NOVELTY - The circuit (5) has voltage detection circuit to detect voltage of unit cell (2). The voltage detection signal is modulated to power line transmission signal. The power line communication circuit (8) is made to superimpose transmission signal between positive and negative output terminals (3,4). The identification signal identifying unit cell causing the abnormality in voltage is output to power line communication circuit, when unit cell exceeds overcharge reference voltage during outputting of detection signal detecting that unit cell is less than reference voltage.    USE - Monitoring circuit for monitoring secondary battery such as lithium ion secondary battery, lead acid battery and nickel-hydrogen battery used in electric vehicle, hybrid vehicle, and fuel cell electric vehicle, used in battery control system (claimed).    ADVANTAGE - Since the identification signal identifying the unit cell causing the abnormality in voltage is output to power line communication circuit, when the unit cell exceeds overcharge reference voltage during outputting of detection signal, the voltage information of several unit cells can be put in block and can be processed in power line communication circuit. Thus the number of wiring can be reduced as well as the number of power line communication circuits can be reduced. Hence the low-cost and reliable battery monitoring circuit can be ensured. The control circuit prohibits the charge or discharge of battery, so that the overcharge or overdischarge of the unit cell can be prevented. The data from the battery monitoring circuit of different assembled battery cannot be collided with each other, thus performing reliable power line communication.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for battery control system.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the battery monitoring circuit. (Drawing includes non-English language text)    Unit cell (2)    Positive output terminal (3)    Negative output terminal (4)    Battery monitoring circuit (5)    Power line communication circuit (8)