Battery Cooling Systems Explained

1. Why Do EV Batteries Need Cooling?

During battery charging and discharging, lithium ions shuttle between the cathode and anode, and the rapid movement of electrons inevitably generates heat. The higher the discharge rate, the more heat is produced. Crucially, lithium-ion batteries are exceptionally sensitive to temperature—the thermal management system must maintain the battery pack within an optimum operating window of 20°C to 40°C, whilst keeping the internal temperature variance to no more than 5°C.

If the temperature variance across the pack is too wide, individual battery cells will charge and discharge at uneven rates, degrading overall pack performance; if the battery overheats or suffers from uneven thermal distribution, severe issues such as accelerated capacity fade, thermal runaway, and even fire or explosion risks may occur. Therefore, an efficient cooling system is paramount to safeguarding battery health and extending its service life.

2. Overview of Battery Cooling Methods

Thermal management systems for high-voltage traction batteries are broadly categorised into four types: natural cooling, air cooling, liquid cooling, and direct refrigerant cooling. Among these, natural cooling is a passive approach, whereas air, liquid, and direct cooling are active solutions, distinguished primarily by their heat transfer mediums. In terms of heat dissipation efficiency, they rank as follows: Natural Cooling < Forced Air Cooling < Liquid Cooling < Direct Refrigerant Cooling.

3. Air Cooling — Simple and Low-Cost, but Reaching Its Limits

Air cooling utilises ambient air as the heat exchange medium and is divided into two formats: passive air cooling (drawing in outside air directly) and active air cooling (conditioning ambient air via the HVAC system before routing it through the battery enclosure). Its operating principle relies on natural airflow or dedicated blower fans paired with an evaporator to lower battery temperatures.

Thanks to its straightforward design and low production cost, air cooling was widely adopted in early mass-market electric passenger cars, such as the Nissan Leaf and Kia Soul EV; today, it is still commonly found in entry-level urban micro-EVs (such as the Wuling Hongguang MINI EV), as well as select electric buses and light commercial delivery vans.

Nevertheless, the drawbacks of air cooling are clear. Its thermal dissipation capacity can no longer keep up with the high-power fast charging and heavy discharge requirements of modern EVs; furthermore, it is heavily dependent on ambient weather conditions, with heat transfer efficiency dropping sharply in severe heat or freezing climates.

4. Liquid Cooling — The Current Industry Mainstream

Liquid cooling employs an antifreeze solution (typically an ethylene glycol-water mixture) as the heat exchange medium. Coolant circulates through internal battery cooling channels to draw away heat generated during operation; in cold climates, the system can also preheat the battery pack to its optimal operating temperature.

System Architecture: A typical liquid cooling circuit comprises water cooling blocks (cold plates), circulating coolant, an electric water pump, piping, an expansion tank, and a heat exchanger (chiller/radiator). The cold plate—fabricated from aluminium or copper with internal channels—makes direct contact with the cells to absorb heat; the water pump circulates the coolant through the circuit, and the heat exchanger expels the heat into the surrounding air. Temperature sensors continuously monitor the pack in real time, allowing the controller to dynamically regulate pump speed and flow distribution.

Key Technical Solutions:

• Serpentine ribbon tubing: Popularised by Tesla, where flat, snake-like cooling ribbons snake between individual cylindrical cells, maximising the surface area available for heat transfer.

• Cooling fin setup: Featured in models like the Chevrolet Bolt, ultra-thin cooling fins (measuring just 1 mm thick) are interleaved between pouch cells to boost surface area and heat transfer rates.

• Multi-circuit switching: Advanced EVs feature multi-loop cooling circuits (radiator loop, air-conditioning chiller loop, PTC heater loop), which are dynamically switched by the Battery Management System (BMS) based on real-time thermal strategies. The Tesla Model S, for instance, employs a switchable series/parallel cooling layout for its electric motor and battery pack—connected in series at low temperatures so motor waste heat warms the battery, and switched to parallel under high loads to cool both powertrains independently.

