Determination of Thermophysical Properties of Power Battery Coolants: Testing Items, Methods, and Ap

Accurate determination of the thermophysical properties of power battery coolants is essential for the design, validation, and quality control of electric vehicle thermal management systems. This technical article explains the key thermophysical parameters of EV battery coolants, the applicable ISO, ASTM, and GB test methods, and the practical requirements for reliable measurement. Whether you are developing glycol-based coolants, dielectric immersion fluids, or qualifying incoming materials, this guide shows how accredited third-party laboratories evaluate thermal conductivity, specific heat capacity, viscosity, density, and related properties.

Technical Overview

Lithium-ion power batteries operate efficiently and safely only within a narrow temperature window, typically between 20 °C and 40 °C, with cell-to-cell temperature gradients preferably below 5 °C. In modern electric vehicles, battery thermal management systems rely on liquid cooling circuits, most commonly cold plates or ribbon-shaped cooling channels bonded to the cell modules, through which a coolant circulates to remove heat during fast charging, high-power discharge, and hot ambient operation, and to provide warming in cold climates.

The heat transfer performance of such systems is governed directly by the thermophysical properties of the coolant: thermal conductivity, specific heat capacity, density, and viscosity. For indirect cooling loops, ethylene glycol or propylene glycol aqueous solutions with corrosion inhibitor packages are the industry standard. For emerging immersion cooling architectures, dielectric fluids such as synthetic esters, polyalphaolefins, silicone oils, and fluorinated fluids are used, and their electrical conductivity, flash point, and material compatibility become as important as their thermal transport properties.

Reliable property data must cover the full service temperature range, commonly from -40 °C to +130 °C or even +150 °C. International and national standards, including ASTM D7896, ASTM E1269, ASTM D445, ASTM D4052, ASTM D1177, ASTM D1120, GB/T 22226, and the GB 29743 series, provide the technical basis for measurement. Accredited third-party laboratories with ISO/IEC 17025 quality systems routinely perform these determinations for formulation developers, OEMs, and battery pack manufacturers.

Testing Samples

Glycol-Based Indirect Battery Coolants

These include ethylene glycol and propylene glycol concentrates as well as pre-diluted mixtures, typically 30 vol% to 60 vol% glycol in deionized water, containing organic acid technology or hybrid inhibitor packages. A 50/50 glycol-water blend typically exhibits a thermal conductivity of 0.35–0.40 W/(m·K) at 25 °C, a specific heat capacity of 3.3–3.5 kJ/(kg·K), and freeze protection down to approximately -37 °C. Both unused (virgin) samples and laboratory-prepared dilutions are accepted for testing.

Dielectric Immersion Cooling Fluids

For direct immersion cooling where cells are in contact with the fluid, synthetic esters, polyalphaolefins, alkylated aromatics, silicone oils, and fluorinated fluids are submitted for characterization. These fluids generally show lower thermal conductivity, around 0.12–0.20 W/(m·K), and lower specific heat, around 1.8–2.2 kJ/(kg·K), than water-based coolants, which must be compensated by higher flow rates or larger contact areas. Their low electrical conductivity, typically below 10 pS/m to 100 μS/m depending on chemistry, and high flash points above 150 °C are critical acceptance criteria.

Engineered Aqueous and Hybrid Fluids

Water-based engineered fluids with drag-reducing or corrosion-inhibiting additives, as well as hybrid formulations designed for high heat flux applications, are tested to quantify how additives shift the baseline thermophysical properties of water, whose thermal conductivity of about 0.60 W/(m·K) and specific heat of 4.18 kJ/(kg·K) represent the theoretical upper limit for aqueous systems.

Development, Incoming, and Field Samples

Beyond new formulations, laboratories also analyze incoming production batches for quality control, competitor benchmark samples, and field-return or aged coolants to assess thermal degradation, contamination, glycol depletion, and inhibitor depletion over the vehicle service life.

