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The reversible (entropic) heat source contributes to the thermal behavior of a lithium-ion battery in particular at the initial state of charge and discharge. One factor that affects the magnitude and direction of the reversible heat is called the entropic coefficient (EC). The objective of this research is to calculate the varying entropic coefficient values of the lithium-iron phosphate battery. A 14Ah lithium ion pouch cell, with a dimension of 220 mm × 130 mm × 7 mm, was studied in both charge and discharge. The SOC levels range from full charge to full discharge in 5% increments. The temperature levels vary from −20 • C to 55 • C in 5 • C increments. It reveals that there is a strong influence of cell temperature on the entropic coefficient when the cell is at its extreme upper or lower SOC level. A correlation was obtained to relate the EC to temperature and the SOC by curve fitting the experimental data. Calorimetric data of the test cell was also presented that shows the influence reversible heating has on the overall rate of heat generation within the cell. The calorimetric data and the EC measurement were combined to determine the irreversible heat generated in the cell.

Infrared thermography shows that the joint between the cathode grid stack and the cell tab is a source of Joule heating within a lithium-ion pouch cell. This can exacerbate thermal gradients within the cell core if the C-rate is sufficiently high. This paper studies the heat generated at the cathode tab joint of a 14Ah lithium iron phosphate (LFP) pouch cell. The heat generation was quantified by using an energy balance equation and the average heat transfer coefficient was calculated by modeling the cell as an isothermal vertical plate in natural convection. The influence of this heat on the cell's thermal gradients was studied during a 3C and 8C rate of discharge. It has been found that removal of this heat at its source can appreciably lower the overall average surface temperature of the cell. However, at a 3C rate discharge, the removal of this heat can induce a greater thermal gradient within the cell core. At an 8C rate of discharge, there is a minimal improvement in the temperature gradient. As a result, a thermal management system which incorporates cathode tab heat removal would most likely be an ineffective design feature. Maintaining uniformity of temperature between cells is an important factor in the thermal management of lithium-ion batteries. It is desirable for this variation to be in a tight range.1 Rugh et.al. stated that temperature gradients should be kept to less than 3• C to 4• C throughout the battery pack.2 Failure to meet this criterion has a direct effect on decreasing the battery pack life. In addition, they also advocate an operating temperature range of 15• C to 35• C for the pack. Excursions beyond this range have repercussions as well. It has been found that the lifespan for a lithium-ion cell is reduced by approximately 2 months for every degree of temperature rise while operating in a temperature range of 30• C to 40 • C. 3 Capacity and power degradation is also accelerated at elevated temperatures. 4-9The lithium iron phosphate (LFP) cell chemistry is gaining wide acceptance in electric vehicle applications.10 Its inherent ability to tolerate abusive conditions and resist thermal runaway is especially attractive to battery pack designers. Battery manufacturers have responded by offering high capacity cells in a pouch format. This format affords better packaging efficiency and offers a very favorable area-tovolume ratio to facilitate thermal management. As a result, this work uses a high capacity LFP pouch cell.The temperature near the cathode terminal of a pouch cell is consistently higher than that at the anode, due to differences in the thermal conductivities of the metals used as current collectors. This is true regardless of the particular lithium-based electrochemistry being used.

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