The operational safety and long-term reliability of Electric Vehicle (EV) battery packs are paramount. While designed to stringent ingress protection (e.g., IP67) standards, the internal microenvironment of an EV battery pack remains susceptible to complex hygrothermal interactions. During service, factors like vibration, thermal cycling, and potential seal degradation can compromise integrity. More subtly, water vapor can permeate through pressure-equalization valves. Coupled with the coexistence of significant heat sources (battery modules) and cold sources (cooling plates), this creates conditions ripe for condensation within the EV battery pack enclosure. The accumulation of condensate, transforming into standing water, poses severe threats including corrosion of electrical components, insulation failure, short circuits, and ultimately, thermal runaway. This work presents a combined experimental and numerical investigation to unravel the transient hygrothermal characteristics and elucidate the mechanisms governing condensation evolution within a representative EV battery pack.
We have constructed a comprehensive experimental platform to simulate the coupled thermal and moisture behavior of an EV battery pack under controlled environmental and operational loads. The core is a scaled model replicating key features of a real EV battery pack: a sealed enclosure with a pressure-equalization valve as the sole mass transfer port, simulated battery modules as controllable heat sources, and a liquid-cooled cold plate as the primary heat sink.
The test EV battery pack model features an aluminum enclosure. Simulated battery modules, consisting of alternating aluminum and acrylic layers with embedded heating films, provide a thermal mass and specific heat capacity analogous to real lithium-ion cells. The heating power is adjustable from 0 to 100 W per module, emulating discharge/charge rates. A multi-channel aluminum cold plate is installed beneath the modules. A cooling system with a pump, a reservoir, and a thermoelectric chiller supplies temperature-controlled coolant (water) to the cold plate. The entire assembly is housed within an environmental chamber capable of precisely controlling temperature and humidity to simulate various climatic conditions. An array of thermocouples, wireless temperature/humidity sensors, and a condensation detection sensor are strategically placed inside the EV battery pack model to capture spatial and temporal variations.
Prior to dynamic tests, the sealing performance and the role of the pressure-equalization valve were verified. With the valve sealed, internal humidity remained stable despite large external humidity swings, confirming the basic integrity of the EV battery pack model. With the valve functional, sensors near it tracked external humidity changes, while those further away remained stable, proving it acts as the dominant pathway for vapor exchange.
Hygrothermal Characterization Under Different Operational Scenarios
Three distinct test cases were designed to isolate the effects of module power and external climate on the internal environment of the EV battery pack. Each test followed a phase sequence: Phase I – simultaneous module heating and cooling system operation; Phase II – cooling system operation alone; Phase III – system idle (soak).
| Test Case | Simulated Module Power | External Climate (Chamber) | Primary Investigation Focus |
|---|---|---|---|
| Test 1 | 1C (for 1 hour) | 20°C, 60% RH (Temperate) | Baseline behavior under moderate load |
| Test 2 | 3C (for 0.5 hour) | 20°C, 60% RH (Temperate) | Effect of high-power (fast-charge) operation |
| Test 3 | 3C (for 0.5 hour) | 40°C, 80% RH (Hot/Humid) | Combined effect of high power and harsh climate |
Test 1 (1C, Temperate): Data revealed a classic thermal gradient within the EV battery pack: warmer at the top of modules and cooler at the bottom near the cold plate. A notable phenomenon was observed at the start of operation: air temperature near the cold plate inlet dropped sharply, accompanied by a transient dip in relative humidity. This suggests instantaneous condensation (dew formation) as the cool coolant first entered the plate, lowering the local air temperature below its dew point. Post-test inspection confirmed the presence of frost-like condensate on the cold plate surface, though droplets were not large enough to trigger the condensation sensor alarm.
Test 2 (3C, Temperate): The high module power dominated the initial thermal environment, causing a general temperature rise inside the EV battery pack. The peak temperature was significantly higher than in Test 1. During the cooling-only phase, temperatures dropped, but humidity changes were minimal. Condensate observed after the test was scant and limited to the cold plate inlet region. This indicates that the substantial internal heating from high-power operation elevates air temperatures, thereby reducing its relative humidity and suppressing the potential for condensation within the EV battery pack, despite the active cold plate.
