What Isolation Impedance Is About
Electrified vehicles are powered by high-voltage energy storage. The energy storage uses vehicle chassis as ground, requiring virtual isolation. Unless otherwise, passengers and a driver will get exposed to high voltage hazards. We have to measure isolation impedance, and it is a minimum of measures between a high voltage positive and chassis ground and between a high voltage negative and chassis ground. As seen in Figure 1, a battery management system (BMS) carries out these measures periodically.
We measure the leakage current flowing to the chassis ground and calculate Ohm’s law to the isolation impedance. In general, an expected value is around 5MΩ at a 500V battery pack; it may vary with the electrical paths of vehicle systems. When the HV Relay is open, the leakage path is created from the battery to the chassis, representing the battery isolation. In contrast, it represents vehicle isolation when the HV Relay is closed.
The degree to which we decide the high voltage exposures is the leakage current, and the standard reference values for warning and error are 500Ω/V and 100Ω/V, respectively. (Refer to Federal Motor Vehicle Safety Standards Part 571.)
