Battery-supercapacitor hybridization represents another research area for the institute. Supercapacitors with high power density and durability are particularly suited to handle rapid power bursts, while battery packs with high energy density can provide average power during vehicle cruising. This combination reduces peak power loads, weight and stress of the battery pack. Another challenge in the design of battery-based electric vehicles is the non-uniformity in the capacity, inner resistance, and thermal characteristics of the different battery cells. This may result in performance degradation, limited by the cell with the poorer performance, i.e., the “weakest-cell problem”. To tackle these challenges, the institute has been investigating new hybrid balancing systems (HBS). The key idea of the HBS is to simultaneously balance the energy in the battery cells and provide an interconnection mechanism with the secondary storage system, namely the supercapacitors (see figure below).
This HBS provides a low-cost hybridization solution, since no additional power converters are needed to actively distribute the energy flow between primary and secondary energy storage units. Smart control strategies for the HBS, based on convex optimization, were also exploited, allowing the minimization of the SOC and thermal variations in the battery cells, while decreasing battery stress.