Due to the intermittency of renewable energy sources, alkaline water electrolyzers are typically operated at partial load compared to the nominal design value. It is well-known that gas crossover is dominant at low current densities leading to higher anodic hydrogen content and higher cathodic oxygen content in the separator tanks. High anodic hydrogen content is tantamount to loss of product hydrogen which results in an explosive atmosphere in the gas phase if the volumetric hydrogen content in oxygen exceeds 4%. We have developed a transient model of a multi-cell stack which can describe the operation of the electrolyzer with mixed electrolyte flows (anolyte and catholyte), separated flows, or a combination thereof (dynamic switching). This is a major extension of the steady-state model developed by Haug et al. (International Journal of Hydrogen Energy, 2017, 42, 15,689–15707). In sharp contrast to the steady-state model by Haug et al., the transient model can calculate the gas crossover as the operating conditions (e.g. electrolyte flow cycles) dynamically change in time. Depending on the size of the stack and the separator tanks, the model estimates different rates for impurities to build up. The transient model is validated using independent experimental results by Haug et al. and Brauns et al. (Electrochimica Acta, 2022, 404, 139,715) The results show that the dynamic model can follow experimental results for fluctuating current densities for a period of several days. We found that the dynamic response and transition time to steady state depend significantly on the geometrical volume of the gas separators with respect to the single-cell stack. For a multi-cell stack, we find that the impurities build-up faster when increasing the number of cells in the stack. This model serves as a tool for sizing and process management of the electrolyzer system and the separator tanks especially with respect to explosion safety.