of charge request alarm based on the detected temperature variably thereby preventing the charge request alarm from being sustained unnecessarily under an environment of high ambient temperature.

When power is supplied from a main power supply 21 by throwing in a power switch SW, the charge control section 31 at a charge monitoring section 30 captures the ambient temperature of a battery measured by a temperature sensor 50 through a temperature detecting section 34. On the other hand operating months counted by an operating month counter 32b is read out from an EPROM 32. Subsequently, a threshold value corresponding to the ambient temperature and the operating months is retrieved from a charging decision table 32a and a threshold value is read out a threshold memory 32c in order to calculate a variable threshold. Subsequently, the voltage of a battery 22 is detected through a voltage operating section 33 and when the detected voltage level reaches the variable threshold, a charge request alarm displayed at an operating panel section 18 is eliminated.