A method and a device for calibrating an interior clock generator installed inside a power monitoring unit of a rechargeable battery. The device includes an input pin for inputting an external clock signal from an exterior clock generator installed outside the power monitoring unit. A calibration timer control circuit is connected to the input pin. A register is connected to the calibration timer control circuit for outputting a start signal to activate the calibration timer control circuit. A counter and a timer are controlled by the calibration timer control circuit to be activated simultaneously therewith to count the outside clock signal and an internal clock signal generated from the interior clock generator, respectively. Such that the timer stops counting when the counter stops counting, a first count of the timer is compared to a second count of the counter to calibrate the interior clock generator.