There are many concepts with advantages and disadvantages for battery cooling in electric vehicles. Here we provide an overview of cooling systems, their application and interfaces to thermal management and control.
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Electric vehicles typically use lithium-ion batteries. The batteries must be operated within a "comfort zone". If the battery is not within this range, its life will be reduced. In addition, if the temperature is too low or too high, the battery cannot provide the required power. If the temperature is too high, it can even be dangerous: it can lead to self-heating and thus to thermal runaway of the battery, in the worst case to the burning of the vehicle.
Lithium-ion batteries differ in their cell chemistry and therefore in their temperature characteristics. The "comfort zone" is typically between 20 and 40 °C. Nickel-manganese-cobalt (NMC) and nickel-cobalt-aluminum (NCA) cells achieve acceptable efficiency even at 0 °C, so the energy that cannot be extracted from the battery is at an acceptable level. Cheaper lithium iron phosphate (LFP) cells are less efficient at such low temperatures. Solid-state batteries promise a much longer range but are still in the research stage. The ranges should not be considered fixed. The efficiency of a cell can continue to increase at temperatures above 40 °C while aging effects increase.
Cooling concepts can be differentiated according to the coolant used and the design, with the largest market share currently held by cooling systems that use water-glycol mixtures as the coolant. The battery is cooled by one or more cooling plates through which the coolant flows. The coolant heats up and transfers the heat to another fluid in a heat exchanger. At low ambient temperatures and low cooling capacity, the heat can be transferred to the ambient air via an ambient heat exchanger in the front end of the vehicle. At high ambient temperatures, the heat cannot be transferred to the air but must be transferred to a refrigerant cycle. This is typically done in a plate heat exchanger called chiller. Refrigerant evaporates in the chiller, absorbing heat from the refrigerant. The refrigerant is then compressed to a higher pressure and temperature level. From this level, the heat can again be transferred to the ambient air. Because the heat is transferred first to a single-phase coolant and then to a refrigerant, this is called an indirect cooling system.
In contrast, direct cooling systems allow the evaporating refrigerant to flow through the cooling plate(s). Direct cooling systems have the great advantage that the temperature of the refrigerant does not change during evaporation. As a result, the battery is cooled uniformly. Another advantage is that no chiller is required. However, the piping in the vehicle for the volatile refrigerant is more complex than for the single-phase coolant, and the amount of refrigerant required is higher than for indirect cooling systems.
In addition to the choice of fluid, the above concepts also differ in how the cooling plates are attached. A cooling plate can be attached to the battery from above or below in a horizontal position; if high cooling capacity is required, two cooling plates can be used as a sandwich. It is also possible to place many small cooling plates vertically between the individual battery cells — the larger and better distributed the cooling surfaces, the more efficient and homogeneous the cooling. However, the space and weight requirements are obstacles. The design of the battery cells, the electrical contacts and the crash behavior must also be considered when selecting the cooling surfaces.
Immersion cooling systems are currently being developed for high-performance vehicles. Here, the battery is placed in a tank filled with fluid. The fluid can be either a single-phase coolant that is pumped in a cycle with a heat exchanger, or a refrigerant that evaporates due to heat input from the battery and releases the absorbed heat elsewhere through condensation. Immersion cooling systems produce homogeneous battery temperatures and can transfer high heat flows. However, the weight of the fluid, flow control, and space requirements are significant challenges. In addition, many of the fluids in question are environmentally harmful PFAS, the use of which is restricted by the EU.
Passive air cooling systems can be used for small, low-power vehicles. In this case, the battery is cooled only by the airflow, either by the air flowing directly around the battery or by the body indirectly absorbing heat from the battery and releasing it into the air.
When the vehicle has cooled down due to low ambient temperatures, such as when driving after a cold night (< 10 °C), the battery should be heated to its comfort zone. Although the battery heats up automatically during operation, it will age excessively until it reaches a temperature in its comfort zone. Outside of this zone, it can only provide a small amount of power. There are also different concepts for heating. With indirect cooling, the coolant can be heated by an electric heater or heat pump to heat the battery. With direct cooling, the refrigerant can be condensed in the cooling plate.
For example, an intelligent charging control system that slowly charges the battery in the morning before starting the journey can heat the battery using the waste heat from the charging process, reducing the energy required for heating. To increase range, it can also be useful to heat the battery with electricity from the grid before driving.
Battery cooling is part of the vehicle's Battery Thermal Management System (BTMS). The BTMS includes the cooling and heating module, as well as the operating strategy, control system and thermal management software. It has large interfaces with the thermal management of the entire vehicle: among other things, it decides how much cooling capacity is used for battery cooling and how much for the passenger compartment air conditioning, or whether waste heat from the battery can be used as a heat source for a heat pump to heat the passenger compartment.
Operating strategy and control are part of the BTMS. It determines whether the battery needs to be cooled or heated. The inlet temperature of the coolant or the evaporating temperature of the refrigerant and the mass flow of the respective fluid are defined to maintain the comfort zone. The higher-level vehicle thermal management system is responsible for meeting these requirements. How the cooling capacity is provided depends on, among other things, the ambient temperature, the cooling capacity required for the air conditioning, and other cooling capacity requirements of the engine or electronic components.
Intelligent operating and control strategies calculate the heat generated in the battery during operation, estimate the future battery temperature, and plan the heating and cooling capacity in advance.
To design such strategies, models are used that range from the individual battery cell to the representation of cooling cycle and powertrain. They are tested in simulations of the entire vehicle. Learn more about simulation, software and services for battery cooling, thermal management and battery safety.
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