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.

II. Thermal Management Control Logic

2.1 Importance of Thermal Management

Electric buses utilize lithium-ion batteries; excessively low temperatures impair charging/discharging performance and driving range, while excessively high temperatures cause uneven cell degradation and shorten service life. Research indicates that as temperature rises, the rate of battery voltage drop increases, internal chemical reactions intensify, and the self-discharge rate accelerates. Electric buses face increasingly stringent requirements regarding energy density, charging speed, battery lifespan, and safety—all of which impose strict limits on operating temperatures—making the battery thermal management system an effective solution for addressing real-world operational challenges.

2.2 Bus Thermal Management System Architecture

Mainstream bus thermal management systems employ an integrated liquid-based heating and cooling solution. Utilizing liquid circulation, the system provides both heating and cooling capabilities; it automatically selects the appropriate operating mode based on the monitored battery pack inlet temperature and employs intelligent temperature control algorithms to maintain the battery within an optimal operating temperature range.

The water circulation module serves as the core component for direct battery pack cooling and heating, connecting the cold plate, heater, plate heat exchanger, and electronic water pump via piping. A water-ethylene glycol mixture is used as the coolant, offering high heat capacity and a broad operating temperature range. The electronic water pump utilizes PWM control for stepless speed regulation, while the PTC heater—chosen for its simple structure and rapid heating capabilities—ensures reliable startup even in extremely low temperatures.

2.3 Thermal Management Control Logic

The core control logic for the liquid-based heating and cooling system comprises the following steps:

Status Assessment: The BMS acquires battery temperature data and determines whether liquid cooling or heating is required.

Mode Switching: If heating is required, the VCU activates the air conditioning system and electric heater; if cooling is required, it activates the air conditioning system and compressor; if neither is required, the system enters a self-circulation mode.

Prevention of Erroneous Operation: Unified control and management of the heating and cooling functions ensure mutual exclusivity, preventing simultaneous activation of cooling and heating systems.

2.4 Special Requirements for the BMS in Bus Thermal Management

(1) Precise Control Across a Wide Temperature Range. Buses must operate under diverse climatic conditions, ranging from freezing winters to scorching summers. Consequently, the BMS thermal management algorithm must possess adaptive adjustment capabilities to maintain stable battery temperatures across varying ambient temperatures. (2) Multi-point temperature monitoring. NTC thermistors must be strategically placed at multiple locations within the battery pack; monitoring temperature gradients allows for the detection of abnormal temperature rise trends, while validating temperature signals helps prevent false triggers.

(3) Coordinated control of liquid cooling and heating. The thermal management unit operates in coordination with the BMS and VCU. Upon receiving a request from the BMS, the VCU commands the contactors in the high-voltage unit to close and initiates the water pump and relevant heating/cooling equipment after a brief delay, ensuring a timely system response without operational conflicts.

(4) Trend toward active thermal management. Current technological trends involve incorporating current data as a control variable; using current data helps overcome the latency issues associated with relying solely on temperature triggers, thereby enabling active thermal management and effectively ensuring the operational safety of electrical components.

(5) Thermal runaway isolation and early warning. The BMS must work with the thermal management system to disconnect contactors during over-temperature events, while physical flame-retardant barriers between modules within the battery pack are required to prevent thermal propagation. In the event of thermal runaway in a single cell, the system must provide at least 5 minutes of warning time to allow personnel to take safety measures. Furthermore, under the framework of the new national standard GB 38031-2025, thermal propagation test requirements have been upgraded to mandate "no fire and no explosion," necessitating a shift in BMS thermal management strategies from passive warning to active defense.