To highly accurately detect a capacity remaining in a secondary battery by forcibly discharging charges and removing the remaining capacity when a calculated value of the remaining capacity becomes not larger than a set value.
A battery pack 10 is provided with a discharger 6 and a discharge switch 7. When a secondary battery 1 is started to discharge, a control part 3 integrates a discharging current Is detected at a current detection part 2, thereby calculating a remaining capacity. The remaining capacity is compared with a set value. When the remaining capacity becomes smaller than the set value, the discharge switch 7 is turned ON to start compulsory discharging by the discharger 6. A discharge speed is accelerated by discharging and compulsory discharging to a load device 4. When the remaining capacity becomes 0, a switch means 5 is turned OFF to stop the current supply to the load device 4, with the compulsory discharging continued until a battery voltage Vc reaches a discharge lower limit voltage. When the secondary battery 1 is completely discharged, a value of the remaining capacity set in a memory is cleared, whereby a calculation error is cancelled. The error is not accumulated even when the charging and discharging are repeated, and highly accurate detection is achieved.
SUZUKI TETSUYA
NAKAO FUMIAKI
SUZUKI KAZUNARI
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