A system for and method of estimating the state-of-age of a secondary cell utilizing an adaptive group filter algorithm, includes battery current, voltage, and temperature sensors, a communication device, and a controller communicatively coupled to the sensors and device, configured to filter data by analyzing only sample data points from instantaneous charge or discharge events, selecting only a sample of n instantaneous points for further regression, including n/2 charge event points and n/2 discharge event points, and separately averaging the charge and discharge event points, and further configured to determine the state-of-age by determining a resistance slope based on the rate of current and voltage change between the averages of the points, and matching the slope to a calibrated scalar or relational database.