Direct Cooling vs. Liquid Cooling for New Energy Vehicle Battery Packs:
Principles, Advantages, and Disadvantages
I. Why are batteries "sensitive to both heat and cold"?
The performance of power batteries is strongly correlated with temperature: Above 45℃, capacity decay accelerates, and cycle life may be shortened by more than 30%; above 60℃, electrolyte decomposes, SEI film ruptures, and in severe cases, thermal runaway is triggered. At low temperatures, internal resistance increases, lithium plating occurs during charging, and range is reduced. The optimal operating range for lithium-ion batteries is generally considered to be 25–40℃, and in engineering, a cell temperature difference of <5℃ within the pack is generally considered an optimization target (high-end models aim for <2℃), but this is not a mandatory national standard. The three main tasks of a thermal management system are: heat dissipation, heating, and maintaining temperature consistency.
Currently, there are four main types of active cooling for passenger vehicles, ranked from most common to most aggressive:
• Passive air cooling (natural cooling): Relies on natural convection through the battery pack's heat exchange fins. It consumes no power and has the lowest cost, but heat dissipation is extremely slow. It has been phased out of mainstream passenger vehicles and is only found in early low-speed cars.
• Active air cooling: Uses a fan to force convection. It has a simple structure and is inexpensive to maintain, but the thermal conductivity of air is only 0.026 W/(m·K), resulting in low efficiency in high temperatures. It is mostly found in entry-level cars priced below 100,000 RMB (such as some Wuling Hongguang MINIs and BAIC New Energy vehicles).
• Liquid cooling: Currently standard in mid-to-high-end vehicles. The coolant (ethylene glycol aqueous solution) circulates to remove heat, accommodating both fast-charging heat dissipation and low-temperature heating, and holds the mainstream market share.
• Direct refrigerant cooling: The air conditioning refrigerant is directly introduced into the evaporator inside the battery pack, achieving "zero-distance" phase change heat absorption, resulting in the highest efficiency.
▸ Immersion Cooling (Emerging Technology): Small-batch installations after 2024, large-scale mass production not yet available; the battery cells are immersed in insulating coolant (fluorinated liquid/synthetic ester), resulting in the most uniform heat dissipation, but with high cost and maintenance difficulty; currently only a small number of models are using it as a pilot option.
Liquid Cooling: Basic Concepts and Principles
Liquid cooling uses a coolant (usually an aqueous solution of ethylene glycol, with a thermal conductivity of approximately 0.5 W/(m·K)) as the heat exchange medium. A water pump drives the coolant to circulate in the channels within the battery pack, absorbing heat before reaching the Chiller (battery cooler) to exchange heat with the air conditioning refrigerant for cooling, and then returning to the cooling plate. Core components: Electric compressor → Chiller → Electric water pump → Liquid cooling plate → Valve body (four-way/eight-way) → PTC heater.
Advantages
• Optimal temperature uniformity, controlling cell temperature difference within ±2℃, directly extending lifespan and improving consistency;
• Mature technology and well-established supply chain, representing the current mainstream solution in the market;
• Deep integration with vehicle thermal management (motor waste heat recovery, cabin heat pump), and the ability to add PTC for low-temperature heating;
• Modularly replaceable cold plates for easy maintenance.
Disadvantages
• Complex system with more components, resulting in approximately 15%–20% higher initial costs;
• An additional water pump and coolant circuit increases vehicle weight, and the water pump's power consumption affects range;
• Risk of coolant leakage, requiring regular coolant level checks;
• Indirect heat exchange results in slower response speeds compared to direct cooling at high charging rates.
Direct Cooling: Basic Concepts and Principles
Refrigerant Direct Cooling highly couples the battery cooling system with the air conditioning system-essentially placing the air conditioner's evaporator directly into the battery pack. The refrigerant (mainstream R134a, R1234yf, latent heat of phase change > 200 kJ/kg) evaporates and absorbs heat within the direct-cooling plate, directly carrying away heat from the battery cell. Core components: electric compressor → condenser → electronic expansion valve → direct-cooling plate → gas-liquid separator, eliminating the need for a water pump, coolant circuit, and chiller.
Advantages
• Eliminates the secondary heat exchange stage, reducing thermal resistance by approximately 40%, and increasing heat exchange efficiency (heat exchange stage) by 20%–50% compared to liquid cooling (multiple simulations and bench tests, varying with operating conditions; this refers to the reduction in thermal resistance in the heat exchange stage, not necessarily a corresponding improvement in overall vehicle energy efficiency);
• Lighter and more compact system (approximately 30% smaller volume), beneficial for vehicles with chassis limitations;
• Eliminates the continuous power consumption of the water pump, potentially reducing energy consumption by approximately 0.8 kWh per 100 kilometers under the same operating conditions (single-vehicle simulation estimate, fluctuating with road conditions and ambient temperature, for reference only);
• Compatible with 800V super-fast charging, capable of sustained 4C or even higher rates without overheating.
Disadvantages
• Direct cooling plates have extremely high pressure resistance and sealing requirements, resulting in high manufacturing and assembly barriers;
• Two-phase flow makes temperature uniformity difficult to control: Under low temperature and low load conditions, the refrigerant is prone to uneven gas-liquid distribution within the direct cooling plate, leading to localized overcooling or insufficient heat exchange, and a significant decrease in temperature uniformity;
• Refrigerant leakage will directly contaminate the battery module; high integration means that repairs often require complete replacement, resulting in high costs;
• Control strategies are complex (anti-frost, anti-overcooling), and the technology maturity is lower than that of liquid cooling.
