The proliferation of electric vehicles and energy storage systems has placed unprecedented demands on the manufacturing quality and consistency of li-ion batteries. The thermal behavior of li-ion batteries during the critical formation and capacity grading processes directly impacts their performance, longevity, and safety. Improper thermal management can lead to accelerated degradation, capacity inconsistencies, and in extreme cases, thermal runaway. The charging and discharging equipment used in production lines is the frontline defense for managing the thermal characteristics of li-ion battery packs.
Among various cooling methods, forced air cooling remains widely adopted due to its simplicity, reliability, and cost-effectiveness. However, ensuring uniform temperature distribution and maintaining peak temperatures within safe limits for densely packed li-ion batteries in such equipment presents a significant engineering challenge. This study delves into the key design factors influencing the thermal performance of air-cooled li-ion battery charging/discharging systems, focusing on the structural design of battery trays and the strategic layout of cooling fans.
Effective thermal management for a li-ion battery pack within production equipment requires meeting two stringent criteria: the maximum temperature of any single li-ion battery should not exceed 40°C, and the temperature difference within a single pack should be less than 4°C. The heat generated by a li-ion battery during operation is a primary concern. The volumetric heat generation rate (q) within a li-ion battery can be described by the Bernardi equation:
$$q = \frac{I}{V} \left( U_0 – U – T \frac{dU_0}{dT} \right)$$
where I is the current, V is the battery volume, U0 is the open-circuit voltage, U is the working voltage, and T is the battery temperature. The dominant mode of heat removal in air-cooled systems is forced convection, where the convective heat transfer (Qcv) from the li-ion battery surface is governed by:
$$Q_{cv} = h A \Delta T$$
Here, h is the convective heat transfer coefficient, A is the surface area of the li-ion battery, and ΔT is the temperature difference between the li-ion battery surface and the cooling air. The efficiency of this process is highly dependent on the airflow distribution and velocity around each li-ion battery.
Numerical Modeling Methodology
Physical and Mathematical Models
The charging/discharging equipment was abstracted into a computational model comprising four main components: the li-ion battery pack, the tray holding the pack, the equipment chassis, and the cooling fans. The li-ion battery pack consisted of 64 cylindrical li-ion batteries arranged in an 8×8 array, with a center-to-center pitch of 60 mm. To balance computational accuracy and cost, each li-ion battery was simplified as a solid, anisotropic cylinder with a constant internal heat generation rate corresponding to a 1.5C (45A) discharge rate. The key parameters of the li-ion battery model are summarized in the table below.
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Dimensions (Diameter x Height) | 46 mm x 95 mm | Nominal Capacity | 30 Ah |
| Internal Resistance | 1.25 mΩ | Nominal Voltage | 3.7 V |
| Charge Cut-off Voltage | 4.2 V | Discharge Cut-off Voltage | 2.8 V |
| Volumetric Heat Generation (1.5C) | 48,800 W/m³ | – | – |
The tray design featured two critical ventilation structures: vent holes and annular vents. Each li-ion battery position had four vent holes (diameter D) on the tray floor and a concentric annular gap (width δ) around the battery’s circumference. The fans, mounted at the bottom of the equipment, forced ambient air (25°C) upward through the tray structures to cool the li-ion battery pack.
Boundary Conditions, Solver Settings, and Validation
Numerical simulations were performed using a commercial finite volume method-based solver. The airflow was assumed to be steady-state, turbulent, and incompressible. The Standard k-ε turbulence model was employed for its robustness and computational economy in such internal flow applications. The governing equations for mass, momentum, and energy conservation were solved using a coupled algorithm with second-order discretization schemes.
The equipment chassis and tray surfaces were assigned a natural convection boundary condition with a heat transfer coefficient of 5 W/(m²·°C). A grid independence study was conducted to ensure the accuracy of the results. Four mesh configurations with increasing cell counts were tested. The results indicated that both the maximum temperature and the maximum temperature difference of the li-ion battery pack stabilized when the mesh count exceeded 4.4 million cells. This configuration was selected for all subsequent simulations.
Experimental validation was performed using a physical prototype. Temperature sensors were attached to the top surface of each li-ion battery in the pack. The pack was discharged at 0.5C, 1.0C, and 1.5C rates, and the temperature rise was recorded. A comparison between the simulated and measured temperature profiles for the hottest li-ion battery channel showed excellent agreement, with a maximum deviation of only 4.2%. This confirmed the high fidelity of the numerical model for analyzing the thermal characteristics of the li-ion battery system.
