In this paper, the cell balance of secondary battery using a large number of MOSFETs is discussed. We can balance the cells by controlling battery charging current with help of MOSFETs. If the cells are not balanced, we can not use the whole energy of the battery while charging and discharging, therefore, the energy efficiency is decreased. To increase the energy efficiency, we propose the MOSFET control algorithm which will perform cell balancing by controlling the charging current.