Battery Thermal Management System (BTMS) for Electric Buses: Selection and Integration Strategy
Battery Thermal Management System (BTMS) for Electric Buses: Selection and Integration Strategy This article introduces common types and configurations of battery thermal management systems (BTMS) for electric buses. As a critical component of electric buses, the BTMS acts as a “guardian” for the battery, ensuring its performance, safety, and longevity. Therefore, a thorough understanding of the different BTMS types and layouts is essential for practical applications. 1. Selection of the Battery Thermal Management System (BTMS) for Electric Buses The battery thermal management system (BTMS) for electric buses regulates the battery’s operating temperature through external equipment, ensuring that the battery always functions within an optimal temperature range. For lithium batteries, the ideal working temperature is between 20°C and 35°C. When the temperature is too low, battery capacity decreases, and power performance declines. When the temperature is too high, the risk of self-discharge increases, and internal side reactions become more frequent, reducing the available battery capacity and decreasing its lifespan and efficiency. Battery thermal management involves cooling the battery in summer to prevent irreversible thermal reactions that could cause safety issues. In winter, it heats the battery to maintain charging and discharging performance while preventing lithium plating at the anode, which could lead to internal short circuits. The selection of battery thermal management equipment should be based on the vehicle’s operating conditions and battery placement to meet the thermal management requirements. Ensuring that the battery remains in an optimal “comfort zone” helps improve its lifespan. Below are common battery thermal management solutions for buses. 1.1 Basic Battery Thermal Management System The basic BTMS unit directs air-conditioned cold air into the unit to exchange heat with circulating antifreeze for cooling. For heating, it uses an electric liquid heater to warm the antifreeze, which is then circulated to the battery pack. After cooling or heating, the antifreeze enters the battery compartment to regulate the battery’s temperature, keeping it within the desired range. Compared to independent and non-independent BTMS units, the basic unit is the most cost-effective and simplest system. It is also relatively safe, as it does not use a vapor compression refrigeration cycle. However, since it relies on the vehicle’s air conditioning system for cooling, it requires the installation of a cooling system. Additionally, when the cooling system first starts, the cold air temperature is relatively high, leading to poor initial cooling performance. The cooling power is generally below 2 kW, making this solution suitable for hybrid buses with slow-charging battery packs and low charge/discharge rates. Basic BTMS Unit System Schematic Diagram Basic BTMS Unit System Structure 1.2 Independent Battery Thermal Management System The independent BTMS unit includes its own compressor, condenser, and plate heat exchanger, forming a separate cooling cycle. Cooling is achieved by exchanging heat between low-temperature, low-pressure refrigerant and the circulating antifreeze in the heat exchanger. Heating is done via an electric liquid heater that warms the antifreeze before circulating it to the battery pack. Compared to the non-independent unit, this system has an additional dedicated compressor and condenser, increasing costs. However, since it operates independently, it features simpler control logic and fewer refrigerant connectors, making it relatively safer. The cooling power of an independent unit is generally above 2 kW, making it suitable for hybrid and fully electric buses with fast-charging battery packs and high charge/discharge rates. Independent BTMS Unit System Schematic Diagram 1.3 Non-Independent Battery Thermal Management System The non-independent BTMS utilizes an external cooling system, where low-temperature, low-pressure refrigerant produced by another cooling device exchanges heat with circulating antifreeze in a plate heat exchanger. Heating is performed using an electric liquid heater, which warms the antifreeze before it circulates to the battery pack. Since it shares the vehicle’s cooling system, it requires the installation of a refrigeration system. Additionally, due to the variable-frequency compressor’s minimum frequency limit, the power output is relatively high, typically above 6 kW. Compared to an independent unit, this system has a more complex control logic due to potential conflicts between battery thermal management and vehicle air conditioning demands. The non-independent unit is suitable for fully electric buses with fast-charging battery packs and high charge/discharge rates. Non-Independent Unit System Schematic Diagram 2. Layout of Battery Thermal Management System for Electric Buses 2.1 Basic Principles for BTMS Layout The layout of battery thermal management equipment is closely related to the placement of the battery itself. The following principles should be followed when arranging the equipment: Proximity to the battery placement – The thermal management equipment should be installed as close as possible to the battery, whether the battery is mounted on the top, bottom, or rear of the vehicle. At the same time, potential disadvantages associated with the chosen placement should be minimized. Installation requirements for different types of equipment – For independent battery thermal management systems, vibration-damping rubber pads should be added during installation. The condenser’s air intake and exhaust must remain unobstructed to prevent air recirculation. For basic thermal management systems, cold air should be drawn from the vehicle’s refrigeration system. The air intake point should be positioned as close as possible to the evaporator outlet of the main cooling system. Coolant circulation considerations – The water pump inlet for circulating antifreeze through the battery box’s cooling plate should be located as close as possible to the expansion tank, which maintains system pressure and allows antifreeze refilling. The expansion tank must be placed at the highest point of the battery cooling system. Additionally, an air vent pipe should be included to remove air released during heating or cooling, preventing difficulties in adding antifreeze. Cooling circuit for multiple battery groups – To minimize temperature differences between different battery packs, the cooling circuit should be arranged in a parallel configuration. Each individual branch should not exceed three battery boxes per loop. PTC Electric Liquid Heater Placement – If a PTC electric liquid heater is installed, it should be positioned downstream of the water pump at a lower point in the cooling circuit. It must not be placed at the highest point of the coolant loop. Optimization