Meaning
Thermodynamic principles that govern the rate at which thermal energy is produced within a battery cell during electrochemical activity. Total heat output results from the combination of reversible entropic heat and irreversible resistive heating caused by internal impedance. Understanding heat generation kinetics is essential for sizing cooling systems and predicting the onset of thermal runaway under high load conditions.
These rates vary significantly based on the state of charge, the current magnitude, and the ambient temperature.
Ohmic Contribution
Joule heating occurs as ions move through the electrolyte and electrons pass through the active materials and current collectors. Higher current densities accelerate heat generation kinetics by increasing the frequency of internal collisions and resistance losses. This component of the heat profile is always positive and grows with the square of the current.
Reaction Entropy
Reversible heat arises from the changes in the crystal structure of the electrodes as ions are inserted or removed. Depending on the specific chemistry, heat generation kinetics can actually show a cooling effect during certain stages of the discharge cycle as the entropy change becomes negative. This behavior is most prominent at low C rates where the resistive heating does not mask the entropic signal.
Active Overpotential
Polarization losses at the electrode surfaces contribute further to the thermal load of the operating cell. As the cell ages, heat generation kinetics typically increase because the internal resistance grows and the electrochemical reactions become less efficient. Monitoring the temperature rise at a constant current provides a clear measure of how much energy is being lost to heat rather than delivered as electrical work.
