Thermal conductivity is one of the most critical performance indicators of power battery coolants, directly determining the heat dissipation capability of electric vehicle battery thermal management systems. This article presents a comprehensive overview of experimental research on coolant thermal conductivity, covering sample types, measurable parameters, standardized test methods such as ASTM D7896 and ISO 22007, and practical engineering applications for coolant developers, battery manufacturers, and testing engineers.
Technical Overview
Power battery coolants are specialized heat transfer fluids circulated through cold plates, cooling channels, or immersion tanks in lithium-ion battery packs for electric vehicles (EVs) and hybrid electric vehicles (HEVs). As battery systems trend toward higher energy density and faster fast-charging rates, the heat generation density within battery packs continues to rise, making the thermophysical properties of the coolant, particularly its thermal conductivity, a decisive factor in overall thermal management performance.
Thermal conductivity, expressed in W/(m·K), quantifies the ability of a fluid to conduct heat. At 25 °C, a typical 50/50 ethylene glycol-water mixture exhibits a thermal conductivity of approximately 0.37–0.40 W/(m·K), while dielectric fluids used for immersion cooling generally fall in the range of 0.12–0.16 W/(m·K). Because thermal conductivity varies significantly with temperature, concentration, and fluid chemistry, systematic experimental characterization across the full operating window, typically from −40 °C to 150 °C, is essential for accurate thermal modeling, coolant formulation, and system design validation.
Experimental research on coolant thermal conductivity is typically conducted by accredited third-party laboratories or in-house R&D facilities using internationally recognized standards published by ISO, ASTM, and other standards organizations, ensuring that measured data are traceable, reproducible, and suitable for engineering decision-making.
Testing Samples
Ethylene Glycol-Water Based Coolants
Ethylene glycol (EG) aqueous solutions with concentration ratios of 30/70, 50/50, and 70/30 are the most widely used coolants in indirect liquid cooling systems for EV battery packs. These samples usually contain corrosion inhibitor packages, buffers, and antifoam agents. Thermal conductivity testing at multiple glycol concentrations allows developers to construct concentration-temperature-property maps for system optimization.
Propylene Glycol-Water Based Coolants
Propylene glycol (PG) based coolants offer lower toxicity and improved environmental compatibility compared with EG-based products. Their thermal conductivity is slightly lower than that of EG solutions at the same concentration, and experimental characterization is required to quantify this difference and assess feasibility for specific thermal management architectures.
Dielectric Immersion Cooling Fluids
Dielectric fluids, including synthetic esters, silicone oils, mineral oils, and fluorinated fluids, are increasingly used in direct immersion cooling of battery cells. Because these fluids make direct electrical contact with cell terminals and busbars, both thermal conductivity and electrical properties such as breakdown voltage and volume resistivity must be evaluated simultaneously.
Nanofluid and Modified Coolant Formulations
Research-stage coolants containing nanometer-scale additives such as alumina, graphene, or carbon nanotubes are tested to evaluate thermal conductivity enhancement. These samples require special handling, including ultrasonic dispersion verification and stability observation, to ensure that measured enhancements are not caused by particle sedimentation or agglomeration artifacts.
Testing Items
A complete experimental study of power battery coolant heat transfer performance typically encompasses the following measurable parameters:
- Thermal conductivity (W/(m·K)) across the temperature range of −40 °C to 150 °C
- Thermal diffusivity (mm²/s) as a complementary heat transport parameter
- Specific heat capacity (J/(g·K)) determined by differential scanning calorimetry
- Density (g/cm³) as a function of temperature, measured by oscillating U-tube densitometry
- Kinematic and dynamic viscosity (mm²/s and mPa·s) for pump power and flow behavior assessment
- Electrical conductivity and dielectric breakdown voltage for immersion cooling fluids
- Thermal conductivity retention after thermal-oxidative aging or long-term circulation
- Freezing point and boiling point verification to confirm the usable operating envelope
Testing Methods
Transient Hot-Wire Method (ASTM D7896)
The transient hot-wire method is the preferred technique for electrically non-conductive and low-conductivity liquids. A thin platinum wire immersed in the sample acts simultaneously as a heat source and a resistance thermometer. By applying a short electrical pulse and recording the temperature rise of the wire, thermal conductivity is derived from the linear relationship between temperature rise and the logarithm of time. Because the measurement completes within seconds, natural convection errors are effectively suppressed, and repeatability better than 2% can be achieved with a calibrated instrument.
