Patent ID: 8207703

Claim:
A protection device for an assembled battery having a plurality of battery cells (i=1 . . . n) connected in series, comprising: at least one sampling capacitor; for battery cell (i), a first switch group including a first switch and a second switch, the first switch being placed between an end of the battery cell (i) and an end of the capacitor, the second switch being placed between an other end of the battery cell (i) and an other end of the capacitor, so that the capacitor samples a voltage of the battery cell (i) and holds the voltage in the capacitor; for battery cell (i), a second switch group including a third switch and a fourth switch, the third switch being placed between the end of the battery cell (i) and the other end of the capacitor, the fourth switch being placed between the other end of the battery cell (i) and the end of the capacitor, so that the capacitor samples the voltage of the battery cell (i) and holds the voltage in the capacitor; a detection unit which detects the voltage of the battery cell (i) of the battery cells (i=1 . . . n) based on the voltage sampled using any one of the first switch group and the second switch group and held on the capacitor and outputs the detected value; a computing unit which computes an average value of detected values corresponding to the battery cells (i=1 . . . n); a comparator which compares detected value of the battery cell (i) with the average value so as to obtain a result of comparison indicating a relation in size between the both; and a controller which controls the first switch group and the second switch group, so that if the detected value of the battery cell (i) is higher than the average value according to the comparison result, the capacitor samples the voltage using one of the first switch group and the second switch group, the one of the first switch group and second switch group being not used for detecting a voltage of a battery cell (i−1), and if the detected value of the battery cell (i) is not higher than the average value according to the comparison result, the capacitor samples the voltage of the battery cell (i) using an other one of the first switch group and the second switch group.