An on-line tracking algorithm for Li-ion batteries' optimal charging frequency

This paper proposes an online tracking algorithm to allocate and track the optimal charging frequency for lithium-ion (Li-ion) batteries in real time under any condition. The optimal frequency refers to the frequency of the sinusoidal component of the charging current at which the internal AC impedance of the battery cell is minimal. The AC impedance depends on the physical properties of the battery's electrodes and electrolyte. It also varies nonlinearly with temperature, state-of-charge (SOC) and state-of-health (SOH). Although the AC impedance can be determined offline using sophisticated AC cell models or measurement equipment, there is no direct way to measure it in real-time. The proposed technique has the capability of allocating and tracking the optimal frequency in real-time without using an AC battery model or measurement equipment. Moreover, the proposed technique is simple, inexpensive to implement, and applicable to any battery cell or pack. Derivation of the proposed technique followed by simulation verification is presented.