Transient Plane Source Method (ISO 22007-2)
The transient plane source technique uses a spiral nickel sensor sandwiched between two volumes of the sample fluid. A constant current pulse heats the sensor while its resistance change records the transient temperature response. This method is suitable for viscous coolants, gels, and thermal interface materials associated with battery cooling systems, and it yields thermal conductivity, diffusivity, and volumetric heat capacity in a single measurement.
Laser Flash Analysis (ASTM E1461) with Derived Conductivity
For reference materials, cured residues, or highly viscous samples that can be shaped into disks, laser flash analysis measures thermal diffusivity. When combined with independently measured specific heat capacity and density, thermal conductivity is calculated through the relationship k = ρ × cp × α. This approach provides valuable cross-validation of direct liquid measurements.
Differential Scanning Calorimetry for Specific Heat (ASTM E1269)
Specific heat capacity is determined by comparing the heat flow of the coolant sample against a sapphire reference during a controlled temperature ramp. Multi-point measurements at discrete temperatures support accurate integration into thermal models.
Temperature-Dependent Measurement Program
A rigorous test protocol measures thermal conductivity at fixed temperature intervals, for example every 10 °C from −40 °C to 150 °C, under controlled heating and cooling cycles. Repeatability checks at selected points and comparison with reference fluids such as water and glycerol verify data integrity. The resulting datasets are fitted into polynomial or piecewise models suitable for direct import into CFD and one-dimensional thermal network simulations.
Applications
- Thermal design of battery cold plates and cooling channel layouts based on verified fluid property data
- Development and optimization of new coolant formulations, including inhibitor systems and nano-additives
- Validation of immersion cooling architectures where dielectric property and thermal conductivity trade-offs must be quantified
- Incoming quality control of coolant shipments at battery plants and tier-one suppliers
- Benchmarking of commercial coolants against competitor products and patent support filings
- Calibration input for battery pack thermal simulation models used in fast-charging strategy development
- Compliance verification against OEM engineering specifications and relevant GB/T and ISO standards for vehicle coolants
FAQ
What is the typical thermal conductivity range of power battery coolants?
Water-glycol mixtures generally range from 0.30 to 0.45 W/(m·K) between −40 °C and 90 °C, while dielectric immersion fluids typically fall between 0.10 and 0.16 W/(m·K). Nano-enhanced formulations under research may reach higher values, which must be confirmed experimentally under realistic operating conditions.
Which test method is most suitable for liquid coolants?
The transient hot-wire method described in ASTM D7896 is generally recommended for engine and battery coolants because of its short measurement time and strong resistance to convection-induced errors. The transient plane source method per ISO 22007-2 is preferred for viscous or paste-like samples.
How much sample is required for testing?
Most transient instruments require approximately 40 to 100 mL of sample per temperature scan. It is good practice to supply at least 200 mL to allow repeat measurements, instrument rinsing, and retesting if anomalous results are observed.
What temperature range can be covered?
Standard laboratory equipment covers −40 °C to 150 °C, which spans the typical operating envelope of EV battery thermal management systems. Extended ranges can be discussed with accredited third-party laboratories for specialized applications such as high-temperature fuel cell coolants.
What measurement uncertainty can be expected?
With calibrated instruments and appropriate reference fluid verification, expanded uncertainty of thermal conductivity is typically within 3%, with repeatability better than 2%. Specific heat capacity measurements by DSC usually carry uncertainty within 5%.
How long does a complete test program take?
A single-temperature measurement takes only minutes, while a full temperature sweep with multiple concentrations and repeat points typically requires three to seven working days, depending on sample conditioning requirements and the number of auxiliary property tests requested.