Test 3 (3C, Hot/Humid): This scenario presented the most severe conditions for the EV battery pack. The initial transient condensation at coolant startup was more pronounced. Crucially, during the operational phases, sufficient condensate formed and grew on the cold plate, eventually triggering the condensation sensor alarm. The condensate was visibly more abundant and widespread than in previous tests. The high external humidity led to greater vapor ingress into the EV battery pack, while the elevated temperature increased the vapor-carrying capacity of the internal air. When this warm, moisture-laden air contacted the cold plate, the driving force for condensation (the temperature difference and the high absolute humidity) was maximized.
The key findings from the experimental characterization of the EV battery pack are:
- Coolant Surge Effect: The initial influx of coolant can cause a rapid local temperature drop, inducing instantaneous condensation near the cold plate inlet.
- Power-Dependent Suppression: High-power operation of battery modules raises the internal air temperature of the EV battery pack, lowering relative humidity and actively inhibiting condensation formation.
- Climate Severity Drives Risk: High ambient temperature and humidity drastically increase condensation risk within the EV battery pack by raising both the internal moisture content and the temperature differential with cooling surfaces.
Development and Validation of a 3D CFD Hygrothermal Model
To gain deeper insight into the spatial-temporal evolution of temperature, humidity, and condensate distribution within the complex geometry of an EV battery pack, a three-dimensional Computational Fluid Dynamics (CFD) model was developed. The model integrates the governing physics for conjugate heat transfer, moist air flow, vapor diffusion, and phase change (condensation/evaporation) on surfaces.
The core conservation equations for energy and species transport in the EV battery pack are solved. The heat transfer encompasses conduction and convection:
$$ \rho C_p \left( \frac{\partial T}{\partial t} + \mathbf{u} \cdot \nabla T \right) = \nabla \cdot (k \nabla T) + Q $$
where $Q$ is the volumetric heat source (from modules), $\rho$ is density, $C_p$ is specific heat, $k$ is thermal conductivity, $T$ is temperature, $t$ is time, and $\mathbf{u}$ is the fluid velocity vector.
Vapor transport through the pressure-equalization valve membrane is modeled via a permeability approach based on Fick’s law:
$$ g_{vapor} = \beta (\phi_{w,ext} – \phi_{w,int}) $$
where $g_{vapor}$ is the vapor flux, $\beta$ is the membrane permeance, and $\phi_w$ represents relative humidity.
The condensation/evaporation mass flux on wetted surfaces (e.g., cold plate, walls) inside the EV battery pack is modeled as:
$$ g_{cond} = M_v \cdot K (c_{sat}(T_s) – c_v) $$
where $M_v$ is the molar mass of water vapor, $K$ is a mass transfer coefficient, $c_{sat}$ is the saturation vapor concentration at the surface temperature $T_s$, and $c_v$ is the vapor concentration in the adjacent air. When $c_v > c_{sat}$, $g_{cond}$ is negative, indicating condensation. The resulting liquid film or droplet concentration on the surface is tracked.
The geometry was simplified for computational efficiency while retaining the critical features of the EV battery pack: enclosure, simplified modules as homogeneous blocks with equivalent properties, integrated cold plate with flow channels, and the valve area. Material properties for the simulated module composite, aluminum enclosure/plate, and coolant were assigned. The model’s boundary conditions mirrored the experimental setup for Test 3 (3C, Hot/Humid), providing a challenging validation case.
The model predictions for air temperature and relative humidity at four internal sensor locations were compared against experimental data from the EV battery pack test. The agreement was excellent across all operational phases, with mean errors generally below 1.5°C and 3.5% RH, validating the model’s fidelity for hygrothermal analysis.
| Sensor Location | Mean Temp. Error (°C) | Max Temp. Error (°C) | Mean RH Error (%) | Max RH Error (%) |
|---|---|---|---|---|
| Near Inlet (THa) | 1.48 | 5.97 | 2.55 | 10.45 |
| Near Valve (THb) | 1.19 | 3.50 | 1.55 | 4.47 |
| Interior (THc) | 1.25 | 3.50 | 1.14 | 3.88 |
| Near Outlet (THd) | 0.81 | 5.35 | 3.21 | 8.82 |
Simulation Insights into Flow Fields and Condensation Evolution
The validated CFD model provides a powerful visual and quantitative tool to analyze processes inside the EV battery pack that are difficult to measure directly.
