Besides the electrical properties, thermal effects are crucial when modeling a lithium-ion battery. Especially investigation of large-format cells is scarce because of the required systems. This poster compares two approaches to measure the specific heat capacity of 46Ah pouch cells, using only standard laboratory equipment.
The first method utilizes an aluminum reference workpiece with known thermal parameters to calculate the heat capacity of the cell. The workpiece has the exact dimensions of the original cell. The reference is heated to 50 °C in a climate chamber using a polystyrene insulated casing. The casing is used to avoid convection cooling and guarantee the same conditions for every test. After the object reaches a constant temperature level, the climate chamber is switched off, and the temperature decrease is measured. The reference is replaced by the battery cell in the second step, and the same test procedure is repeated. While the temperature decreases, the difference between the cell surface temperature and the temperature inside the climate chamber is fitted to obtain the heat capacity value.
The latter method, called Electro-Thermal Impedance Spectroscopy (ETIS), utilizes the internal heating of the cell, by irreversible heat generation, for example, the ohmic losses. The test is implemented analog to the well-known Electrochemical Impedance Spectroscopy, which uses a sine signal of different frequencies to build an impedance spectrum. Since the thermal parameters are more inert than the electrical, the used frequencies range from 30 mHz to 15 μHz. The heat generation also depends on reversible effects triggered by entropy change. The modulated current signal has to maintain the state of charge in a quasi-stationary state. Therefore, a second sine signal with a frequency in the low Hz range is superimposed. The test procedure results in a spectrum with the shape of a quarter circle in the negative imaginary part of the Nyquist plot. This spectrum is fitted with a least-squares regression method to obtain a value for the heat capacity.
As a result, the usage of the test, including a reference workpiece, is of good accuracy when validating with literature data from other pouch cells. This result leads back to the test setup, an adaptation of the calorimetric test. On the other hand, the ETIS procedure is not as consistent, caused by uncertainties, e.g., from convection and electrical connections. The ETIS shows a specific heat capacity about 30 % higher than the reference test and the values stated in the literature.
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