Hybrid energy storage systems or HESS are typically coupled with power converters through a DC or AC network. Different converters are used depending on the application. Power converters are used to control the power flow among the different storage elements. There are different ways of coupling different batteries using power converters. Both series and parallel converter arrangements are available. Depending on the complexity of the control strategies, the use of power converters and microcontrollers can be expensive. As a result, the trade-off between economic feasibility and technical advantages is crucial in determining the financial and technical viability and implementation of HESS.
One major challenge in HESS is designing the energy management controllers for real-time implementation to yield a good power split performance. An imbalance often develops between the DC link bus and the battery bank voltage as a result of the battery change in SOC. This change occurs even if the battery voltage sizing was previously carefully determined. A fixed structure of an HESS bank seems insufficient to solve this issue. Although a run time reconfiguration of the energy storage banks has been proposed in the past, a comparative study in terms of energy efficiency, capacity utilization, scalability, flexibility, hot swap capability, cost as well as the overall systems enhancement has yet to be established. Other critical areas in the study of HESS include charge allocation, charge replacement and charge migration within the energy storage systems.