In addition, some key modules in an electric vehicle include the onboard charger, battery management system (BMS), HVAC system, traction inverter, and DC/DC converter. Accurate monitoring of current and voltage values is essential to ensure proper system operation.
This article is a summary of a presentation by John Wilson, Senior Product Line Manager at Skyworks. Wilson explains the importance of effectively measuring and monitoring voltage and current in EVs to ensure optimal operation.
Measurement and its purpose
To monitor the voltage across a module and the current flowing through it, current and voltage sensors are placed at appropriate locations in the circuit. For example, to measure the current flowing through the battery pack, a current sensor measures the voltage across a series shunt resistor and generates an output signal proportional to the magnitude of the current flowing in the circuit. Similarly, to measure the voltage across the battery pack, a voltage sensor detects the voltage at that point and generates an output proportional to the voltage. These sensor readings are then sent to the BMS (Battery Management System), which makes decisions regarding effective operation. Different modules in a vehicle require measurements from various nodes around the vehicle for feedback, control, safety, and charging level assessment.
Various challenges arise when using sensors to measure different power-related parameters in circuits. A primary challenge is maintaining electrical isolation between the sensor and the power supply circuitry to prevent power supply fluctuations from affecting the measurement. Efficient isolation also helps maintain measurement accuracy in high-frequency switching circuits, which are highly susceptible to ground loop noise. Low response times and fluctuating temperatures can also affect measurement accuracy. Isolation also facilitates level shifting, allowing two logic levels to operate independently on the same circuit board. Most modules use CMOS-based capacitive coupling to couple signals across isolation boundaries. Its advantages include long-term reliability, low propagation delay, high noise immunity, and higher efficiency compared to optocouplers.
To better understand the measurement process, Wilson demonstrated a BMS monitoring a lithium-ion battery pack, where two battery management ICs monitor the voltage and current levels of two individual cells. An additional voltage and current monitoring system allows the entire system to remain safe and redundant. The main BMS measures the system voltage via a voltage divider circuit and voltage sensors, as shown in Figure 1. Individual cell measurements are also sent to the BMS for further monitoring. All sensors and the BMS are isolated.
Figure 1: Voltage and current measurements in BMS
Consequences of measurement errors
Accuracy in determining various system parameters is crucial for EV functionality. Inaccurate measurements can lead to inconvenience and even danger. Some common problems caused by inaccurate voltage and current measurements include incorrect estimates of driving range and battery charge levels, incorrect estimates of remaining time for a full charge, mismatches between actual and required temperatures, and over- or under-voltage DC/DC converter modules. While some protection features can help mitigate these issues, they are not permanent solutions; addressing the root cause is essential.
There are two basic types of errors that can occur: static errors and dynamic errors. Static errors occur when there is a difference between the actual value and the measurement of a sensor that measures a static or slowly changing parameter. Dynamic errors occur when the sensor measures a distorted or noise-sensitive parameter. Static errors include gain error, offset error, and nonlinearity. To effectively measure the different components of static errors, they need to be converted to standard units; for a BMS in an electric vehicle, this is the full-scale percentage of the maximum battery capacity. The square roots of these errors are then summed to represent the data in a statistically accurate manner.
Other key systems requiring precise measurement include on-board charging systems, DC/DC converters, traction inverters, and HVAC systems. For each of these systems, power parameters are measured at different stages of power conversion to maintain redundancy and accuracy.