Influence of Tray Ventilation Structure
The tray’s ventilation design dictates the path and distribution of cooling air around the li-ion batteries. Two primary structural features were investigated: the vent hole diameter (D) and the annular vent width (δ).
Effect of Tray Vent Hole Diameter (D)
With the annular vent width fixed at δ = 0 mm, simulations were run for vent hole diameters of D = 20, 22, 24, and 26 mm. The temperature distribution followed a consistent pattern across all cases: the central region of the li-ion battery pack was consistently hotter than the peripheral regions. This is typical in air-cooled arrays where airflow tends to bypass the core, leading to heat accumulation.
| Vent Hole Diameter, D (mm) | Max. Battery Temp. (°C) | Max. ΔT in Pack (°C) | System Pressure Drop (Pa) | Airflow Rate (m³/s) | Avg. Turbulent Kinetic Energy at Battery Surface (m²/s²) |
|---|---|---|---|---|---|
| 20 | 46.6 | 6.3 | High | Low | Low |
| 22 | 45.1 | 5.0 | — | — | — |
| 24 | 43.8 | 3.9 | — | — | — |
| 26 | 42.8 | 3.1 | Low | High | Moderately Low |
As shown in Table 2, increasing the vent hole diameter D improved thermal performance. The maximum temperature of the li-ion battery decreased from 46.6°C to 42.8°C, and the temperature uniformity improved by 50.8%. This enhancement is attributed to the reduced flow resistance and increased overall airflow rate through the tray. However, even at D = 26 mm, the peak li-ion battery temperature (42.8°C) exceeded the 40°C target. Furthermore, the turbulent kinetic energy near the li-ion battery surfaces, a key indicator of convective heat transfer intensity, remained relatively low. This implies that simply enlarging the vent holes provides limited benefit because the airflow does not effectively impinge on or sweep across the critical li-ion battery surfaces.
Effect of Tray Annular Vent Width (δ)
With the vent hole diameter fixed at D = 20 mm, the annular vent width was varied: δ = 0, 0.5, 1.0, 1.5, and 2.0 mm. The introduction of the annular vent had a dramatic and positive impact on the thermal management of the li-ion battery pack.
| Annular Vent Width, δ (mm) | Max. Battery Temp. (°C) | Max. ΔT in Pack (°C) | System Pressure Drop (Pa) | Airflow Rate (m³/s) | Avg. Turbulent Kinetic Energy at Battery Surface (m²/s²) |
|---|---|---|---|---|---|
| 0.0 | 46.6 | 6.3 | High | Low | Low |
| 0.5 | 42.4 | 2.8 | — | — | — |
| 1.0 | 40.4 | 1.9 | — | — | — |
| 1.5 | 39.0 | 1.4 | — | — | — |
| 2.0 | 38.3 | 1.2 | Low | High | High |
The results in Table 3 demonstrate the critical role of the annular vent. For δ ≥ 1.5 mm, the maximum temperature of every li-ion battery in the pack fell below the 40°C threshold, and the temperature difference met the sub-4°C requirement. While the system pressure drop and total airflow followed trends similar to the vent hole study, the key differentiator was the turbulent kinetic energy at the li-ion battery surface. The annular vent, positioned directly adjacent to the li-ion battery wall, forces a portion of the cooling air to flow through the narrow gap along the battery’s cylindrical surface. This direct impingement and acceleration of flow significantly intensify local turbulence and shear, thereby drastically improving the convective heat transfer coefficient (h) as per Equation (2). The increase in surface turbulent kinetic energy with the annular vent was approximately 1.8 times greater than the effect achieved by increasing the vent hole diameter alone. This confirms that guiding airflow directly over the heat-generating surfaces of the li-ion battery is far more effective than merely increasing the open area for general airflow.
Influence of Fan Layout Strategy
The layout of the cooling fans, which provide the motive force for the airflow, is another pivotal factor in the thermal management system for the li-ion battery pack. Two aspects were studied: fan alignment and fan quantity.
