A processor executes a program to calculate values of the internal resistance R of a battery and updates a database of parameters defining the dependence of internal resistance on the battery state of charge (SOC) and temperature. The database is utilized to obtain the information needed to make accurate remaining run-time calculations. The processor may also execute a program to effectuate entry of a database into memory representative of characteristics of the battery including values of measured open circuit voltages (OCV) of the battery and determinations of “starting SOC” values that correspond to the most recently measured values of OCV after the battery has stabilized. The processor may also execute programs to determine the present SOC of the battery, the present battery capacity, and the remaining run-time of the device powered by the battery.