Computer aided design of battery management systems requires mathematical models and software to simulate the storage devices under investigation. For lead-acid batteries a suitable model and an appropriate software package have been developed at the Control Engineering Department of the Institut für Solare Energie-versorgungstechnik in Germany. The software represents the physical and electrochemical structure of those batteries and can be initialized by some geometrical data, the amount of substances in the electrodes and the values of porosity for the separators and the electrodes in charged state. Originally the model has been designed for condition monitoring systems, but for some time, it has been used also by some enterprises of the automotive industry to simulate complete electrical networks. Of course it can be used for cell design applications, too. This contribution describes the basic structure of the model and demonstrates the accuracy of the developed software by comparing some experimental data to appropriate calculations and gives a short example of an interesting application.