An ambulatory infusion device comprising a replaceable energy storage for electrically powering its functional units is presented. Control units provide to a monitoring system data about energy consuming activities or consumed energy by the functional units. Based on this data, the monitoring system determines energy consumed by the functional units and triggers measurement of the voltage of the energy storage at a point in time which depends on the determined energy consumed in a time interval preceding the point in time. Adapting the time of measurement to the actual energy drawn from the energy storage makes it possible to detect reliably and early a low energy state of any conventional standard battery, regardless of its electrochemical design and electrical behavior.
