# Battery Management System Hardware Concepts: An Overview
## 6 Safety and Reliability
### Insulation Measurement  
Usually, the high voltage system in electric- or part electric vehicles is constructed as IT net. This means that no deliberate connection between vehicle chassis and high voltage system is given. A single case of failure does not lead to a potentially dangerous situation. However, a second case of failure does. Thus, the first fault needs to be detected so the system can act appropriately. Figure 8a depicts a battery powered high voltage system in IT-net configuration, indicated by a grey box, which is surrounded by a chassis, symbolized by a dash dot line. With respect to a battery pack, an electric insulation or rather an electrically insulating material is generally characterized by its electric resistance. This insulation resistance is distributed over the whole electric circuit. To describe the insulation practically, the model in Figure 8a can be used, where \(R_{p}\) and \(R_{n}\) are the concentrated insulation resistances between the positive and the negative pole of the high voltage system and the chassis. The same applies to the capacitances \(C_{p}\) and \(C_{n}\), and these are the sum of parasitic cable capacitances and possibly placed EMC filter capacitances. Due to transient measurement methods, these capacitances should be taken in to account. Several techniques to measure the insulation resistance are common. [PERSON] et al. [(76)] measures common mode currents using a toroidal coil, which is externally excited with an alternating signal. When a common mode current flows through the coil, the excitation signal will be changed and, in combination with a voltage measurement, the insulation resistance can be calculated. However, in order for this method to work, the main current conductors have to be guided through the coil, which is a disadvantage of this system. A more common method is described in [PERSON] et al. [(77)]. As shown in Figure 8b, only two additional switches and two resistors are needed to determine the insulation resistances \(R_{p}\) and \(R_{n}\). In terms of the measurement of insulation resistances, the capacitances \(C_{p}\) and \(C_{n}\) are contributing as disturbances. The two switches are able to produce different states where the chassis' potential is shifted with respect to the system's potential. For example, when switch \(S_{p}\) is closed, a current \(I_{sp}\) flows through \(R_{sp}\). Because this current gets added to the current through \(R_{n}\), the voltage \(V_{n}\) increases, and, as a consequence, \(V_{p}\) decreases. In short, the chassis potential will be shifted with respect to the high voltage system. By measuring the different states' voltages, the insulation resistance can be calculated. To sum up, the presented insulation measurement system only needs two switches, two concentrated resistors and two voltage measurement units. A slightly different system is presented in [(78)]. The described solution uses four switches and four concentrated resistors, so it is able to create more different states. As a result, only one voltage  
Figure 7: Equivalent circuit model of cell voltage acquisition front-end, showing the detectability of sense line faults. measurement is needed. Ref. [79] presents a quite simple solution that uses only one switch. However, it is not able to detect symmetric insulation resistance errors. As mentioned, the insulation capacitance is a kind of disturbance for the insulation resistance measurement. Whenever a change in the potential between chassis and HV system occurs, these capacitances get recharged. The insulation capacitance and the insulation resistance form an \(RC\) circuit with a time-constant depending on \(R\) and \(C\). Therefore, after a new measurement state has been set, the voltage between chassis and HV system changes very slowly. The measurement unit has to wait, for instance, five time constants, until it is capable of determining the correct insulation resistance value.