Design Guidelines of Passive Balancing Circuit for Li-Ion Battery for Bleeding Current Adjustment Using PWM Technique

In a conventional passive balancing system for the Li-Ion battery, the drained current is determined by the installed bleeding resistors, which are fixed by a constant resistor. The fixed drained current may not support the increased demand for bleeding power when the Li-Ion cells age. An advanced cell balancing circuit should possess the ability to adjust the drained current to support the increased unbalanced SoC among the aged battery cells. The pulse width modulation (PWM) switching technique can be applied to control the duty cycle of the bleeding switch and, consequently, to regulate the averaged bleeding current. The bleeding current can be controlled by adjusting the duty cycle of the PWM switching from 0 to 100 percent of the maximum drained power. The passive balancing circuit controlled by PWM technique will be typically designed with a relatively high maximum bleeding current (low bleeding resistance) in order to obtain the highest flexibility in controlling the bleeding current. Under the requirements of high bleeding current and the ability to control the bleeding current using the PWM technique, this paper discusses technical challenges and design consideration of the passive balancing circuit, i.e. operating frequency and balancing MOSFET switch behavior. As an output of this paper, a design guideline for implementing the passive balancing circuit with the capability of bleeding current adjustment for the Li-Ion battery is suggested.