Hygrothermal Flow Fields: The simulations reveal distinct flow patterns during different phases. During simultaneous heating and cooling, warm, buoyant air rises from the modules, while cooler, denser air descends near the cold plate, setting up convection loops. The flow is influenced by the coolant direction, with slightly stronger downdrafts on the coolant inlet side. During the cooling-only phase, these convective motions weaken as temperature gradients reduce, but the general trend of air cooling and settling continues. In the final soak phase, the environment inside the EV battery pack becomes nearly uniform and stagnant.
Condensation Mechanism and Mapping: The model quantifies the condensate accumulation over time on the cold plate surface. The evolution clearly shows a three-stage process:
- Phase I (Heating/Cooling): Condensation initiates almost exclusively on the cold plate region near the coolant inlet. This is driven by the lowest local surface temperature and the immediate exposure to incoming humid air.
- Phase II (Cooling Only): As the entire EV battery pack interior cools, the air temperature drops, increasing its relative humidity universally. Condensate growth accelerates and begins to propagate across the cold plate surface from the inlet towards the outlet region. This phase is critical for significant water accumulation.
- Phase III (Soak): With cooling stopped, the cold plate temperature begins to rise slowly. However, the now-cooled, moisture-laden air continues to settle onto the plate, allowing condensation to persist and even expand to cover most of the plate surface for a period before eventual re-evaporation as temperatures equalize.
This simulated progression—localized inception near the inlet, followed by propagation across the cold plate during sustained cooling—aligns perfectly with the experimental observations from the EV battery pack tests, confirming the cold plate as the primary condensation site and outlining its dynamic growth pattern.
Conclusions and Implications for EV Battery Pack Design
This integrated study, combining targeted experimentation on a representative model with high-fidelity CFD simulation, provides a clear understanding of the hygrothermal dynamics within an EV battery pack. The work specifically illuminates the condensation mechanism, a critical yet often overlooked failure precursor.
The key conclusions are:
- The internal environment of an EV battery pack is dynamic and highly sensitive to operational power and external climate. The coexistence of heat and cold sources within a sealed, vapor-permeable enclosure creates an inherent risk for condensation.
- The initial surge of coolant into the cold plate can cause a transient but significant local temperature drop, triggering immediate condensation. This “cold-start” effect for the thermal management system is a vulnerable moment for the EV battery pack.
- High-power operation of the battery modules acts as a suppressing factor for condensation within the EV battery pack by elevating the internal air temperature and lowering its relative humidity, despite the presence of the cold plate.
- High ambient temperature and humidity constitute the most severe environmental loading for an EV battery pack. They work synergistically to increase both the internal moisture load (via the pressure-equalization valve) and the driving potential for condensation (via a larger temperature difference between warm humid air and the cold plate), leading to prolific condensate formation and accumulation.
- The primary site for condensation is the surface of the cooling plate, with initiation typically at the coolant inlet region where temperatures are lowest. The condensate then spreads as the overall pack environment cools during and after operation.
The insights derived necessitate a holistic approach to environmental management within the EV battery pack. Moving beyond traditional thermal management, “hygrothermal management” should be considered. Recommendations for mitigating condensation risk in EV battery pack design include:
- Mitigating the Coolant Surge: Implementing control strategies to moderate the initial cooling rate or temperature of the coolant upon system activation could reduce the instantaneous condensation risk.
- Active Humidity Management: Integrating humidity sensors within the EV battery pack alongside the Battery Management System (BMS) could enable real-time risk diagnosis. Conditional actions, such as initiating localized heating on the cold plate or activating controlled ventilation (if designed), could prevent or dispel condensation.
- Passive Design for Redirection: In high-risk areas identified (like the cold plate inlet zone), incorporating hydrophobic coatings, micro-structured surfaces to promote droplet shedding, or dedicated drainage channels could help manage any formed condensate, preventing its accumulation and random dripping onto sensitive components.
- Climate-Specific Design: EV battery packs destined for markets with hot and humid climates may require enhanced sealing, more sophisticated valve technology with selective permeation, or adjusted thermal management setpoints to operate safely within a “no-condensation” window.
In summary, safeguarding the EV battery pack against the insidious threat of internal condensation requires a nuanced understanding of the coupled heat and mass transfer processes. This work provides a foundational framework and methodological approach for analyzing and addressing this critical aspect of EV battery pack safety and durability.