Testing Items

The core thermophysical testing program for power battery coolants typically includes the following parameters, each reported as a function of temperature across the specified service range:

• Thermal conductivity, W/(m·K), typically from -40 °C to +150 °C
• Specific heat capacity (isobaric), kJ/(kg·K) or J/(g·K)
• Density and temperature-dependent density profile, kg/m³
• Kinematic viscosity, mm²/s, and dynamic viscosity, mPa·s
• Coefficient of volumetric thermal expansion, 1/K
• Thermal diffusivity, mm²/s, measured or derived from k, cp, and density
• Freezing point and pour point behavior, °C
• Boiling point and vapor pressure characteristics, °C and kPa
• Flash point (closed cup), °C, especially for dielectric immersion fluids
• Electrical conductivity, for immersion fluids and low-conductivity formulations
• Surface tension, mN/m, relevant to wetting, two-phase cooling, and microchannel design
• Viscosity-temperature behavior and viscosity index for flow prediction

Supplementary items frequently requested alongside thermophysical characterization include pH and reserve alkalinity per ASTM D1121, refractive index for concentration verification, foam tendency, corrosion behavior, and thermal-oxidative aging stability, because long-term property retention is a key procurement requirement for EV coolants.

Testing Methods

Thermal Conductivity — Transient Hot-Wire Method

Thermal conductivity is most commonly measured with the transient hot-wire technique in accordance with ASTM D7896 for engine coolants and related fluids, or GB/T 22226 for liquids in general. A thin platinum wire immersed in the sample acts as both heat source and resistance thermometer; the temperature rise slope in the first hundreds of milliseconds yields the thermal conductivity with a typical uncertainty of ±2% to ±3%. Measurements are performed at discrete set points, for example -40, -20, 0, 25, 50, 80, 100, and 150 °C, and the results are fitted to a temperature-dependent regression for direct use in CFD models.

Specific Heat Capacity — Differential Scanning Calorimetry

Isobaric specific heat capacity is determined by differential scanning calorimetry using the sapphire reference method per ASTM E1269. Sealed hermetic pans prevent evaporation of aqueous samples, and a slow heating rate, commonly 10 °C/min, is applied over the target range. Modulated DSC can improve accuracy for fluids exhibiting slow thermal events. Typical expanded uncertainty is ±3% to ±5%. The measured cp, together with thermal conductivity and density, defines the thermal diffusivity and the volumetric heat capacity ρcp that determines how much energy a given coolant flow can transport.

Density and Thermal Expansion — Oscillating U-Tube Densitometry

Density is measured with oscillating U-tube digital density meters per ASTM D4052, with hydrometer methods per ASTM D1298 and ASTM D1122 as classical alternatives. A temperature-controlled measurement from -20 °C or lower up to 100 °C provides the density-temperature curve, from which the coefficient of volumetric thermal expansion is derived. Thermal expansion data are essential for sizing expansion tanks and predicting pressure build-up in sealed battery cooling loops; a 50/50 glycol-water coolant expands by roughly 5% to 7% between 25 °C and 90 °C.

Viscosity and Rheology — Capillary and Rotational Methods

Kinematic viscosity is measured with glass capillary viscometers per ASTM D445 or equivalent GB/T 265 procedures at standard temperatures such as -40, -20, 0, 20, 40, and 100 °C, while rotational and Stabinger viscometers per ASTM D7483 cover low-temperature behavior and non-Newtonian additive systems. Viscosity dominates pumping power and laminar-flow heat transfer behavior; for example, a 50/50 glycol mixture may rise from about 3–4 mPa·s at 25 °C to more than 50 mPa·s near -30 °C, which directly constrains cold-start flow distribution through parallel cooling channels.

Phase-Change and Volatility Properties

Freezing point of aqueous engine coolants is determined by the refractive or gravimetric procedure of ASTM D1177, while boiling point is measured per ASTM D1120. Flash point of combustible immersion fluids is determined by the closed-cup Pensky-Martens procedure of ASTM D93 or the small-scale closed-cup method of ASTM D7094, supporting thermal runaway mitigation assessments and compliance with EV safety frameworks such as ISO 6469 and the IEC 62660 series context for cell and pack behavior.

