Abstract: A rechargeable battery pack has a battery voltage detector, a full charge detecting circuit detecting a full charge state, a switch interposed in a charge current circuit that opens the charge current circuit when detecting a fall charge state, a memory for storing a closed circuit voltage immediately before the charge current circuit is opened, an internal resistance detection circuit for finding a value corresponding to an internal resistance of the rechargeable battery cell from an open circuit voltage and the closed circuit voltage stored in the memory after the charge current circuit is opened, and a lifetime indicator to predict lifetime. This enables accurate determination of the lifetime of the rechargeable battery cell under stable conditions without wastefully consuming the charging capacity.

Abstract: A rechargeable battery pack provided with a battery voltage detector for detecting a terminal voltage of a rechargeable battery cell, a −&Dgr;V detection circuit or other full charge detecting means for detecting a full charge state when charging the rechargeable battery cell, a switch device interposed in a charge current circuit of the rechargeable battery cell and opening the charge current circuit when detecting a full charge state, a memory for storing a closed circuit voltage of the rechargeable battery cell immediately before the charge current circuit is opened, an internal resistance detection circuit for finding a value corresponding to an internal resistance of the rechargeable battery cell from an open circuit voltage and the closed circuit voltage stored in the memory after the charge current circuit is opened, and a lifetime indicating means for referring to a battery lifetime table in accordance with a value corresponding to the internal resistance of the rechargeable battery cell to pred