Optimal Cell Equalizing Control Based on State of Charge Feedback for Lithium-ion Battery Pack

This paper presents a cell optimal equalizing control method for Lithium-Ion battery pack formed by many cells connected in series in order to extract the maximum capacity, maintain the safe operation requirements of pack, and prolong the cells cycle life. Using the active cell to cell equalizing method, the energy levels of two adjacent cells will be equalized based on the principle of the bidirectional Cuk converter with the pulse width modulation (PWM) control variables. The strategy to determine the optimal PWM variables is based on the solution of nonlinear optimal problem with the state of charge (SoC) feedback satisfying the constraints of the SoC levels, amplitude of charge/discharge current, equalizing currents, and control variables. The nonlinear optimal problem is solved by appying the sequential quadratic programming (SQP). Experimental optimal cell equalizing control results for Lithium-Ion battery pack with 7 Samsung 22P 18650 battery cells connected in series with charge/discharge alternately scenarios show that the cell optimal equalizing control proposed is quite effective and satisfied all the desired operation requirements of pack in the practice.