State of Health Assessment of Spent Lithium–Ion Batteries Based on Voltage Integral during the Constant Current Charge

Lithium–ion batteries (LIBs) are used in many personal electronic devices (PED) and energy-demanding applications such as electric vehicles. After their first use, rather than dispose of them for recycling, some may still have reasonable capacity and can be used in secondary applications. The current test methods to assess them are either slow, complex or expensive. The voltage integral during the constant current (CC) charge of the same model of LIBs strongly correlates with the state of health (SOH) and is faster than a full capacity check. Compared to the filtering requirement in the incremental capacity (IC) and differential voltage (DV) or the complex analysis in the electrochemical impedance spectrum (EIS), the voltage integral offers a simple integration method, just like the simple capacity Coulomb’s counter that is installed in many BMS for estimating the SOC of LIBs. By obtaining the voltage integral of a relatively new cell and an old cell of the same model with known SOH at a given ambient temperature and CC charge, the SOH of other similar cells can be easily estimated by finding their voltage integrals.