Sinusoidal chargers use a DC-shifted sinusoidal current profile to charge the battery at a frequency at which the battery's AC impedance is minimum. In this paper, a new method is proposed for allocating and tracking the optimal charging frequency in real-time for lithium-ion (Li-ion) batteries for sinusoidal chargers. The proposed method is based on tracking the maximum power point of the charging power by auto-varying the charging frequency in order to get almost a unity battery input power factor. A high battery power factor means that the real power used for charging is large, which means that the impedance is almost purely resistive, while a low battery power factor means that the real power used for charging is low due to the high input impedance of the battery. Details on the proposed method followed with simulation results and experimental verification on a 3.8V, 2600mAh Samsung Li-ion battery cell are presented.