Voltage Relaxation Methods for State of Charge, and State of Health Estimation in Lithium-Ion Batteries

Abstract

Lithium ion batteries are the dominant technology for energy storage, and only projected to grow as Electric Vehicles gain higher adoption rates. Monitoring of the state of charge and the degradation metrics (state of health and internal resistance growth) in lithium-ion batteries without destructive investigation is an important and difficult task. For this reason, an in-situ monitoring technique could be a useful method to determine state of charge and degradation metrics. Voltage relaxation is a method that could be such a monitoring technique. Voltage relaxation is the process by which operational voltage changes to a steady-state open-circuit voltage after current stops flowing in a battery. There are two important metrics with voltage relaxation: magnitude and curve shape. Magnitude is amount of voltage that is relaxed, where curve shape describes how it approaches steady-state open-circuit voltage. Two studies were performed on three popular chemistries of lithium ion-batteries with the purpose of: (1) finding a battery model to accurately curve fit voltage relaxation; (2) finding the relationships between degradation metrics and voltage relaxation metrics; (3) determine methodologies for evaluation of state of charge, state of health, and internal resistance from voltage relaxation. A new model was adapted from the equivalent circuit model. It can estimate state of charge within 0.5% for nickel based chemistries and 2.27% for the Lithium iron phosphate (LFP) chemistry while achieving acceptable curve fit error. The studies showed that for nickel based chemistries the magnitude of voltage relaxation is a function of state of charge and has a useful relationship to degradation metrics. This means that if state of charge is known, or determined, degradation metrics can be estimated from a voltage relaxation period. Methodologies were created and trialed that used these relationships to determine the state of charge, state of health and IR from voltage relaxation periods with considerations for practical applications. Voltage relaxation in LFP batteries is a function of state of charge, but insufficient degradation was achieved to analyze degradation metrics. For this reason, further studies should include full cycle life of LFP batteries to find these relationships and determine the sensitivity required of voltage relaxation analysis of degradation metrics.