US Patent 6892148 Circuit and method for measurement of battery capacity fade

The remaining run-time (t_rem) of a battery is determined independently of the condition of the battery and the amount of load on the battery by obtaining the value of a present total zero-current capacity (Qmax) of the battery based on relaxed-battery OCV values measured just before and just after a charging or discharging cycle. A current through the battery is integrated to determine a transfer of charge (Q) from the battery, and a value of total run-time (t_total) that would be required to reduce the open circuit voltage of the battery to a predetermined lower limit (Vmin) is determined. The remaining run-time (t_rem) is determined by subtracting the duration of the integrating from the total run-time (t_total).