A Novel Real-Time State-of-Health and State-of-Charge Co-Estimation Method for LiFePO_4 Battery

The state of charge (SOC) and state of health (SOH) are two of the most important parameters of Li-ion batteries in industrial production and in practical applications. The real-time estimation for these two parameters is crucial to realize a safe and reliable battery application. However, this is a great problem for LiFePO_4 batteries due to the large constant potential plateau in the charge/discharge process. Here we propose a combined SOC and SOH co-estimation method based on the experimental test under the simulating electric vehicle working condition. A first-order resistance-capacitance equivalent circuit is used to model the battery cell, and three parameter values, ohmic resistance (R_\rm s), parallel resistance (R_\rm p) and parallel capacity (C_\rm p), are identified from a real-time experimental test. Finally we find that R_\rm p and C_\rm p could be utilized to make a judgement on the SOH. More importantly, the linear relationship between C_\rm p and the SOC is established to make the estimation of the SOC for the first time.