The essential task of a battery management system (BMS) is to consistently operate the high-voltage battery in an optimum range. Due to the safety-critical nature of its components, prior testing of a BMS is absolutely necessary. Hardware-in-the-loop (HIL) simulation is a cost-effective and efficient tool for this. Testing the BMS on a HIL test bench requires an electronics unit to simulate the cell voltages and a scalable real-time battery model. This paper describes a HIL system that enables comprehensive testing of BMS components. Hardware and software solutions are proposed for the high requirements of these tests. The individual components are combined to make a modular system, and safety-critical aspects are examined. The paper shows that the system as developed fulfills all the requirements derived from the different test scenarios for BMS systems.