An Optimal Charging Method for Li-Ion Batteries Using a Fuzzy-Control Approach Based on Polarization Properties

Battery charging is growing in importance as it has direct influence on battery performance and safety. This paper develops a generalized online estimation method for a charge-polarization-voltage-based resistance-capacitance (RC) circuit model to simulate the charging behavior of Li-ion batteries. The effects of charging current, initial state of charge (SOC), initial polarization state, and aging on the charge polarization voltage are quantitatively analyzed in both time and SOC domains. It is demonstrated that the charge polarization voltage is nonlinearly related to these impact factors. In the SOC domain, the change in the charge polarization voltage is also analyzed with the gradient analytical method, and the relations between current, polarization voltage amplitude at the inflection point, and SOC are quantitatively established. This can be used to estimate battery SOC and polarization voltage accurately. A constant-polarization-based fuzzy-control charging method is proposed to adapt charging current acceptance with battery SOC stages. Experimental results demonstrate that the proposed charging method significantly shortens charging time with no obvious temperature rise compared with the traditional constant-current-constant-voltage charging method.