Study on cell balancing circuit of lithium-ion batteries with charge-transfer structure between adjacent cells

Abstract

Nowadays, a number of applications require various rechargeable batteries as an energy storage element and power source. To meet the voltage and power demands of applications, multiple cells should be connected in series and parallel. Parallel-connected batteries naturally have the same voltage and state of charge, but series-connected batteries do not. Cell imbalances among series-connected batteries can occur due to inevitable electrical and chemical properties such as manufacturing tolerances, differences in aging characteristics, and uneven ambient temperatures. When these unbalanced batteries are left in use without any control such as cell balancing, the energy storage capacity of batteries decreases severely. In the worst case, they may explode or catch fire. Therefore, cell balancing circuit should be strongly required to guarantee safety and to extend the life time of the batteries. To achieve the cell balancing of a battery string, many techniques have been developed. Cell balancing circuits are classified into two types: passive cell balancing circuits and active cell balancing circuits. Passive cell balancing circuits can be easily implemented with low production costs and a small size using minimal resistors and switches. However, these schemes show a limited balancing current due to the heat problem from the power consumption of bleeding resistors. To overcome the drawbacks of passive cell balancing, active cell balancing techniques and circuits using charge-transfer among the cells have been studied and applied. Active cell balancing circuits can be classified into two methods: a pack-to-cell balancing method and a cell-to-cell balancing method. Cell-to-cell balancing method can be categorized into two methods: a direct cell-to-cell method and an adjacent cell-to-cell method. Through the immediate charge distribution and the direct charge-transfer, pack-to-cell and direct cell-to-cell balancing methods show outstanding balancing performance and...