Patent ID: 8614871

Claim:
A battery system comprising: a first battery block including a plurality of batteries connected in series; a second battery block including a plurality of batteries connected in series; a battery state detector configured to detect voltages of the batteries in the second battery block; a switch connected to the first battery block in series; a battery protection circuit, comprising detection circuits, configured to detect overvoltage in the batteries of the first battery block and make the switch open when the overvoltage is detected; and a switch controller configured to control open and close states of the switch on the basis of the signals from the detection circuits; wherein the first and second battery blocks are electrically connected in parallel, the number of the batteries in the second battery block is smaller than that of the first battery block, the battery state detector is installed integral with the second battery block, the protection circuit comprises an overvoltage protection circuit comprising Zener diodes which are respectively connected to the batteries of the first battery block, and each of the overvoltage detection circuits comprises a photocoupler, wherein the photocoupler transmits a control signal to the switch controller when the Zener diode breaks down.