Patent ID: 7808244

Claim:
A method of determining a true internal resistance R of a battery when the battery is inserted into a user device which draws current from the battery comprising: providing a connection to receive voltage from a battery which will draw current from the battery; determining an initial voltage, V 0 , upon insertion of the battery; determining an initial estimate of internal resistance of a battery; determining an initial current, I 0 , upon insertion of the battery; determining a state of charge for the battery using the initial estimate internal resistance of the battery and the initial current; determining a state of charge, SOC 1 , associated with a measured relaxed state voltage of the battery; determining a startup state of charge, SOC 0 , from SOC 1 , a charge/discharge rate Q 1 , and a maximum battery charge, Qmax; determining an undistorted initial voltage OCV 0 for the SOC 0 ; and determining a true value of the internal resistance R from the undistorted initial voltage OCVO, initial voltage VO and the initial current IO.