A Lithium battery manager based on modular multilevel converter (MMC) is researched. The charging/discharging control and voltage balance management are unified inside one single converter instead of two independent ones, which solves the problem of complex circuits. The operating principle is analyzed. The function requirements are set according to the example of a 12 W battery pack. Control strategy and energy management strategy are designed accordingly, then the control block diagram and program flowchart are obtained. Afterwards, an experimental prototype and its key parameters are designed. Eventually, simulations and experiments are conducted. The simulation results indicate the bidirectional charging/discharging current control, output voltage control and the voltage balance can be realized, and the above-mentioned analysis and design methods are verified. The bidirectional charging/discharging current control and output voltage control are realized in the prototype, while the long-term performance of voltage balance still needs to be tested.