Thermal & Structural Design
of Battery Packs
Temperature is the single most powerful variable governing battery performance, degradation, and safety. This chapter covers heat sources, all four cooling approaches, thermal interface materials, propagation prevention, and full structural design requirements.
Of all the engineering disciplines that converge in battery pack design, thermal management is the one where errors are most unforgiving. Poor electrical design reduces efficiency. Poor mechanical design reduces durability. Poor thermal design can kill people.
Temperature is the single most powerful variable governing every aspect of a battery's behaviour — its immediate performance, its long-term degradation rate, and its safety margin against catastrophic failure.
Thermal design is not simply about preventing cells from getting too hot. It is about maintaining the entire cell population within a narrow operating window — typically 20°C to 40°C — while managing heat generated during operation, heat absorbed from or lost to the environment, and heat that must be supplied in cold climates to enable safe charging. Non-uniform temperature creates non-uniform degradation, which creates imbalance that compounds over thousands of cycles.
The rate of calendar aging in lithium-ion cells roughly doubles for every 10°C increase in operating temperature (Arrhenius relationship). A pack operating consistently at 45°C ages approximately 4× faster than one at 25°C. This is not a minor performance difference — it is the difference between a 10-year battery and a 2.5-year battery.
Before designing a thermal management system, the designer must understand where heat comes from. There are three primary heat generation mechanisms, each with different characteristics in terms of magnitude, location, and controllability.
Even with an active thermal management system, temperature will not be perfectly uniform across all cells in a pack. Understanding how temperature gradients form, where hotspots occur, and what determines their severity is fundamental to thermal design.
Automotive applications typically require the temperature difference between any two cells in a module to remain below 5°C during normal operation. Exceeding this gradient drives SOC imbalance, resistance imbalance, and capacity fade imbalance — all forms of aging heterogeneity that degrade pack performance cumulatively over thousands of cycles.
There are four fundamentally different cooling approaches used in battery packs, each with distinct trade-offs in performance, complexity, cost, weight, and packaging.
| Approach | Cooling Capacity | Complexity | Cost | Fast Charge? | Best Application |
|---|---|---|---|---|---|
| Air Cooling | Low | Minimal | Lowest | No | Consumer, early EV, controlled ESS |
| Liquid Cooling | Very High | Moderate | Medium | Yes | Premium EV, PHEV, fast-charge ESS |
| PCM | Transient only | Low | Medium | Limited | Bus, industrial, hybrid systems |
| Cold Plate | Very High | Moderate | Medium | Yes | Automotive standard — most EVs |
Thermal interface materials are applied between the cell surface and cooling structure to improve heat transfer across the interface. Without a TIM, air gaps created by surface roughness act as thermal insulators, dramatically reducing cooling effectiveness.
The choice of TIM has a significant and often underappreciated impact on serviceability. A thermal adhesive that bonds cells to the cold plate provides excellent thermal performance and mechanical stability — but makes individual cell replacement essentially impossible. A non-adhesive gap pad allows cell removal. If cell-level replacement is a planned service operation, this trade-off is a primary factor in TIM selection — not an afterthought.
Thermal runaway propagation — the spread of thermal runaway from a single cell to adjacent cells — is one of the most critical safety design challenges in battery engineering. Automotive standard UN ECE R100 requires that a single-cell thermal runaway event does not result in fire or explosion at the vehicle exterior within 5 minutes, allowing occupants to evacuate.
The structural design of a battery pack must satisfy requirements that are in direct tension: strong enough to protect cells under mechanical loads, yet light enough not to compromise vehicle range; stiff enough to maintain dimensional integrity, yet compliant enough to accommodate cell swelling.
The physical components that contain, position, and protect the cells form a hierarchy of structural elements — from the cell holder at the individual cell level, to the module frame at the group level, to the pack enclosure at the system level.
The IP (Ingress Protection) rating — defined by IEC 60529 — provides a standardised measure of protection against solid particle and liquid ingress. For automotive battery packs, IP rating is not a marketing specification — it is a safety requirement.
Many premium packs: IP6K9K
Achieving IP67 requires careful sealing of every penetration through the pack enclosure — HV connectors, LV connectors, coolant inlet/outlet fittings, service access panels, and pressure equalisation vents must all be sealed to the required standard using moulded rubber gaskets, O-rings, or formed-in-place (FIP) silicone seal beads. The sealing interfaces must maintain integrity across the full temperature range and after the thermal cycling and vibration the pack experiences over its full service life — typically 10–15 years in automotive applications.
| IP Rating | Liquid Protection | Application |
|---|---|---|
| IP54 | Splash from any direction | Industrial ESS in sheltered locations |
| IP65 | Low-pressure water jets | Outdoor ESS cabinets |
| IP67 | Temporary immersion (1m/30min) | EV battery pack minimum |
| IP68 | Continuous immersion (defined depth) | Submersible / marine applications |
| IP6K9K | High-pressure/steam cleaning | Premium EV — wash-safe servicing |
The structural design of a battery pack must balance two competing objectives: minimising components and assembly complexity (for manufacturing cost and weight reduction) while maintaining sufficient accessibility for any required maintenance, inspection, or repair operations.
CTP architectures that eliminate the module level dramatically reduce weight and improve volumetric energy density — but they also make cell-level replacement essentially impossible in the field. This is a deliberate engineering trade-off: the cost reduction and performance benefit of CTP is deemed worth the increased end-of-life replacement cost by most manufacturers adopting this architecture. Understanding this trade-off and communicating it clearly is part of a battery designer's responsibility to the product team.
