BTMS for Electric Buses: Selection and Integration Strategy
Shanghai OE Industrial Co., Ltd.
Leading EV Solution Provider for Commercial Vehicles
In our previous article, we explored the structure and working principles of common battery thermal management systems (BTMS) in battery electric buses (BEBs). Read here: https://www.linkedin.com/pulse/battery-thermal-management-systems-electric-bus-overview-llrbc
The BTMS in electric vehicles regulates battery operating temperatures through external equipment, ensuring optimal performance within a suitable temperature range. For lithium batteries, the ideal working temperature is between 20°C and 35°C.
1. Selection of BTMS for Electric Buses
The selection of BTMS must align with the vehicle’s operating conditions and battery placement, ensuring an optimal “thermal comfort” environment to extend the battery lifespan. Below are common BTMS options for electric buses:
1) Basic Battery Thermal Management System
Working Principle:
Cooling: Utilizes cold air from the vehicle’s air conditioning system to exchange heat with circulating antifreeze.
Heating: Uses an electric liquid heater to warm the circulating antifreeze.
The cooled or heated antifreeze then circulates into the battery pack for thermal management.
Advantages:
Lowest cost and simplest system among all BTMS options.
No vapor-compression refrigeration cycle, making it safer.
Limitations:
Requires a vehicle-mounted cooling system to function.
Cooling capacity is limited as air conditioning systems take time to reach low temperatures.
Application:
Typically less than 2 kW cooling power, making it suitable for low-charge/discharge rate, slow-charging hybrid buses.
2) Independent Battery Thermal Management System
Working Principle:
Cooling: Uses a dedicated compressor, condenser, and plate heat exchanger to create a closed-loop refrigeration cycle. The low-temperature, low-pressure refrigerant exchanges heat with circulating antifreeze.
Heating: Uses an electric liquid heater to warm the antifreeze before it is circulated into the battery pack.
Advantages:
Fully independent system with simpler control logic than non-independent units.
Fewer refrigerant connections, reducing potential leak points and improving safety.
Limitations:
Higher cost due to the dedicated compressor and condenser.
Application:
Scalable cooling capacity, typically above 2 kW. Suitable for high charge/discharge rate, fast-charging hybrid, and fully electric buses.
3) Non-Independent Battery Thermal Management System
Working Principle:
Cooling: Uses low-temperature, low-pressure refrigerant from an external cooling system to exchange heat with circulating antifreeze in a plate heat exchanger.
Heating: Uses an electric liquid heater to warm the antifreeze before circulation.
Advantages:
Utilizes an existing cooling system, reducing hardware costs.
Limitations:
Requires a vehicle-mounted cooling system to function.
More complex control logic compared to independent units, as it must balance battery thermal management with overall vehicle cooling needs.
Typically above 6 kW cooling power due to minimum operating frequency constraints of variable-speed compressors.
Application:
Suitable for high charge/discharge rate, fast-charging fully electric buses.
2. Layout of BTMS for Electric Buses
2.1 Basic Principles of BTMS Layout
The layout of BTMS is closely related to the placement of the battery itself. The following principles should be followed when arranging the system:
Proximity to the battery placement
The BTMS 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 BTMS
For independent BTMS, 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 BTMS, 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 of coolant piping
The pipes should be as short as possible with large turning radii to reduce flow resistance. Insulation should be applied to minimize heat loss during antifreeze circulation. Pipe connectors should be made of stainless steel or nylon, and copper should not be used to prevent corrosion of the cooling plates inside the battery box, ensuring no coolant leakage occurs.
Ease of maintenance
The layout should facilitate easy access for inspection and repairs of the system.
Below is a general schematic diagram of the battery thermal management system.
2.2 Advantages and Disadvantages of Different BTMS Layouts
2.2.1 Roof-Mounted Layout
The following picture shows the roof-mounted layout of an independent battery thermal management system in a pure electric bus. The system includes a cooling unit, water-cooling circulation system, electric liquid heater, and water pump, all installed on a roof-mounted support frame along with the battery pack. The support frame is fixed to the roof using pre-installed bolts embedded in the roof skeleton.
Design Considerations:
The water pump should be positioned at the lowest point of the coolant loop.
The height difference between the expansion tank and the coolant circuit should be maximized to ensure proper coolant circulation.
Since the support frame is removable, the thermal management system and its piping can be assembled on the ground before being lifted and secured onto the roof. This provides more space for assembly, improving efficiency.
Advantages:
Cleaner air environment – Less dust accumulates on the condenser surface, preventing heat exchange efficiency loss.
No impact on the vehicle’s structural space – The system does not require modifications to the main body structure, allowing for more flexible integration.
Optimized coolant circulation – The cooling pipes remain mostly on the same plane, reducing resistance caused by uneven piping and ensuring uniform water flow through all battery packs, helping maintain battery temperature consistency.
Disadvantages:
Increased structural load – The roof skeleton and side frames must bear a higher load, requiring stronger body frame strength.
Higher vehicle center of gravity – This affects vehicle stability.
Risk of exceeding height limits – The additional height may increase the risk of the vehicle exceeding road height restrictions.
2.2.2 Bottom-Mounted Layout
The following pictrue shows a bottom-mounted layout where the cooling unit, water-cooling circulation system, and water pump are all fixed to the chassis, with piping routed underneath.
Advantages:
Lower center of gravity – Improves vehicle stability.
Aesthetic benefits – The system is hidden, maintaining a sleek exterior design.
No interference with rooftop air conditioning or sunroof layout.
Disadvantages:
Inconsistent coolant pipe heights – The piping must pass through the chassis, creating uneven elevation differences, which adds resistance and reduces water flow consistency among battery packs. Poor layout may cause temperature variations between batteries.
Limited installation space – Modifications to the vehicle body and chassis are required.
Water ingress concerns – Since high-voltage components are installed at the bottom, the system must account for the vehicle’s wading depth.
Difficult maintenance – The tight space makes repairs more challenging compared to other layouts.
2.2.3 Rear-Mounted Layout
The following picture illustrates a rear-mounted layout in a hybrid bus, where the cooling unit, water-cooling circulation system, electric liquid heater, and water pump are integrated with the battery pack in the rear compartment.
Advantages:
No impact on vehicle exterior design.
Easier maintenance – Rear compartment placement provides more convenient access for servicing and repairs.
Lower external heat load – Compared to the bottom-mounted system, the rear-mounted system is less exposed to ground heat. Compared to the top-mounted system, it is less affected by solar radiation.
No impact on wading depth – Unlike the bottom-mounted layout, high-voltage components are kept away from potential water exposure.
Disadvantages:
Reduced passenger space – The rear compartment takes up space that could otherwise be used for seating.
Affects vehicle axle load distribution – Additional weight at the rear changes the vehicle’s balance and load distribution.
Conclusion
Currently, the most common types and layouts of battery thermal management systems in electric buses are those discussed in this article. As a critical component, the battery thermal management system serves as a "guardian," ensuring optimal performance, safety, and longevity of the power battery. Therefore, gaining a comprehensive understanding of its various types and configurations is essential for effective application.
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/
Business inquiry: contact@brogenEVSolution.com