Thermal loads are heat sources. The most significant thermal loads in EVs are the major powertrain components, including the battery pack, motor, power electronics, and charger.
Additional thermal loads include the cabin heating, ventilation, and air conditioning (HVAC) system, brakes, in-car electronics, solar radiation through glazed areas of the cabin, and passenger metabolic load.
Thermal management is critical to the safe, efficient, and reliable operation of an EV. Maintaining an optimal temperature is particularly important for the battery pack, which experiences short-term performance loss at low temperatures and permanent degradation if operated at high temperatures.
The battery pack is one of the most significant heat sources in an EV, and also one of the most sensitive to temperature. At low temperatures, battery packs may require pre-heating to achieve the required performance.
During driving and charging, heat is generated, which must be dissipated to ensure the temperature doesn’t rise to a level where battery degradation occurs, or there may be a risk of thermal runaway. The optimum operating temperature for common batteries is between 15° and 35° C.
The electrochemical reactions within batteries can result in temperature gradients across the battery, which can cause stress and separator tearing.
When a battery is operated at higher temperatures with rapid rates of charge/discharge, dendrite formation and separator piercing occur, causing significant performance loss. At extremely high temperatures off-gassing and separator collapse can result in thermal runaway, resulting in fire and explosion.
A battery thermal management system (BTMS) is a cooling system that may use liquid cooling, air cooling, or phase change materials. The electric motor and power electronics are also significant thermal loads, requiring cooling to maintain good performance and service life.
Modern EVs use motors with a high power density, high torque density, and high speed. These characteristics tend to infer high thermal load, with the potential for short circuits, demagnetization, and other degradation issues. If the motor is not cooled, this performance loss can occur rapidly. Motors may be air or liquid-cooled.
Power electronics are also susceptible to damage if operated at elevated temperatures. Capacitors are a particular source of failure, with overheating accelerating electrolyte evaporation, reducing the life of these components.
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