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Nowadays, the tendency towards electric vehicles is increasing and various studies are being carried out in order to eliminate the shortage of battery life. One way to improve battery life is to overcome imbalances in battery packs. The imbalances due to structural differences arising from production and changing operating conditions, reduces the utilization of available energy. Passive and active equalization methods are preferred in order to overcome these imbalances. Due to the slow equalization and energy inefficiency of passive balancing circuits, the trend towards active balancing circuits is increasing. In this thesis, an active cell balancing model and a new active cell balancing algorithm that is able to transfer excess energy to the cells with weak energy via the charge transfer-bus (CTS) is proposed for a electric heavy commercial vehicle (EHCV) equipped with large battery packs (LBPs). To test and evaluate the performance of the active balancing system (AcBS), an active balancing circuit (AcBC) that includes 11 modules which consist of 16 cells equipped with bidirectional dc-dc converters and switch matrixes have been simulated in MATLAB/SIMULINK environment. The study has been carried out in two stages. In the first study, a model is presented for LBPs which can be applied in EHCVs. CTS is used for transferring excess charge from source cell to destination cell. Unlike topologies in literature, presented model doesn't need for external storage element because it uses the existing vehicle battery to buffer the CTS. The presented LBP model consists of 176 cells connected in series. Thus, twenty-two bidirectional dc-dc (BiDC) converters are used in the simulation because every 8 cells are combined with one dc-dc converter. Controlling the switching signals of BiDC converters involves high runtime/complexity. Therefore, current sources are used instead of dc-dc converters by simplifying the model in the simulation. In the second study, a new balancing algorithm is proposed. Compared to existing approaches, the algorithm with features of fast balancing, smaller voltage imbalance range, and multiple charge transfers is suitable for LBPs. By comparing the proposed algorithm with the existing algorithm, the presented algorithm has been shown to have better results for LBPs. Key words: Active balancing, large battery pack, heavy electric vehicle.