Battery Thermal Management System

Battery Thermal Management System (BTMS) refers to an engineered technical solution that regulates the operating temperature of traction batteries within an optimal range via cooling, heating, and thermal insulation. As a core subsystem of an electric vehicle's Battery Management System (BMS), the BTMS plays a vital role in ensuring battery safety, extending service life, and preserving optimal charging and discharging performance.

Functions and Necessity

The performance of lithium-ion batteries is exceptionally sensitive to operating temperatures. Ideally, battery temperatures should be maintained between 20°C and 50°C, with the maximum temperature variance across modules kept within 5°C. Excessive heat accelerates cell degradation and heightens the risk of thermal runaway, whereas sub-optimal low temperatures reduce ion mobility and usable capacity. Furthermore, cold-weather charging can trigger lithium plating, resulting in permanent, irreversible cell damage.

The primary functions of a BTMS include: precise temperature monitoring and telemetry; effective heat dissipation and ventilation under high thermal loads; rapid pre-heating in low-temperature conditions; hazardous gas venting; and maintaining uniform temperature distribution across the entire battery pack.

Cooling Technology Classifications

Based on the heat transfer medium employed, battery cooling technologies are broadly categorised into the following methods.

Air Cooling uses air as the medium to dissipate heat via natural convection or forced induction (fans/blowers). The system features a simple architecture, low manufacturing costs, and easy maintenance, but offers limited cooling capacity and poor thermal consistency. It was widely adopted in early passenger EVs and remains common in electric buses and commercial logistics vehicles.

Liquid Cooling relies on liquid convection for heat transfer, delivering high heat transfer coefficients, substantial thermal capacity, and rapid cooling rates. Depending on coolant contact methods, it is divided into indirect liquid cooling (using liquid cooling plates mounted against battery surfaces) and direct liquid cooling (immersion cooling, where cells are submerged directly in dielectric fluid). Liquid cooling represents the current mainstream benchmark for electric passenger cars, widely deployed in models from Tesla, BMW (i3), and GM (Volt).

The main engineering challenges of liquid cooling involve system complexity, higher production costs, and potential coolant leakage risks. Because cooling plate channel geometry directly dictates heat dissipation performance, novel designs incorporating biomimetic leaf-vein channels, honeycomb structures, and topology optimisation are being actively researched.

Phase Change Material (PCM) Cooling leverages the latent heat absorbed or released during material state transitions to achieve passive thermal regulation. While PCM requires zero external power consumption and provides excellent thermal uniformity, its relatively low thermal conductivity means it is typically coupled with liquid cooling circuits or heat pipes.

Direct Refrigerant Cooling utilises the latent heat of evaporation by integrating the air-conditioning system evaporator directly into the battery pack, delivering three to four times the cooling efficiency of conventional liquid cooling. However, due to its high engineering complexity, adoption remains limited to select models such as the BMW i3.

Heating and Thermal Insulation

In low-temperature climates, battery heating systems are essential to safe charging. Cold-temperature charging causes lithium ions to form dendrites on the negative electrode, which can puncture the separator and induce internal short circuits. Common heating methods include electrothermal film heaters, PTC heaters, and liquid heating circuits. Heat pump systems represent a highly efficient modern solution, delivering 1.8 to 2.4 times the efficiency of PTC units and halving the driving range penalty associated with heating. Meanwhile, dedicated insulation materials and thermal enclosure designs minimise parasitic thermal management energy consumption.

Active and Passive Management

Thermal management systems are categorised into active and passive setups based on whether they possess active regulation capabilities. Passive systems rely on natural heat transfer; while cost-effective, their performance is heavily influenced by ambient conditions. Active systems proactively regulate temperatures via mechanical or electrical hardware such as fans, pumps, and heaters—delivering superior thermal precision at the expense of higher energy consumption and maintenance overheads.

In automotive engineering, hybrid active-passive architectures represent a key development pathway. Research by the Guangzhou Institute of Energy Conversion (Chinese Academy of Sciences) demonstrates that a combined PCM-liquid cooling system subjected to a high-rate 3C discharge can limit peak battery temperatures to 39.7°C with a maximum variance of just 4.9°C, while slashing coolant pump power consumption by 80.8% compared to continuous liquid cooling.

Technological Frontiers

The technological frontier of battery thermal management is evolving towards greater intelligence and high energy efficiency. Rapid simulation methodologies such as thermal resistance network modelling have slashed thermal simulation runtimes from hours to just tens of seconds, significantly accelerating design cycles. Next-generation systems based on thermoelectric effects can convert battery waste heat into recovered electrical energy while meeting thermal targets, achieving dual "active temperature control" and "energy harvesting" capabilities. Notably, the "phase-change thermosiphon bi-directional heat flow control" dual-loop thermal management technology developed by Henan University of Science and Technology has achieved commercialisation through industry partners like CALB, generating cumulative sales exceeding RM2,300,000,000.