A method of monitoring a state of charge of a battery of a field device wherein the field device includes the battery, a voltage converter and a battery-powered component connected to the voltage converter, is disclosed. The method includes enabling the voltage converter and subsequently operating the voltage converter in duty cycles based on a current flowing from the battery to the voltage converter by repeatedly: disabling the voltage converter when the current exceeds an upper limit and enabling the voltage converter again when the current drops below a lower limit; and at least once determining and providing a state of charge of the battery determined based on a duration of at least one first time interval during which the voltage converter was enabled during one of the duty cycles and a previously determined dependency of the durations on the state of charge of the battery.