Performance Estimation Method of Li-Ion Starting-Lighting-Ignition Batteries of Electric Vehicle through Lab-Scale Pouch Cell Experiment

Abstract

CCA (cold-cranking amps) is used as one of the standards to define the power of a SLI (starting-lighting-ignition) battery of electric vehicles (EVs) by applying a large discharge current for a short time. It is easier to start an engine at a high temperature than a low temperature, so tests are usually conducted at a low temperature to meet the demands of users. However, in a large cell, the internal temperature of the cell rises due to the heat of the cell when a large current is applied. This phenomenon can increase the overall cell power performance because it has the effect of reducing ohmic overvoltage and improving electrode kinetics. Therefore, it is difficult to distinguish the effect of the temperature rise and applied current on the voltage curve during the consecutive pulse discharge.

In this study, three times of 10 C discharge current pulse (~ 7 mA/cm2 in this study) is applied on a cell for 30 seconds, with rest periods of 600s, 30s included between each pulse. Evaluations were conducted from -18 ℃ to 50 ℃, with a LiFePO4/graphite lab-scale pouch cell rather than an actual large cell. Therefore, the temperature of the cell remained constant under the evaluation conditions in which heat generation is almost zero, so the voltage curves can be interpreted by excluding the effect of the temperature rise of a cell. Based on these experimental results, voltage curves were simulated depending on each internal resistance that related to Joule heating of the actual large cell. It is possible to predict the performance and degree of temperature rise of large cells. Also, this study proposes an effective and economical method to distinguish the effect of the temperature rise of the large cell and the applied current by using a lab-scale pouch cell with controlled temperature.