Patent ID: 8803481

Claim:
A battery pack comprising: one or a plurality of secondary batteries; a current detection resistor provided in a current path through which a current flows into each secondary battery; a control FET provided in the current path so as to control the current; and a controller detecting the current flowing in the current detection resistor and controlling the control FET on the basis of the detection result, wherein the controller compares the current detected by the current detection resistor with a threshold value indicating an overcurrent detection current with respect to the current, and when the detected current is equal to or larger than the threshold value, determines that an overcurrent state is detected, the controller includes a timer which counts the time of the overcurrent state and has a delay time set with respect to the counted time such that when the overcurrent state is detected, the controller activates the timer to start to count the time of the overcurrent state, and performs control such that the control FET is not turned off until the counted time reaches the delay time, even though the time counted by the timer exceeds the delay time, when the current is equal to or larger than the threshold value, the controller performs control such that the control FET is turned off, before the time counted by the timer reaches the delay time, when the current becomes smaller than the threshold value, the controller ends time counting by the timer and performs control such that the control FET is not turned off, and after the control FET is turned off, the controller measures a potential difference between each end of the current detection resistor and if the controller determines that there is no potential difference, the controller turns on the control FET.