Introduction
As Plug-in Hybrid Electric Vehicles (PHEVs) continue to bridge the gap between traditional combustion engines and fully electric vehicles (EVs), battery thermal management has emerged as a critical challenge in ensuring safety, longevity, and optimal performance. Unlike pure EVs, PHEV batteries must operate under complex thermal conditions, as they interact with internal combustion engine (ICE) exhaust systems, ambient environmental factors, and rapid charging cycles.
Keywords:
Plug-in Hybrid Electric Vehicle battery cooling, thermal runaway prevention in PHEVs, battery heat dissipation strategies, MSC Cradle scFLOW simulations, OPTUMATICS Thermal Runaway Model, high-fidelity CFD simulations for batteries, battery thermal management systems (BTMS), conjugate heat transfer in PHEV batteries, thermal propagation in hybrid vehicle batteries, battery pack safety in hybrid electric vehicles, advanced cooling strategies for PHEV batteries, Vehicle-level thermal analysis for PHEVs.
Why is Battery Thermal Management Crucial?
The battery pack is the heart of any PHEV, but excessive heat can lead to:- Reduced battery lifespan – Prolonged exposure to high temperatures degrades lithium-ion cells.
- Performance inefficiencies – Uneven temperature distribution affects charge cycles and power delivery.
- Safety hazards – Uncontrolled heat buildup can trigger thermal runaway, leading to fire or explosion.
- EV batteries can generate a lot of heat, especially during fast charging and high-performance driving.
- Battery temperatures can reach over 100 degrees Celsius (212 degrees Fahrenheit) if they are not properly cooled.
- Thermal runaway is a rare event, but it can be catastrophic.
- Once thermal runaway begins, it is very difficult to stop.
Understanding Heat Transfer in PHEV Battery Systems
In the previous section, we highlighted the critical challenges of managing PHEV battery temperatures—from thermal runaway risks to heat accumulation from external sources. Now, let’s take a closer look at how heat moves within and around the battery pack and why a comprehensive heat transfer strategy is essential.
The Three Modes of Heat Transfer in PHEV Batteries
A PHEV battery pack experiences multiple heat transfer modes simultaneously, making its thermal management more complex than traditional internal combustion engine (ICE) cooling systems.
1.Conduction: Heat Flow Within the Battery Pack
- The primary heat transfer mechanism in high-voltage battery systems.
- Occurs within cells, between modules, and through structural materials (casings, busbars, compression pads).
- Key Challenge: Uneven thermal distribution can lead to localized hotspots, accelerating cell degradation.
2.Convection: Heat Dissipation Through the Cooling System
- Battery cooling systems rely on air (natural/forced) or liquid cooling to dissipate heat.
- PHEV-Specific Factor: The battery pack can be exposed to additional heat from the exhaust system, requiring precise thermal shielding.
- Key Challenge: If coolant flow is uneven, certain modules may overheat, affecting performance and safety.
3.Radiation: Heat Exchange Between Battery and Surrounding Components
- Often overlooked, thermal radiation affects battery surfaces, housing, and nearby structures.
- PHEV-Specific Factor: Radiant heat from engine components and underbody structures can elevate battery temperatures beyond safe limits.
- Key Challenge: If radiation is not controlled, it can lead to unexpected temperature rise in specific battery regions.
Why Simulation is Critical for PHEV Battery Thermal Control?
Managing PHEV battery temperatures requires an in-depth understanding of how heat is generated, transferred, and dissipated under different driving and environmental conditions. Traditional testing methods are costly, time-consuming, and often fail to capture transient behaviors that occur during real-world operation.
To overcome these limitations, engineers rely on Computational Fluid Dynamics (CFD)-based simulations, which enables:
- Accurate modeling of battery pack heat generation and dissipation.
- Predicting the impact of external heat sources, including exhaust radiation.
- Optimizing cooling strategies for various operating conditions.
Vehicle-Level Thermal Runaway (TR) Modeling Using High-Fidelity CFD Simulations
Using OPTUMATICS’ Thermal Runaway Model, seamlessly integrated into Hexagon AB’s MSC Software CFD solution (Cradle CFD), engineers can conduct vehicle-level transient simulations to predict, analyze, and mitigate thermal runaway events in PHEV battery packs.
This coupling allows for comprehensive 10-minute transient simulations of vehicle-level thermal propagation effects. This enables a multi-scale, multi-physics analysis, accounting for:
- Multi-Physics Integration – A complete Conjugate Heat Transfer (CHT) model incorporating conduction, convection, and radiation.
- Electrochemical Reactions Coupling – Accounts for battery heat generation from internal resistance, charge-discharge cycles, and external thermal influences.
- Full-System Analysis – Evaluates battery pack interactions with surrounding components, such as underbody structures, insulation materials, and vehicle wiring.
- Thermal Containment and Shielding – Optimizes heat-resistant materials and barrier placement to minimize heat propagation.
Key Insights from a Toyota Hybrid SUV Case Study
A 10-minute vehicle-level thermal runaway simulation was conducted using OPTUMATICS’ TR Model within Cradle CFD on a Toyota Hybrid PHEV SUV. The study focused on how thermal runaway propagates within the battery pack and its impact on critical vehicle components.
Vehicle Components Temperature and Thermal Runaway Propagation (cell-to-cell) with Jet Ejecting outside the Battery Pack. Courtesy: OPTUMATICS LLC.
Findings from the Simulation:
- Battery Internal Heat Generation: Internal cavity heat and mass accumulation accelerated thermal runaway inside the pack.
- Vented Hot Gases: The ejection of heated gases from failing cells caused a rapid rise in temperatures for adjacent components.
- Vehicle Component Temperature Rise: Components such as the fuel tank, floor, and carpeting showed varying levels of heat exposure, depending on shielding effectiveness.
Simulation results highlight the need for:
- Improved thermal barriers to minimize heat transfer to critical vehicle structures.
- Optimized airflow pathways to enhance heat dissipation and prevent localized overheating.
- Advanced containment materials to delay or halt thermal runaway propagation.
By leveraging OPTUMATICS’ Thermal Runaway Model with Cradle CFD, engineers can proactively design safer and more efficient PHEV battery systems—ensuring longevity, performance, and safety compliance in real-world driving conditions.
Conclusion: Advancing PHEV Battery Thermal Management
The thermal management of Plug-in Hybrid Electric Vehicle (PHEV) batteries is a complex challenge that requires a multi-physics approach to ensure safety, longevity, and efficiency. Given the risks associated with thermal runaway, external heat sources, and cooling inefficiencies, adopting high-fidelity CFD simulations is critical for designing robust battery systems.
By leveraging OPTUMATICS’ Thermal Runaway Model, integrated into Hexagon AB’s MSC Software Cradle CFD, engineers can accurately simulate and analyze thermal propagation at both the battery pack and vehicle levels. This predictive modeling approach helps in:
- Identifying thermal runaway risks early in the design phase.
- Optimizing cooling strategies for enhanced battery performance.
- Developing thermal shielding solutions to prevent component overheating.
The Toyota Hybrid PHEV case study demonstrates how advanced vehicle-level simulations can provide crucial insights into the impact of heat transfer mechanisms on battery safety. The study highlights the need for:
- Better thermal containment strategies to mitigate heat propagation risks.
- Optimized cooling designs for improved heat dissipation.
- Enhanced thermal shielding materials to prevent overheating of critical components.
As PHEV adoption grows, simulation-driven thermal management strategies will be essential for developing next-generation battery systems that meet stringent safety, performance, and durability standards.