Battery system and method for detecting current restriction state in a battery system

A battery system includes a battery pack 10, a detecting portion 5, a storage portion 6, and a determining portion 7. The battery pack 10 includes parallel battery units 1. The unit 1 includes a current restriction portion 9 and a plurality of battery cells 2. The cells 2 are connected to each other in parallel. The detecting portion 5 detects voltage and current of each unit, and calculates the accumulated current value of each unit. The storage portion 6 stores reference voltage values to be associated the accumulated current value. The determining portion 7 reads one of the reference voltages corresponding to the accumulated value, and compares the read reference voltage with the detection voltage whereby determining that the current restriction portion is brought in a current restriction state if the difference between the detection voltage and the read reference voltage is larger than a predetermined value.