Experimental investigation of the influence of electrical contact resistance on lithium-ion battery testing for fast-charge applications

With the rising demand for electric vehicles with a fast-charging ability, high currents are applied to lithium-ion batteries to develop accurate battery models and intelligent fast-charging strategies. In order to achieve reliable results for automotive applications, single cell test conditions should be as close as possible to the conditions present in the battery pack of the battery electric vehicle. As cells are irreversibly connected in a battery pack, electrical contact resistance (ECR) is usually in the magnitude of <1 mΩ, and thus far lower than in reversible contacts during lab-testing. An interesting question arises as to whether this ECR has any unintended influence on the battery cell during testing. In this article, various experiments with high charge rates of up to 5 C are performed in order to assess the impact of the ECR of the measurement setup on the cells’ behavior. Two different commercial contact probes with different ECRs are tested on a 18650 lithium-ion battery, and compared to a laser-welded cell as a benchmark. ECRs of the lab-testing setups are measured with a micro ohmmeter, and the temperature evolution of all cells studied is measured at both cell tabs and the cell mantle during cycling. The results show that high peak temperature differences due to parasitic joule heat at the lithium-ion battery tabs occur when applying full charge cycles from 0.5 C to 5 C. Repetitive cycling with a multistage fast-charging strategy indicates a correlation of ECR with peak temperatures and aging spread. As a consequence, high ECRs could negatively affect drawn conclusions of cycle life tests. They should therefore be taken into account and kept low in any examination of fast-charging strategies.