Meaning
Spatial variation in temperature arises from the uneven distribution of internal resistance and current density within a battery cell during high power discharge or charging cycles. This joule heating thermal gradient affects the aging rate of different regions within the electrode stack. It governs the efficiency of the thermal management system and the overall safety of the battery pack.
The term stops applying when the cell reaches thermal equilibrium with its surroundings.
Heat Generation
Internal resistance of the current collectors, active materials and electrolyte causes the conversion of electrical energy into heat. This joule heating thermal gradient is exacerbated by the local variations in the state of charge and the reaction kinetics. High current density at the tab connections leads to localized hot spots that can damage the separator.
The amount of heat produced is proportional to the square of the current and the resistance of the path. If the heat is not removed quickly, the temperature of the cell will rise and may trigger exothermic side reactions. Designers use electrochemical models to predict the heat distribution under various operating conditions.
Temperature Distribution
Conduction through the cell components and convection from the surface determine the shape of the temperature profile. This joule heating thermal gradient is typically most pronounced in large format cells where the distance to the cooling surface is greater. The center of the cell often becomes much hotter than the edges which leads to non uniform aging.
This variation in temperature causes different parts of the electrode to degrade at different rates. The resulting imbalance in capacity and resistance further distorts the current distribution. Thermal imaging and internal sensors are used to monitor the temperature map of the battery.
Accurate mapping is essential for the design of effective cooling plates and heat sinks.
Thermal Management
Active cooling systems must be designed to minimize the temperature difference across the cell to ensure uniform performance. This joule heating thermal gradient is reduced by using materials with high thermal conductivity and optimizing the flow of the coolant. Liquid cooling is more effective than air cooling for managing the high heat loads of electric vehicle batteries.
The control strategy for the cooling system relies on the accurate estimation of the internal temperature profile. Excessive gradients can lead to mechanical stress and the delamination of the electrode layers. Long term reliability of the battery pack depends on maintaining a stable and uniform temperature.
The final thermal design is a primary factor in the life cycle cost of the energy storage system.