Effect of Fan Alignment
Two fan alignment configurations were compared, keeping D=20 mm and δ=1.5 mm. In the first configuration, fans were positioned directly underneath the li-ion batteries. In the second, they were positioned directly underneath the tray’s vent holes (i.e., aligned with the gaps between batteries).
| Fan Alignment | Max. Battery Temp. (°C) | Max. ΔT in Pack (°C) | Airflow Rate (m³/s) | Avg. Turbulent Kinetic Energy at Battery Surface (m²/s²) |
|---|---|---|---|---|
| Aligned with Batteries | 39.0 | 1.4 | Baseline | Baseline |
| Aligned with Vent Holes | 39.6 | 1.7 | +31.4% | Lower |
Counter-intuitively, aligning fans with the vent holes increased the overall system airflow by 31.4% because the air encountered less immediate obstruction. However, this configuration degraded the thermal performance of the li-ion battery pack. The maximum temperature rose to 39.6°C, nearing the critical limit, and temperature uniformity worsened by 21.4%. The analysis of the flow field revealed the cause: when fans are aligned with gaps, the air jet preferentially takes the path of least resistance, flowing freely through the open channels between li-ion battery rows rather than being directed toward the battery surfaces themselves. This reduces the flow interaction with the li-ion battery walls, leading to a decrease in surface turbulence and, consequently, a lower convective heat transfer rate. This finding underscores that for effective cooling of a li-ion battery array, airflow must be actively directed toward the batteries, not just through the assembly.
Effect of Fan Quantity
Using the optimal parameters (D=20 mm, δ=1.5 mm, fans aligned with batteries), the number of fans was varied from 6 to 9 to assess the trade-off between cooling performance and energy consumption.
| Number of Fans | Max. Battery Temp. (°C) | Max. ΔT in Pack (°C) | Airflow Rate (m³/s) | Avg. Turbulent Kinetic Energy at Battery Surface (m²/s²) | Relative Energy Use |
|---|---|---|---|---|---|
| 6 | 40.0 | ~2.0 | Baseline (Lowest) | Baseline (Lowest) | Lowest |
| 7 | 39.5 | ~1.7 | +12% | +8% | +17% |
| 8 | 39.2 | ~1.5 | +18% | +15% | +33% |
| 9 | 39.0 | 1.4 | +23.8% | +21.6% | +50% |
The data in Table 5 shows a clear positive correlation between fan quantity and the thermal performance of the li-ion battery pack. Increasing the number of fans boosts the total airflow and the surface turbulence around each li-ion battery, enhancing convective cooling. With 9 fans, the pack exhibited the best thermal characteristics. However, a system with 6 fans still managed to maintain the maximum li-ion battery temperature at the 40°C threshold while significantly reducing the temperature difference compared to baseline poor designs. From a system design perspective, selecting between 6 and 8 fans represents a practical optimization point. It achieves the necessary thermal management goals for the li-ion battery pack—keeping it within safe and consistent temperature limits—while offering substantial savings in energy consumption, fan cost, and operational noise compared to a full 9-fan configuration.
Conclusion
This comprehensive numerical study identifies and quantifies the critical design factors for optimizing the thermal performance of air-cooled charging/discharging equipment for li-ion battery packs. The key conclusions are:
- Tray Ventilation Structure is Paramount: The design of the tray holding the li-ion battery pack is crucial. While increasing the size of vent holes (parameter D) improves airflow and offers moderate thermal benefits, its effect is limited. The introduction of an annular vent (parameter δ) around each li-ion battery is the most influential factor. By forcing cooling air to flow directly over the li-ion battery surface, it dramatically increases local turbulence and the convective heat transfer coefficient. An annular vent width (δ) of 1.5 mm or more is sufficient to maintain the li-ion battery pack below 40°C with excellent temperature uniformity.
- Strategic Fan Alignment is Essential: The position of cooling fans must be carefully considered. Aligning fans directly with the li-ion batteries, rather than with gaps or vents, ensures that the airflow is effectively directed toward the primary heat sources. Although this alignment may slightly reduce total volumetric flow compared to an unobstructed path, it maximizes the heat transfer efficiency from the li-ion battery surfaces.
- Fan Quantity Presents a Trade-off: Increasing the number of fans monotonically improves the cooling performance of the li-ion battery pack. However, a point of diminishing returns exists from a system efficiency perspective. A configuration with 6 to 8 fans, combined with an optimized tray design (δ ≥ 1.5 mm), can successfully meet the stringent thermal management requirements for li-ion batteries during formation/grading processes while minimizing the equipment’s energy consumption and cost.
These findings provide actionable guidelines for the design and development of efficient, reliable, and economical air-cooled thermal management systems for industrial-scale li-ion battery production equipment, ultimately contributing to the manufacturing of higher quality and safer li-ion batteries.