Understanding Electric Bus Battery Management Systems
The Battery Management System (BMS) is the primary electronic control and protection system for an electric bus high-voltage battery. It continuously monitors individual cell voltages, battery temperatures, pack current and other operating conditions to keep the battery within safe operating limits. The BMS also communicates battery status and allowable charge and discharge limits to other vehicle controllers.
What Does a Battery Management System Monitor?
A battery management system monitors individual cell voltages and temperatures throughout the battery pack. It compares cells to identify voltage imbalance, overvoltage, undervoltage, excessive temperature and abnormal operating conditions. This information allows the BMS to protect individual cells while also managing the battery pack as a complete energy-storage system.
Cell Voltage and Temperature Monitoring
Cell voltage monitoring is one of the most important BMS functions. The BMS measures the voltage of individual cells or cell groups and compares them with one another. A cell that becomes significantly higher or lower than the others can indicate imbalance, deterioration or an developing fault. Temperature sensors provide additional protection by allowing the BMS to detect overheating, excessive temperature differences and operating conditions that could damage the battery.
State of Charge and State of Health
The BMS calculates State of Charge (SOC) to estimate how much usable energy remains in the battery. SOC is similar to a fuel gauge and is calculated using cell voltage, current flow, temperature and accumulated charge and discharge data. State of Health (SOH) represents the battery's condition compared with its original capability. SOH can be affected by battery age, charge and discharge cycles, temperature exposure, cell imbalance and operating history.
Contactors, Precharge and Pack Protection
The BMS controls and supervises the high-voltage battery contactors that connect the battery pack to the vehicle high-voltage bus. During startup, the precharge circuit gradually raises bus voltage before the main contactors close, limiting damaging inrush current. The BMS verifies voltage conditions and contactor operation before allowing the pack to energize. If an unsafe condition is detected, the BMS can prevent contactor closure or command the contactors to open to protect the battery and vehicle.
Cell Balancing and BMS Fault Detection
Cell balancing helps keep the individual cells within a battery pack at similar states of charge. The BMS identifies cells that are becoming higher or lower than the rest of the pack and may use balancing circuits to reduce these differences. The BMS also detects faults involving cell voltage, temperature, current, contactors, communication and other battery conditions. Diagnostic trouble codes and live BMS data can help technicians identify whether a problem is associated with an individual cell, battery module, sensor, wiring circuit or another part of the high-voltage battery system.