A method for determining system resistance of at least one power supply of a handheld medical device, the method including: a) generating at least one excitation voltage signal, wherein the excitation voltage signal comprises at least one direct current (DC) voltage signal, wherein the excitation voltage signal has a fast transition DC flank of 20 ns or less; b) applying the excitation voltage signal to at least one reference resistor having a predetermined or pre-defined resistance value, wherein the reference resistor is arranged in series with the power supply; c) measuring a response signal of the power supply; d) determining a signal flank from the response signal and determining an ohmic signal portion from one or both of shape and height of the signal flank; and e) determining the system resistance of the power supply from the ohmic signal portion.