2 Optimizing Safety Signal Management

The Battery Management System performs a great amount of voltage, current, and temperature monitoring in order to keep the battery healthy and provide efficient control. It is also important to keep the end user safe around these high voltages and currents associated with electric vehicles, which makes safety and robustness key design goals in Battery Management Systems.

Logic is often seen handling safety signals such as over-current (OC), over-voltage (OV), and over-temperature (OT) signals to help indicate fault conditions. These logic circuits, however, are typically used as a reduntant path to help reach the safety metric target set by the OEM. Figure 2-1 illustrates how Logic and Translation can be utilized in this manner to help meet these targets.

Figure 2-1 Example of Logic and Translation Improving System Robustness

In the example above, the battery monitor device has its own current sensing capabilities to monitor the current through the battery stack. When it senses an OC fault condition it can then communicate this to the safety MCU using a level shifter to match the different I/O voltages. The safety MCU will then address the fault by changing the state of the contactor or relay. This main path may not meet the required safety metric due to the compexity of the devices involved. A second path is then created to perform the same task, but this path includes simple devices with lower failure risk.

Discrete logic devices do not have complex internals, making them ideal for this application. Though simple in design, these devices can be packed with features like back drive protection and schmitt-trigger inputs that can improve design robustness even more. The following sections will help with selecting the right function and features to best optimize system performance.