A method of operating a fuel cell system, the fuel cell system comprising: a direct current, DC, bus; a first fuel cell segment electrically connected via a first input connection to a first DC converter, wherein the first DC converter is connected via a first output connection to the DC bus; at least one second DC converter (612, 614, 616) connected via at least one second output connection (622, 624, 626) to the DC bus; at least one second fuel cell segment (604, 606, 608) electrically connected via at least one second input connection (642, 644, 646) to the at least one second DC converter (612, 614, 616), and wherein the first output connection and the at least one second output connection (622, 624, 626) connect the first DC converter and the at least one second DC converter (612, 614, 616) to the DC bus in parallel; and a processor connected to the first DC converter and the at least one second DC converter (612, 614, 616), wherein the method comprises: - injecting, by the processor controlling a controller of the first DC converter of the first fuel cell segment selected for impedance testing, an electrochemical impedance spectroscopy, EIS, test waveform into the first fuel cell segment to generate a first resulting ripple to the DC bus; and - injecting, by the processor controlling a controller (632, 634, 636) of the at least one second DC converter (612, 614, 616), at least one second offset ripple (908, 912, 916) to the DC bus from the at least one second DC converter (612, 614, 616), wherein the at least one second offset ripple (908, 912, 916) at least partially offsets the first resulting ripple on the DC bus. a direct current, DC, bus; a first fuel cell segment electrically connected via a first input connection to a first DC converter, wherein the first DC converter is connected via a first output connection to the DC bus; at least one second DC converter (612, 614, 616) connected via at least one second output connection (622, 624, 626) to the DC bus; at least one second fuel cell segment (604, 606, 608) electrically connected via at least one second input connection (642, 644, 646) to the at least one second DC converter (612, 614, 616), and wherein the first output connection and the at least one second output connection (622, 624, 626) connect the first DC converter and the at least one second DC converter (612, 614, 616) to the DC bus in parallel; and a processor connected to the first DC converter and the at least one second DC converter (612, 614, 616), wherein the method comprises: - injecting, by the processor controlling a controller of the first DC converter of the first fuel cell segment selected for impedance testing, an electrochemical impedance spectroscopy, EIS, test waveform into the first fuel cell segment to generate a first resulting ripple to the DC bus; and - injecting, by the processor controlling a controller (632, 634, 636) of the at least one second DC converter (612, 614, 616), at least one second offset ripple (908, 912, 916) to the DC bus from the at least one second DC converter (612, 614, 616), wherein the at least one second offset ripple (908, 912, 916) at least partially offsets the first resulting ripple on the DC bus. - injecting, by the processor controlling a controller of the first DC converter of the first fuel cell segment selected for impedance testing, an electrochemical impedance spectroscopy, EIS, test waveform into the first fuel cell segment to generate a first resulting ripple to the DC bus; and - injecting, by the processor controlling a controller (632, 634, 636) of the at least one second DC converter (612, 614, 616), at least one second offset ripple (908, 912, 916) to the DC bus from the at least one second DC converter (612, 614, 616), wherein the at least one second offset ripple (908, 912, 916) at least partially offsets the first resulting ripple on the DC bus.