The charge drawn from a battery during each switching event (pulse) of a pulse frequency modulated DC-DC converter is determined during a calibration period. based on differences in pulse rate with different current loading. Another approach calibration approach determines charge drawn from the battery by measuring voltage across a sense resistor while measuring the total pulse rate and while adding sufficient load current to ensure that the voltage is much larger than the residual offset of the measurement system. During operation, the system counts number of pulses are counted and the total charge drawn from the battery is determined based, at least in part, on the charge transferred per pulse during calibration, the operational mode, the battery voltage during calibration and operationally and the output voltage. Based on the total charge drawn and temperature (for temperature dependent battery types), the battery state of charge is estimated.