To provide a power supply control method for accurately predicting the remaining charge of a secondary battery and charging the secondary battery, while reducing the cost.

A battery pack 71 includes a positive-side terminal t1 for providing power to a portable device 72 from secondary batteries 14a to 14c and a voltage control terminal t3 for outputting a signal, having substantially the same electrical potential as the battery voltages of the secondary batteries 14a to 14c. In a standby state, a protective circuit 75 of the battery pack 71 blocks power supply from the positive-side terminal t1 to the portable device 72 and outputs a signal for making a standby power supply 73 of the portable device 72 activated from the voltage control terminal t3.

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