The performance of the EV battery system is a key factor in a battery electric bus's overall efficiency, range, and reliability. Since battery temperature directly affects performance, lifespan, and safety, a well-designed Battery Thermal Management System (BTMS) is crucial for optimizing operation and ensuring long-term durability. This article explores the structure and working principles of common battery thermal management systems in BEBs, offering valuable insights for their design.
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Temperature has a significant impact on the performance of lithium-ion batteries:
The battery thermal management system regulates battery temperature through cooling or heating methods. The two main tasks of the BTMS are:
The primary focus of the thermal management system is cooling, while heating and insulation are considerations for lithium-ion batteries operating in extremely cold regions.
The common cooling methods for EV batteries include natural air cooling, forced air cooling, liquid cooling, and direct refrigerant cooling.
A comparison of different cooling methods is as follows:
Although directly refrigerant cooling offers high heat exchange efficiency, in electric buses which have large battery capacity and multiple battery packs, the piping layout for direct refrigerant cooling is complex and poses a risk of leakage.
The common battery heating methods include:
Directly heats the battery cells inside the battery pack.
The effectiveness of this heating method depends on the ambient temperature. When the ambient temperature is above 0°C, the electric heating film works efficiently, does not take up extra space, requires no additional control, consumes no energy, and is cost-effective and easy to implement.
However, when the ambient temperature drops below 0°C, its heating efficiency significantly decreases, making it generally unsuitable for cold environments.
A liquid heater is integrated into the system’s coolant circuit to heat the antifreeze liquid, which then transfers heat to the battery cells.
This method provides effective and uniform heating, has a compact design, and only requires minimal additional space. While the cost is relatively higher than electric heating films, it offers a mature control system, high process reliability, and ease of implementation.
Due to its efficiency and reliability, liquid heating is currently the most commonly used battery heating solution.
To maintain optimal performance and longevity, EV traction batteries in electric buses operate within an ideal temperature range of 25°C ± 5°C, regardless of seasonal variations. In winter, the battery thermal management system heats the coolant to maintain the target temperature of 25°C ± 5°C. In summer, the system cools the coolant to the same temperature range to prevent overheating.
Electric buses commonly use three types of liquid-based thermal management systems, which integrate both heating and cooling functions. These systems adjust the battery temperature based on environmental conditions and operational needs:
A basic BTMS unit consists of the plate heat exchanger, water pump, fan, and PTC electric liquid heater.
When the basic BTMS receives a cooling signal:
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When the system receives a heating signal:
In addition to heating and cooling, the water-cooled unit also features a self-circulation function, which helps regulate temperature differences within the battery pack.
The basic BTMS unit is simple in structure and relatively inexpensive. However, because it lacks an independent refrigeration system and relies on cool air from the cabin for cooling, it has limited cooling capacity. Additionally, its cooling performance depends on the air conditioning system, which restricts its application. Due to its low cooling power (typically less than 2 kW), this system is suitable for hybrid buses with slow-charging batteries that operate at low charge-discharge rates.
A non-independent battery thermal management system integrates with the vehicle’s air conditioning system to manage battery temperature.
Both evaporators are arranged in parallel, sharing a common compressor, condenser, and dryer bottle. The refrigerant flow is managed as follows:
Operation Modes
The non-independent unit does not require a separate cooling system, which helps reduce the cost of thermal management equipment. However, it has several limitations:
An independent unit functions as a small-scale, fully electric air conditioning system, with a complete, self-contained refrigeration system.
The independent unit requires a dedicated cooling system. When the system receives a cooling signal, the fan and water pump start operating. The refrigerant exchanges heat with the antifreeze coolant inside the unit's plate heat exchanger. The cooled antifreeze is then pumped into the battery’s internal heat exchanger, effectively lowering the battery temperature.
For heating, when the system receives a heating signal, the PTC liquid heater and water pump activate, warming the antifreeze. Following the same circulation principle as cooling, the antifreeze transfers heat to the battery’s internal heat exchange plates, raising the battery temperature.
Independent battery thermal management units can be designed with different power capacities, allowing them to be paired with compressors, evaporators, and other components of corresponding power levels. This makes them adaptable to various cooling demands, providing greater flexibility in application.
Compared to non-independent systems, independent units offer a faster response to temperature changes within the battery, as they do not need to account for the air conditioning requirements of the passenger compartment. Additionally, they allow for more flexible installation.
Unlike non-independent systems, independent units include a dedicated compressor and condenser for cooling, which increases costs. However, since they operate as self-contained systems, their control logic is simpler than that of non-independent setups.
The cooling capacity of independent systems can be adjusted as needed, typically starting at 2 kW or higher. This makes them particularly suitable for hybrid and fully electric buses equipped with fast-charging batteries that require high charge/discharge rates.
At Brogen, we offer both independent liquid-cooled top-mounted BTMS and bottom/skirt-mounted BTMS, designed to meet the thermal management needs of buses ranging from 6 meters to 18 meters, as well as electric locomotive batteries.
For enhanced functionality, an optional PTC liquid heater is available, supporting standby, cooling, heating, and self-circulating modes. Additionally, the system utilizes CAN bus communication, enabling real-time fault self-diagnosis and continuous monitoring by uploading operational status and fault information.
Discover our BTMS solutions here: https://brogenevsolution.com/battery-thermal-management-system-btms/
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