Advantages: Compared to air, liquid coolant delivers dozens of times higher specific heat capacity and a significantly superior heat transfer coefficient; by substantially reducing operating temperatures and cell-to-cell thermal variance, overall pack efficiency, stability, and durability are noticeably enhanced; liquid cooling provides a well-balanced benchmark across thermal dissipation, temperature uniformity, and sealing reliability.

Disadvantages: Complex hardware layout that adds weight and volume to the battery pack, eating into cabin and boot packaging space whilst reducing overall volumetric energy density; manufacturing and maintenance costs are also higher.

5. Direct Refrigerant Cooling — Highly Efficient and Ultra-Compact

Direct cooling uses air conditioning refrigerant (such as R134a or R1234yf) directly as the heat transfer medium. Its fundamental principle leverages the massive latent heat absorbed during the refrigerant's liquid-to-gas phase change—since latent heat of evaporation is significantly greater than sensible heat capacity—which can increase thermal exchange efficiency by over three times compared to conventional coolant solutions.

System Architecture: Direct cooling taps directly into the vehicle's onboard air conditioning system, primarily comprising an electric compressor, dual evaporators, a condenser, dual electronic expansion valves (EXVs), refrigerant lines, and a receiver-drier.

Direct cooling can be more than three times more efficient than cold-plate liquid cooling, ensuring the battery remains in its optimal temperature band even under prolonged high-load driving or DC fast charging; meanwhile, eliminating separate water pumps, reservoirs, and coolant plumbing reduces system complexity and saves valuable packaging space. Direct cooling stands out for being compact, lightweight, and thermally efficient.

In terms of thermal uniformity, direct cooling performs exceptionally well—bench testing shows that under a standard 25°C ambient temperature with a 500W thermal load, the maximum temperature variance across the cold plate surface can be maintained within just 0.4°C.

Direct refrigerant cooling is already deployed in various production vehicles, including the entire BYD DM-i hybrid range, BYD e-Platform 3.0 models (such as the Dolphin), BMW i3, and Jeep 4xe PHEV models.

Disadvantages: Because it operates as a dual-evaporator system, it cannot directly heat the battery pack in sub-zero conditions (requiring a secondary heating method); precise control of refrigerant temperature and pressure is more demanding, and system longevity requires rigorous validation. However, data from CATL indicates that in -10°C ambient temperatures, direct cooling consumes 35% less heating energy than liquid setups because it can harness heat pump thermal energy directly, eliminating the need for high-draw PTC auxiliary heaters.

6. Emerging Next-Generation Technologies

Beyond the three mainstream active cooling methods, several cutting-edge thermal management technologies are gaining serious momentum:

• Phase Change Material (PCM) cooling: Harnesses latent heat absorption during material phase transitions to keep battery temperatures within an ideal range for extended periods without parasitic electrical draw. However, because its latent capacity is finite, it is generally paired alongside active cooling systems.

• Immersion liquid cooling: Directly submerges the battery cells in a non-conductive dielectric fluid, utilising convective liquid flow to draw away heat. It offers a compact layout, ultra-fast heat dissipation, and class-leading temperature uniformity; crucially, it acts as an effective fire suppressant—if a single cell experiences thermal runaway, heat is instantaneously dissipated into the surrounding fluid, which itself possesses flame-retardant properties.

• Coupled thermal management systems: Combines multiple cooling methodologies (e.g., PCM + liquid cooling or immersion cooling + PCM) to leverage their complementary strengths. Lightweight construction, thermal safety, robust reliability, integrated cooling architectures, and intelligent predictive control represent the key focal points of future EV R&D.

7. Summary

The battery thermal management system is the vital cornerstone of electric and electrified vehicles. From early air cooling to today's dominant liquid cooling architectures, and onward to the rapid rise of direct refrigerant cooling, cooling technologies are steadily evolving towards higher efficiency, tighter packaging, and smarter control. Liquid cooling will remain the mainstream benchmark for the foreseeable future thanks to its balanced performance and mature supply chain; meanwhile, advanced solutions like direct cooling and immersion cooling represent the definitive frontier for next-generation EV battery thermal management.