Apparatus used with processor of portable device and arranged for performing at least one part of fuel gauge operation for battery by using hardware circuit element(s) when processor enter sleep mode

An apparatus used with a processor of a portable device to perform fuel gauge operation for a battery of the portable device, comprising:

• a voltage measuring circuit, for periodically measuring and recording a battery pack voltage for the battery according to time information received from the processor of the portable device after the processor enters a sleep mode from a non-sleep mode;
• a control circuit, coupled to the voltage measuring circuit, for generating an accumulation result by calculating and accumulating at least one value of battery characteristics according to the battery pack voltage periodically measured and recorded, and for comparing the accumulation result with a reference threshold of the 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 for the battery.