Electrical Conductivity and Surface Tension

Electrical conductivity is measured per ASTM D1125 with appropriate cell constants, distinguishing conventional aqueous coolants from ultra-low-conductivity immersion fluids where stray currents through the fluid could disturb cell voltage monitoring. Surface tension is determined by du Noüy ring or Wilhelmy plate tensiometry per ASTM D1331, informing wetting behavior on cold-plate surfaces and flow boiling stability.

Data Treatment, Derived Quantities, and Reporting

From the measured properties, laboratories compute thermal diffusivity a = k/(ρcp) and Prandtl number Pr = cpμ/k for convection modeling. Coolant performance ranking is often expressed through the Mouromtseff number, for example Mo = ρ^0.8 · k^0.67 · cp^0.33 / μ^0.47 for turbulent internal flow, which quantifies convective merit in a single figure. Reports issued by accredited third-party laboratories include temperature-dependent data tables, regression equations, measurement uncertainty budgets, sample conditioning, and instrument traceability to national standards.

Applications

Thermophysical property data for power battery coolants are used throughout the electric vehicle value chain:

• Thermal management system design: cold-plate and channel sizing, pump selection, pressure-drop calculation, and CFD model calibration for battery packs
• Coolant formulation development: optimizing glycol concentration, inhibitor systems, and nano-additive strategies to maximize heat transfer while retaining pumpability
• Immersion cooling validation: screening dielectric fluids for combined thermal, electrical, flammability, and material compatibility performance
• Incoming material quality control: batch verification against datasheet limits for thermal conductivity, viscosity, density, and freezing point on the production line
• Benchmarking and supplier qualification: objective comparison of competing coolant products under identical, traceable measurement conditions
• Service-life and condition monitoring: periodic re-testing of field samples to detect thermal-oxidative degradation, dilution, or contamination
• Regulatory and safety support: generating data packages aligned with OEM specifications, GB 29743 series requirements, and international EV safety standards

FAQ

Why is thermal conductivity so important for battery coolants?

Thermal conductivity determines how effectively heat is transported from the cooling channel wall into the bulk fluid. Higher thermal conductivity reduces the convective thermal resistance and allows smaller, lighter cold plates, which directly improves cell temperature uniformity and enables faster DC charging without exceeding cell temperature limits.

Over what temperature range should properties be measured?

For most EV applications, a range of -40 °C to +130 °C covers cold-start, normal operation, and fast-charging conditions. Immersion cooling fluids and fluids targeted at high-heat-flux applications may require data up to +150 °C or higher to support worst-case and abuse-condition analysis.

How much sample is required and what is typical turnaround?

A full thermophysical characterization generally requires 100 mL to 500 mL of homogeneous sample depending on the number of set points and repeat measurements. Standard programs are typically completed within five to ten working days at accredited third-party laboratories; low-temperature viscosity and high-temperature DSC work may require additional conditioning time.

Do dielectric immersion fluids require different testing than glycol coolants?

Yes. While thermal conductivity, specific heat, density, and viscosity are measured with the same instrument platforms, immersion fluids additionally demand low electrical conductivity verification, closed-cup flash point determination, and compatibility assessment with cell materials, busbars, and sealing elastomers, because the fluid is in direct contact with live electrical components.

How is accuracy ensured across different measurement techniques?

Laboratories operating under ISO/IEC 17025 accreditation validate instruments with certified reference materials such as water, toluene, and sapphire, apply defined uncertainty budgets, and report both repeatability and reproducibility. Cross-verification between transient hot-wire, DSC, and densitometry results ensures internal consistency of the derived quantities such as thermal diffusivity and Prandtl number.

In summary, systematic determination of the thermophysical properties of power battery coolants, thermal conductivity, specific heat capacity, density, viscosity, thermal expansion, and their temperature dependence, provides the quantitative foundation for efficient, safe, and durable battery thermal management. Engaging accredited third-party laboratories that apply recognized ISO, ASTM, and GB test methods ensures that the resulting data are traceable, comparable, and suitable for design, validation, and regulatory submission purposes.