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D3.2 Cell characterization and combination with thermal management system technology - Full report

This report contains a summary of the work carried out in Task 3.1a, T3.1b and T3.5 within WP3. Specifically, these tasks focus on the electrical and thermal characterisation of the initial candidate battery cells, including modelling of their performance to feed BMS development activities in WP4, as well as the initial design of the cooling system for the battery pack refrigeration. Within the execution of these tasks, it has been found that the performance of the cells was in-line with manufacturer specifications. Additional experimental data has fed the electrochemical and equivalent circuit to reach precision below 30 mV.

Three different electrochemical models have been used to reproduce the cell behaviour. The validation process has shown the NTGK model to be the most suitable for the goals of the project. Five different discharge rates have been simulated: 0.05, 0.33, 0.5, 1 and 2 C. The maximum heat generated corresponds to the 2C rate discharge, with a value of 41 kW/m3, which is the most important information for the future design of the cooling circuit. A linear fit has also been obtained relating the total heat generated and the discharge rate.

In terms of electrical (voltage) response, a 2-RC-equivalent circuit model (ECM) has been developed. The parameters have been calculated based on the HPPC-measurement data (current pulse at different SoC-steps and operation points) by a least-squares algorithm. This resulted in a set of model parameters for every operation point (T, SoC, I), which can then be used to simulate the cell’s electrical response at any driving profile. To assess the model accuracy, a WLTC profile was simulated for a representative cell, which showed satisfying results considering the 30 mV error window. In contrast, the ECM cannot predict the temperature evolution of the cell as accurate as the electrochemical model, which leads to the conclusion, that both models will be combined with their respective advantages to build an overall modelling approach with high accuracy.

The TMS can be characterised by a top and bottom cooling design. With aluminium fins in between the cells a homogeneous temperature distribution can be achieved within the BP. Open cell aluminium foam can improve the thermal conductivity to fulfil the project requirements from the thermal standpoint.