Abstract: A battery expansion system and a battery charging and discharging control method configure the cells as an expansion form to connect to a system host such that the charger IC of the system host treats the battery expansion system as a single battery cell for DC charging and discharging control. The battery expansion system utilizes a controller in control of the charge/discharge route for each battery cell. In such way, each battery cell is capable of providing power for the system host or charging another cell until the battery cells are balanced with one another. With the control of a current control device and a charging switch in the charging/discharging route, the controller may effectively limit the charging current toward each cell under the maximum design charging current once a current impulse or in-rush current occurs.