The continuous growth of the electric vehicles market and the increasing environmental awareness impose to search for innovative solutions to reuse the exhausted vehicle batteries in different applications. One of the main problems of second-life batteries is the very high mismatch among their cell capacities which reduces the overall battery performance. The mismatch effect can be overcome by using dynamic equalization. This technique aims to keep balanced as much as possible the State of Charge of the cells during the battery operation, i.e. in both the charge and discharge phases. In order to do this, the dynamic equalization approach requires a high-current active energy balancing system able to move a quantity of charge among the battery cells much higher than the active balancing circuits sometimes used in first-life batteries. The use of a high equalization current increases the design complexity of the balance system.
A methodology to study and compare the main balance system topologies suitable for second-life batteries with dynamic equalization approach is presented in this work. The Adjacent Cell-to-Cell, Direct Cell-to-Cell, Cell-to-Pack, and Pack-to-Cell active balance topologies are analyzed considering the case study of a second-life battery composed of 10 series-connected cells with capacity values uniformly distributed around
15 % of the nominal value.
The investigation proves that the Direct Cell-to-Cell topology has the best performance. This balancing topology improves the usable battery capacity of around 16 % with respect to the case in which no dynamic equalization is applied if a DC/DC converter with a power efficiency of only 70 % is used. Finally, the results show that the Adjacent Cell-to-Cell performance is strongly affected by the position of the cell mismatches in the battery.
Keywords: Second-life battery applications; Dynamic balancing approach; Active energy balance architectures; Battery Management System
