After a Lithium battery pack is used for a period, the inequality of battery cells will increase and the performance of the pack will decrease. The lack of appropriate user maintenance measures results in the scrap of the whole battery pack and resource waste. To solve the above problems, this paper designed an intelligent lithium battery monitoring and maintenance system based on the relative capacity estimation of battery cells, and proposed a pack usability monitoring method and a battery selecting method both based on the average capacity of a battery pack and the capacity of the battery cells. This system is a balanced / imbalanced dual-mode system, which can improve reduce the inequality of battery cells under the balanced mode, and detect the performance of the pack and cells under imbalanced mode. Experiments show that this system can improve the utilized efficiency and performance of batteries, detect the battery pack performance and select the bad battery cells.