This study is research upon the over-potential of Li-iron batteries. By performing many cycles of charging-discharging experiments under controlled ambient temperature, adjusted depth-of-discharge, and discharging current rates, the dynamic voltage variation data is collected. A model based on the cubic over-potential differential equation is proposed. This model is able to predict the gradually prolonged time-constant dynamics of the battery loaded voltages with fewer parameters. By this way, a set of procedures is also proposed to adjust the parameters as the battery ages. The experimental result is used to verify the feasibility of this model. Even for the aged batteries, the prediction error is small and the result is satisfactory.

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In this paper, we propose a novel charge-equalization circuit for series-connected batteries. The circuit is composed of an intermediate resonant energy tank, formed by an inductor and a capacitor, and selection switches connected to each battery. The advantage of the proposed circuit is that an exchange of unbalanced charge can be established between any two batteries via the intermediate energy tank. In addition, more than one adjacent battery can serve as a charge-transference party on either side through proper activation of the associated selection switches. Moreover, by virtue of the resonant tank, zero-current switching can be achieved to significantly reduce the transference losses. A laboratory circuit with a control unit was designed for eight 2.5 Ah lithium iron phosphate (LiFePO4) batteries in series. The test results demonstrate that the proposed charge-equalization circuit can realize charge balance effectively. With the charge-equalization circuit, the worst open-circuit voltage difference can be reduced to less than 20 mV under offline conditions, which helps the batteries operate more efficiently.