Protection circuit for battery monitoring device, and battery monitoring device

A protection circuit for a battery monitoring device that monitors a battery module having a configuration in which a plurality of unit batteries are connected to each other in series, the protection circuit comprising:

• a signal line group including a plurality of voltage signal lines, which are electrically connected to inter-battery electrode portions between the plurality of unit batteries that are connected to each other in series, or electrode portions at end portions of the battery module;
• a first protection circuit portion including a plurality of Zener diodes that are connected to each other in series, the Zener diodes being connected in parallel to the unit batteries between the plurality of voltage signal lines, an anode of each of the Zener diodes being connected to one of the voltage signal lines on a negative electrode side of a corresponding one of the unit batteries that is connected in parallel to the Zener diode, and a cathode being connected to one of the voltage signal lines on a positive electrode side;
• a resistor portion group including a plurality of resistor portions for restricting a current, the resistor portions being interposed between the Zener diodes and the unit batteries on the respective voltage signal lines;
• and
• a second protection circuit portion including a plurality of varistors that are connected to each other in series, the varistors being connected in parallel to the unit batteries between the plurality of voltage signal lines, and terminals of the varistors being connected to the voltage signal lines between the resistor portions and the unit batteries,
• wherein if an overvoltage occurs at both ends of the battery module when an open fault has occurred in some of the plurality of unit batteries, a current flows through the varistor and the Zener diode that are arranged in series with respect to the unit battery in which the open fault has occurred.