Section 5 · Topic 17
Thermal Management Systems and Architecture
Thermal management is a performance requirement before it is a safety one, because the same temperature window that maximises safety also maximises cycle life and efficiency.
Heat sources and the temperature window
A cell generates heat two ways: entropic heat from the reactions themselves, and ohmic heat — I²R — from current driven through internal resistance, the latter scaling with the square of C-rate. Below 0 °C the cell suffers increased resistance and plating risk; between ~15 °C and 35 °C it sits in the window of maximum cycle life; above ~45 °C it undergoes accelerated SEI growth, electrolyte decomposition, and gassing. The rule of thumb that reaction rate roughly doubles per 10 °C means 45 °C degrades the cell about twice as fast as 35 °C.
Cooling architectures
Extracting that heat to hold the window admits a spectrum of architectures matched to the application’s C-rate and environment.
Active liquid cooling circulates a water–glycol mixture through cold plates, exploiting the coolant’s high specific heat to manage the sustained I²R load of fast charge and high discharge. Dielectric immersion submerges the cells directly, removing the thermal-interface resistance between cell and coolant and flattening gradients enough to support extreme fast charge.
Phase-change materials do not continuously extract heat — they absorb latent heat as they melt, buffering temperature swings. Placed between cells, their role is to delay the cell-to-cell propagation of thermal runaway (Section 2), not to cool the pack in normal operation.
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