Patent ID: 7525284

Claim:
A method for calculating charged rate of a secondary battery which is adapted to supply electric power to a load, comprising the steps of: providing a time function for the open circuit voltage of the secondary battery as a function of time with its coefficients undetermined; measuring voltage samples of the open circuit voltage of the secondary battery at a plurality of different time points within a predetermined period of time after termination of charge or discharge to obtain plural measured voltage samples along time axis; using the plural measured voltage samples to determine the coefficients of the time function to thereby determine the time function; calculating a convergent value of the open circuit voltage of the secondary battery based on the determined time function; and calculating the charged rate based on the convergent value of the open circuit voltage; wherein the step of providing the time function comprises providing a combination of at least two exponential decay functions in which the combination of N exponential decay functions, hereinafter referred to as multi-exponential decay function, where N is an integer of at least two is given by Y = a ⁢ ⁢ 1 ⁢ ⁢ exp ⁡ ( - b ⁢ ⁢ 1 · X ) + a ⁢ ⁢ 2 ⁢ ⁢ exp ⁡ ( - b ⁢ ⁢ 2 · X ) + a ⁢ ⁢ 3 ⁢ ⁢ exp ⁡ ( - b ⁢ ⁢ 3 · X ) + a ⁢ ⁢ 4 ⁢ ⁢ exp ⁡ ( - b ⁢ ⁢ 4 · X ) + … + an ⁢ ⁢ exp ⁡ ( - bn · X ) + c wherein (2N+1) coefficients a1 to an, b1 to bn and c are undetermined, Y denotes the open circuit voltage, and X denotes time; and wherein the step of using the plural measured voltage samples comprises using the measured voltage samples to determine the (2N+1) coefficients to determine the multi-exponential decay function.