An apparatus used with a processor to perform fuel gauge operation for a battery of a portable device includes a voltage measuring circuit and a control circuit. The voltage measuring circuit periodically measures and records a battery pack voltage for the battery according to time information received from the processor after the processor enters a sleep mode. The control circuit generates an accumulation result by calculating and accumulating at least one value of battery characteristics according to the battery pack voltage, and compares the accumulation result with a reference threshold of battery characteristics to determine whether to trigger an interrupt to wake up the processor, so as to cause the processor to update a battery cell voltage and an internal battery resistance of the battery for performing fuel gauge operation.