The battery bank is an important device in a substation DC operation system， which supplies uninterrupted power for loads when the AC power is off. It is required to install real time battery status monitoring devices in all important substations. A RS-485 bus based distributed monitor device， comprising several measuring units and an inspector， is presented. Each battery is equipped with a measuring unit and each unit contains a single chip microcomputer with AD function to monitor the battery terminal voltage. The monitor unit calls up to get real time data from the measuring unit， and sends internal resistance detection commands to it. The measuring unit detects corresponding battery internal resistance on-line using twi-discharge method.