State of charge method and apparatus

A method for determining a state of charge (SOC) in a battery system based on determining values of a resistance R and an open circuit voltage Voc, respectively, comprising:

• creating at least two data points based upon measurement or prior computation;
• establishing a linear relationship using said resistance R and said open circuit voltage Voc;
• applying least square analysis upon said linear relationship;
• formulating a recursive relationship, wherein a first state at a backward time increment solely determines a current state after said backward time increment, said recursive relationship being the summation of said first state manifested in a recast summation derived from integrals, and a data point at current time.

A vehicle powertrain control system, comprising:

• a battery pack;
• a power inverter coupled to said battery pack;
• a controller controlling said power inverter, said controller monitoring a state of charge (SOC) of said battery pack;
• and
• a program product for computing a resistance and an open circuit voltage both being functionally related to said SOC, said program product being associated to said controller.