Abstract
With regard to safety, efficiency and lifetime of battery systems, the thermal behavior of battery cells is of great interest. The use of models describing the thermoelectric behavior of battery cells improves the understanding of heat generation mechanisms and enables the development of optimized thermal management systems. In this work, a novel experimental approach is presented to determine both the irreversible heat due to ohmic losses and the reversible heat due to entropy changes directly via heat flow measurements. No additional information about thermal properties of the battery cell, such as heat capacity or thermal conductivity, are required. Thus, the exothermic and endothermic nature of reversible heat generated in a complete charge/discharge cycle can be investigated. Moreover, the results of the proposed method can potentially be used to provide an additional constraint during the identification process of the equivalent circuit model parameters. The described method is applied to a 23 Ah lithium titanate cell and the corresponding results are presented.
Highlights
The phenomenological modeling approach using an equivalent circuit model (ECM)to describe the electrical behavior of Li-Ion cells is widely used in academia and industry.The standard ECM of a Li-Ion battery cell consists of an ideal voltage source whose voltage depends on the state of charge (SoC), an internal ohmic resistance R0, and one or more parallel resistor-capacitor (RC) networks that approximate transient effects with various time constants
The results of the measured heat flow will be discussed on the basis of an exemplary chosen data set, namely the 3C experiment between 60% to 70% SoC
The total amount of heat generated during the discharge pulse is significantly higher than that during the charging phase. This already indicates the presence of reversible processes in this particular state of charge
Summary
The standard ECM of a Li-Ion battery cell consists of an ideal voltage source whose voltage depends on the state of charge (SoC), an internal ohmic resistance R0 , and one or more parallel resistor-capacitor (RC) networks that approximate transient effects with various time constants. A voltage hysteresis dependent on the current flow direction and a Warburg impedance can be added to increase the model accuracy. Accurate determination of the internal resistance and RC components is essential because they significantly affect the terminal voltage of the cell. RC models such as [1,2,3] among others, the capacitor of the RC-network is assumed to be constant when formulating the differential equation, which leads to the well known form published maps and institutional affildVi I Vi = − , dt Ci Ri · Ci iations
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