ETD

• balancing
• BMS
• capacity
• EV
• LiFePO4
• SoC

The charge imbalance in Lithium battery packs is an important problem to deal with in the design of an energy storage system. The main imbalance sources are different series resistances and self-discharge rates of the cells constituting the battery, but also different working environments. This work is part of a larger project the purpose of which is to realize an energy storage system for electric off-road vehicles. The main aim of this thesis is to deal with the charge imbalance problem of a battery pack. First a battery characterization with a detailed study of Lithium-iron-phosphate cells has been carried out. Then the main functions of a Battery Management System have been developed, implemented and tested on a module of 4 series connected cells to guarantee the optimum utilization of the pack. In particular an innovative method for estimating capacities of series connected LiFePO4 batteries based on charging characteristics has been proposed. The method brings to an effective approach to the imbalance problem and to an accurate individual cell state of charge estimation. A new definition of balanced battery and a new balancing method are proposed. Their aim is to keep the battery as far from an imbalance situation as possible. Finally, the strategies to balance the cells of a battery module and to balance modules each other are studied and characterized, to find the best algorithm. The complete system has been integrated and tested on a 4 batteries module BMS.