This material focuses on three core technical aspects of the power battery management system (BMS) for buses: high-current balancing strategies, thermal management control logic, and fault warning thresholds based on extreme values. Buses are characterized by long operating hours, high charge/discharge rates, heavy loads, and complex road conditions; these factors impose stricter safety requirements on the performance metrics of the BMS. With the upcoming implementation of the national standard GB 38031-2025 ("Safety Requirements for Power Batteries for Electric Vehicles") on July 1, 2026, the thermal propagation test criteria have been revised. The requirement has shifted from merely "providing a thermal event warning signal five minutes prior to fire or explosion" to "no fire or explosion (though a warning is still required) and ensuring smoke does not harm occupants," presenting a new challenge for the safety management capabilities of the BMS.

I. High-Current Balancing Strategies

1.1 Basic Concepts of Balancing Strategies

In bus battery packs, multiple cells are connected in series to form a high-voltage pack. Due to unavoidable differences in capacity, internal resistance, and aging rates among cells, disparities accumulate as charge/discharge cycles progress, leading to "cell mismatch." Without effective balancing management, the weakest cell reaches voltage limits first—hitting the upper limit too early during charging and the lower limit too early during discharge. Consequently, the BMS is forced to cut off the entire pack to protect the weak cell, preventing the full utilization of healthy cells' capacity and significantly reducing the overall pack capacity utilization rate.

The essence of balancing is to align the energy levels, bring the State of Charge (SOC) values closer together, and equalize the termination voltages across different cells. Balancing methods are primarily categorized into passive balancing and active balancing.

1.2 Passive Balancing (The method used in our BMS)

Passive balancing employs a resistive discharge approach, dissipating excess energy from high-voltage cells as heat through bleed resistors. Typical balancing currents for passive balancing range from 20 mA to 100 mA; this method can only be activated at high charge levels (near full capacity), offers low balancing efficiency, and results in energy loss through heat dissipation. From an engineering design perspective, passive balancing places high demands on component heat resistance and longevity; discharge resistors require a designated heat dissipation path (power resistors with a package size of 2512 or larger are recommended), MOSFET switch arrays require gate series resistors to prevent oscillation, and NTC thermistors are needed to monitor the temperature near the balancing circuitry.

Passive balancing is suitable for scenarios where cell-to-cell variation is minimal; however, for high-capacity, high-C-rate applications like electric buses, the balancing capability of passive balancing is clearly insufficient.

1.3 Active Balancing (Overview)

Active balancing achieves energy transfer between cells using inductors, capacitors, or dedicated chips, "shuttling" excess energy from high-voltage cells to low-voltage cells. It can operate during charging, discharging, and idle states; balancing currents can reach the ampere level, and energy transfer efficiency ranges from 85% to 95%.

There are three mainstream active balancing schemes:

Inductor-based active balancing: Uses power inductors (10–68 µH) for energy storage and transfer; capable of transferring energy between adjacent cells, with mass-produced balancing currents reaching up to 5A.

Capacitor-based active balancing (charge shuttling): Uses high-capacity non-polarized capacitors as energy transfer carriers; balancing efficiency is limited, with mass-produced balancing currents around 300mA.

DC/DC bidirectional balancing: Enables energy transfer between any two cells with excellent balancing results; mass-produced balancing currents can reach 5A, making it suitable for large-scale traction battery packs.

1.4 Special balancing requirements for high-capacity bus battery packs

(1) High balancing current requirement. Bus battery packs typically exceed 100 kWh in capacity. Based on the rule of thumb: Balancing Current ≈ Battery Pack Capacity × Max ΔSOC ÷ Balancing Time. Active balancing should be prioritized for battery packs exceeding 10 kWh. In actual bus operation, the time window for balancing is limited, necessitating high-current balancing capabilities. Currently, commercial bus BMS slave modules include mature products supporting 5A bidirectional active balancing, which are widely used in pure electric buses and hybrid buses.

(2) Balancing must remain active throughout the entire operation cycle. Buses operate for extended periods during the day and undergo centralized charging at night, leaving a limited window for cell balancing; consequently, balancing must be active throughout the entire charging, discharging, and resting phases to promptly correct disparities between cells.

(3) Adaptation to battery aging characteristics. After 200–300 cycles, disparities between battery cells tend to widen. The performance gap between active and passive balancing schemes increases as the battery ages; therefore, the bus BMS should employ an active balancing solution to address cell mismatch issues during long-term operation.

(4) Engineering design considerations for balancing strategies. High-current balancing imposes stringent thermal management requirements: inductor saturation can cause balancing current to drop; high-frequency MOSFET switching demands superior switching speeds and gate-driving capabilities; and Schottky diodes must be managed to prevent reverse leakage and excessive temperature rise. BMS design must incorporate adequate heat dissipation paths and implement real-time temperature monitoring for the balancing circuitry.