The requirements for stack monitoring devices are becoming more strict as the fuel cell and battery technologies reach an advanced stage of development and move towards commercialisation. Different applications put restraints on such devices when it comes to cost, weight and size. No commercial products can meet the requirements with respect to both cost and performance. Individual cell voltage measurements are crucial to protect the fuel cell stack and ensure maximum stack lifetime. Different concepts for measuring individual cell voltages in large fuel cell stacks or battery stacks and their potential accuracy are discussed. A novel low cost, lightweight and compact multiplexer circuit was implemented based on a resistor-diode circuit. Based on this circuit a prototype 80-channel multiplexer device was built and tested on a fuel cell stack with satisfactory speed and accuracy.