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PCCM for Peak Thermal Load Shaving of LIB During 6C Fast Charge

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Summary:

Lithium-Ion Secondary Batteries (LIB) have a narrow operational window in terms of cell temperature. The optimum working temperature is between 15 and 45°C. Thus, modern battery systems for electric vehicles have complex active thermal management systems allowing for heating and cooling to operate the LIB in its desired temperature range. These systems often need to be oversized in order to take the high thermal load during fast charging. An inexpensive solution to buffer thermal energy during high loads allows to downsize the overall active cooling and to save weight and cost.
In this work, a phase change compound material (PCCM) serving as thermal buffer is developed and evaluated. The PCCM consists of a carbon structure that can host a paraffin. The compound is form stable and does not release paraffin during cycling. Due to the carbon content, the PCCM overcomes the weak thermal conductivity of paraffins while maintaining a high share of the enthalpy of fusion of the paraffin.
For evaluation, a battery-module-like test bed was designed using four automotive high-power pouch cells with 30 Ah nominal capacity. It allows for i) no, ii) tab, iii) bottom active cooling with 10°C cooling fluid with and without PCCM, and is equipped with 34 temperature sensors (Pt100). The PCCM is designed as a set of plates with the size of the pouch cell’s surface. The ratio of carbon to paraffin is also varied to investigate the influence of the thermal conductivity and storage capabilities of the materials. The melting point of the paraffin is 37°C. Realistic load profiles were defined containing low- and high- thermal load scenarios. Fast charge capability is evaluated up to 6C which allows a 0 to 80% charge in less than 10 min. The different cooling concepts are evaluated with and without the PCCM at a controlled ambient temperature of 33°C which is considered as worst-case scenario for middle European weather conditions.
It is shown that in terms of maximum temperatures, the highly thermally conductive PCCM combined with an active cooling circuit at the bottom is superior to the other cases. The tab cooling leads to gradients over the cell surface of up to 5K, so maximum temperatures are reached at an earlier stage than for the bottom cooled cells. The PCCM, as expected, improves the power capabilities of all designs. Most significant is the impact on the no-cooling scenario where it allows for doubling or tripling the number of achievable cycles before reaching the temperature threshold. For the other concepts it brings significant improvements. In combination with bottom cooling it allows for continuous 1C discharge, 6C charge cycles without exceeding the temperature limit. Using a PCCM with higher carbon content is beneficial, because of the improved thermal conductivity.
Overall it is concluded that the PCCM can smooth thermal profiles of LIB and thus reduce peak load on cooling systems. PCCM leads to improved heat conduction compared to pure paraffin, is stable in form and thus suited for real-life application.

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