Thermochemical Approach to Determining Battery’s Heat Release: RI2 Formula

Abstract

At discharge, a battery releases the stored energy of the redox chemical reaction partially by passing electric current throughout the external circuit, and partially dissipating it in the form of heat. The amount of that heat depends on the discharging current. This heat primarily warms the battery itself. Knowing its total amount as well as the heat flow is important for the safety of batteries, especially lithium-ion ones. In battery engineering practice, it is often suggested that the heat flow P follows the Joule-Lenz law that together with the Ohm law has the form P = I 2 R (I is the current and R is the battery’s internal resistance); we showed that this is not true. Both total heat and heat flow can be determined relatively easily by applying general ideas of thermochemistry and thermodynamics when the battery operates under projected use conditions by comparing the dependencies of the open circuit voltage (OCV) and voltage profile of the discharging battery upon the state of charge (SoC) or depth of discharge (DoD). Although the theoretical background is universal, being applicable to any kind of battery, our paper presents the results of examining three lithium-ion batteries of extensively used chemistries: lithium iron phosphate-graphite, lithium cobalt oxide-graphite, and lithium nickel cobalt aluminum oxide-graphite. Particular attention is given to the effect of the entropy of the reactions on batteries’ thermal behavior.