Characterization of NMC Lithium-ion Battery Degradation for Improved Online State Estimation

This paper presents a characterization of Nickel Manganese Cobalt (NMC) Lithium-Ion (Li-ion) batteries based on empirical tests to improve online state estimation. It investigates how battery degradation affects the battery management systems (BMS) ability to estimate the state of charge (SOC) and state of health (SOH). The testing is conducted on 8 NMC 18650 Li-ion battery cells split up in two groups of four new and four degraded cells with an SOH of 90%. As both the SOC and the SOH estimations are dependent on the relation between the SOC and the open circuit voltage (OCV), the necessary relaxation time to achieve a stable OCV as well as the SOC-OCV relations dependency on temperature and ageing are derived. The relaxation time needed to achieve a stabilised terminal voltage is, in the worst case, found to be equal to 20 minutes for both the new and the degraded cells. The SOC-OCV curve is not changing significantly when the operation temperature is increased from 23°C to 45°C, which is also the case for both the new and the degraded cells. The internal battery resistance and capacitance are however found to depend significantly on both the SOC and operation temperature. A first order RC representation is derived for different SOC and temperatures levels.