| Summary: | In this study, the characteristic properties of a lithium ion battery cell which is in the dimension standard of “18650’’, creates the battery pack of a hybrid vehicle are determined. The test setup that is able to measure instantaneous current, open circuit voltage and circuit voltage of the bat-tery cell during charging and discharging is prepared. The battery cell is periodically subjected to charge and discharge processes on the test set-up and current, battery voltage and open circuit voltage values are ob-tained. Based on the results of the test, internal resistance and efficiency values are calculated depending on status of the battery charge ratio by using ‘‘Equivalent circuit’’ and ‘‘Rint method’’. With the test results ob-tained, efficiency values were calculated depending on the battery charge rate during the charge and discharge process of the battery. Internal re-sistance and efficiency graphs of the battery cell are created based on the status of the battery charge ratio after the result of the process performed during the charging and discharging. It is determined that if the battery cell charge level is between 15% to %95, the average internal resistance value is 51,452 mOhm during discharhing and 57,48 mOhm during charg-ing. It is also determined that if the battery cell charge level is between 15% to %95, the average efficiency is 98,58 % during discharhing (1A) and 98,496 % during charging (1A).
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