Control device for a fuel cells stack

The invention relates to a control device () for a fuel cell stack (), the control device being set up to actuate the fuel cell stack () using a predetermined current (I) and to measure the resulting voltage (U) and to compare this with a reference voltage (U). The predetermined current (I) depends on the reference voltage (U) by means of a sum of at least two exponential functions whose argument in each case contains the reference voltage (U). The control device () is designed to ascertain the reference voltage (U) by approximately reversing the correlation between the predetermined current and the reference voltage (U) using an iterative approximation algorithm.