the present invention concerns a power supply comprising a primary cell adapted to provide a primary cell current . the primary cell comprises an anode , a cathode current collector and an electrolyte , whereby a passivation layer is formed on a surface of the anode as a result of a chemical reaction between the anode and the electrolyte . to ensure that the passivation layer remains essentially intact for the lifetime of the primary cell , the power supply further comprises a current limiter that is configured to prevent the magnitude of the primary cell current exceeding a value that would damage the passivation layer .