As Electric Vehicles (EVs) continue to scale globally, the performance, safety, and longevity of lithium-ion battery packs remain key considerations for product engineers. Among the many engineering challenges that EV manufacturers face, thermal management stands out as a fundamental element of battery efficiency, reliability and lifecycle cost.
A well-engineered Thermal Management System (TMS) is not just about design optimisation, it’s a necessity for commercial viability.
In this article we examine the technical importance of a TMS in the context of electric vehicle battery packs and why it is so important when it comes to the performance, durability and safety of electric vehicle batteries.
Thermal Sensitivity of Lithium-Ion Batteries
The optimum operating temperature for lithium-ion cells is a relatively narrow band, typically between 20°C to 40°C.
Deviating from this range, particularly when it comes to prolonged exposure to high temperatures, can lead to the following:
- Accelerated capacity fade due to electrolyte decomposition and SEI (solid electrolyte interphase) layer growth.
- Reduced charge/discharge efficiency as internal resistance increases.
- Thermal runaway risks at high temperatures, posing serious safety concerns.
- Substantial impedance growth and lithium plating at low temperatures, especially during fast charging.
Temperature gradients within a pack can be equally damaging. Even a 5°C cell-to-cell delta can cause uneven aging, reduced capacity, and thermal imbalance, ultimately shortening the life of the battery.
Functions of an EV Thermal Management System
A properly designed TMS in EVs performs several integrated functions:
- Heat dissipation during high-load operations, including acceleration, hill climbing, and fast charging.
- Heat retention in cold conditions to bring the pack up to optimal temperature.
- Thermal uniformity across cells and modules to prevent uneven aging.
- System integration with cabin HVAC and power electronics for energy efficiency.
Depending on the vehicle architecture, duty cycle, and pack configuration, engineers employ active (liquid or forced-air cooling) and/or passive (phase change materials, heat pipes) solutions to achieve these functions.
Liquid Cooling vs. Air Cooling
For high-energy-density battery systems, liquid cooling has become the favoured option, offering superior thermal conductivity and more precise temperature control.
| Parameter | Air Cooling | Liquid Cooling |
| Heat transfer efficiency | Low | High |
| System complexity | Low | High |
| Maintenance | Low | Moderate |
| Energy consumption | Low (passive) | Medium (active) |
| Temperature uniformity | Poor | Excellent |
Liquid-cooled systems, often using a glycol-water mixture, can be embedded between cells or use cold plates for module-level management. When designing coolant loops, it is important that we consider pressure drop, pump sizing, and flow rate distribution.
Fast Charging: A Thermal Bottleneck
With more and more consumers choosing EVs, expectations around fast charging have grown.
C-rates measure how fast you charge compared to battery size. Higher C-rates (1.5C and above) mean faster charging, but also more heat and stress on the battery. Without an effective TMS, cell temperatures can surpass 50°C within minutes, accelerating degradation or triggering protective shutdowns.
Thermal management becomes especially critical when designing systems for ultra-fast charging (e.g, 350 kW+), where cell temperature rise can exceed 2°C per minute. Advanced TMS designs must account for:
- Transient thermal loads during charge cycles.
- Localised cell heating near terminals or higher resistance regions.
- Integration with battery thermal management system (BTMS) to modulate charging power based on thermal state.
Cold Weather Impacts and Preconditioning
Sub-zero temperatures present the opposite challenge: reduced ionic conductivity and increased cell impedance can often lead to power limitations and reduced range. Lithium plating during charging at low temperatures is also a serious concern, which can irreversibly damage cells.
Thermal management in cold climates includes:
- Battery preconditioning before charging or driving.
- Waste heat utilisation from power electronics and motors.
- Thermal insulation and passive retention during vehicle parking.
Active thermal control is essential for user experience and battery longevity, especially in regions with harsh winters.
Holistic System Integration
Modern EVs demand tight integration between TMS, BTMS, powertrain, and HVAC systems. Engineers are increasingly leveraging model-based systems engineering (MBSE) tools to simulate thermal dynamics and optimise cooling architecture early in the design phase.
Key integration points include:
- Predictive thermal algorithms to optimise cooling cycles and energy usage.
- Shared cooling loops with inverter and onboard charger to reduce system weight.
- Smart control strategies to balance performance, comfort, and efficiency.
Some systems use a centralised thermal controller with multiple valves and heat exchangers to dynamically route coolant across different subsystems based on thermal demand and drive cycle.
Emerging Trends in TMS for EVs
As EV technology continues to develop, thermal management is becoming more sophisticated:
- Dielectric immersion cooling (immersion of cells in non-conductive fluids) for extremely fast charging or motorsports applications.
- Phase change materials (PCMs) to buffer peak thermal loads.
- Active thermal balancing at the cell level using micro-pumps or thermoelectric elements.
- AI-driven thermal control systems that learn user driving patterns and climate conditions to precondition the pack intelligently.
These innovations are poised to enable higher power density, faster charging, and longer-lasting batteries.
The Calatherm Battery Thermal Management System
With all eyes on Electric Vehicles, we believe the biggest innovation gaps and opportunities lie around thermal management. For product engineers working on EVs, the Thermal Management System is far more than an auxiliary subsystem – it is a core enabler of battery performance, safety, and durability.
In response, in July 2025 we launched our own Battery Thermal Management System (BTMS).
As demands on battery systems grow, particularly with fast charging and high-performance applications, TMS innovation will be a key differentiator. Engineering teams that prioritise early thermal modelling, real-world testing, and smart integration will lead the next wave of EV excellence, and our BTMS is a game-changing element for OEMs to consider.
At Calatherm, we recognise that there is not a ‘one-size-fits-all’ solution, and our BTMS represents just one offering within our product portfolio. From heat pumps to cooling packs, HVAC units to control modules – we have a comprehensive and diverse portfolio of thermal management solutions designed to match the unique requirements of our customers across a diverse range of sectors and applications.