NOVELTY - A state-of-age of secondary cell is estimated utilizing an adaptive group filter algorithm, by analyzing current signal with respect to time to determine a rate of current change and an instantaneous charge or discharge event, determining correlative pair of candidate current and voltage drop values, averaging current and voltage drop values of sample data points, and determining change in current value between the average of charge and discharge events divided by the change in voltage drop value between average of charge and discharge events to calculate a resistance slope.    USE - Estimation of state-of-age of secondary cell for use with a vehicle having a cranking engine (claimed). Can also be used in any electrical device which uses a battery with high transient (current and/or power) demands throughout the device life.    ADVANTAGE - The algorithm is more efficient compared to conventional algorithm including battery state estimator (BSE) modules, and is more robust to input data; saves computation time; and requires less stringent control of inputs and data sample rates. The method does not require that a model be developed for the battery, thus making it more portable between applications in less time. The algorithm acts as a compound filter, thus eliminating the need for additional filtering structure. The algorithm is fully adaptive, thus making it insensitive to fluctuations and applicable over all battery operating states including temperature and state-of-charge. The algorithm provides reduced excitation thresholds in input data compared to existing BSE modules. The method requires minimal controller memory and utilizes conventional current and voltage sensors, thus requiring minimal hardware additions and/or modifications.    DETAILED DESCRIPTION - Estimation of state-of-age of secondary cell utilizing an adaptive group filter algorithm, comprises:    (A) concurrently sensing a current supplied to and voltage drop across the cell over a sampling period to generate synchronized current and voltage signals;    (B) analyzing current signal with respect to time to determine a rate of current change and an instantaneous charge or discharge event based on the rate of current change;    (C) determining correlative pair of candidate current and voltage drop values based on current and voltage signals when an instantaneous event is determined;    (D) accessing data bucket to determine a number of previously determined sample data points from like instantaneous events within sampling period, and storing the pair as sample data point in bucket, if the number of previously determined sample data points is less than a minimum quantity or if candidate current value is greater than each of current values of previously determined sample data points;    (E) averaging current and voltage drop values of sample data points, when the number of stored points reaches minimum quantity for each of instantaneous charge and discharge events;    (F) determining change in current value between the average of charge and discharge events divided by the change in voltage drop value between average of charge and discharge events, to calculate a resistance slope; and    (G) determining a battery state-of-age based on the slope.    An INDEPENDENT CLAIM is included for a system for estimating state-of-age of a battery within completed circuit and selectively alerting an operator.    DESCRIPTION OF DRAWING(S) - The drawing shows a plot and line graph of a sample of data points taken from a complete battery cycle of a 24-module Cobasys 1000 series nickel metal hydride battery pack at 80% state-of-charge and negative 30 degrees Celsius.