Effects of Depth-of-Discharge, Ambient Temperature, and Aging on the Internal Resistance of Lithium-Ion Battery Cell

The energy and power capabilities of lithium-ion batteries degrade over time. This degradation is quantified by the state-of-health (SOH), which is a function of multiple battery characteristics. Among these characteristics, remaining capacity and internal resistance are the two dominant ones. However, measuring the internal resistance of a lithium-ion battery is not easy because lithium-ion batteries are not pure ohmic devices. The capacitive and inductive behaviors of lithium-ion batteries make the internal resistance measurement more difficult. In this paper, the direct wave pulse method is used to measure the internal resistance of the cell. The effects of depth-of-discharge (DOD) (between 10-90^%), ambient temperature (-25 to 50 degrees Celsius), and aging (up to 800 cycles) on the internal resistance of a 20Ah lithium-ion polymer battery cell are investigated. Result shows that the internal resistance of the cell increases from average 0.0041 ohm to average 0.0311 ohm as the ambient temperature decreases from 50 to -25 degrees Celsius. Also, an aged cell after 800 aging cycles has average 0.0188 ohm internal resistance comparing to average 0.0031 ohm when the cell is brand new. The DOD level has smaller effect on the internal resistance, but the internal resistance can be doubled when the DOD approaching 90% for an aged cell (after 600 aging cycles). This paper provides a better understanding of the cause for the capacity shrinkage of lithium-ion batteries and also provides supporting information for the battery SOH estimation.

I. Introduction

Even though lithium-ion batteries feature advantages such as high energy density, high specific energy, low self-discharge rate, and minimal memory effect, one of their major drawbacks is the durability [1], [2]. The energy and power capabilities of lithium-ion batteries degrade over time. This degradation is quantified by the state-of-health (SOH), which is a function of multiple battery characteristics [3]. Among these characteristics, remaining capacity and internal resistance are the two dominant ones, because they assess the batteries' capabilities of storing energy and delivering power, respectively. The remaining capacity of a lithium-ion battery can usually be measured easily if the battery's specifications are known. However, measuring the internal resistance of a lithium-ion battery is not easy because lithium-ion batteries are not pure ohmic devices. The capacitive and inductive behaviors of lithium-ion batteries make the internal resistance measurement more difficult [4].