In the past two decades, the performance of battery energy storage systems (BESS) has been significantly improved with the utilization of advanced architectures and control methods and new electronic devices. However, the increasing demands imposed by BESS applications still necessitate the need for additional performance improvement and/or create new issues that need to be addressed. These can be summarized as follows: (1) The imbalance in the state-of-charge (SOC) between cells might occur, which might degrade the performance of the battery system. (2) In a BESS, with the increasingly advanced functions and control methods, the number of required components is increased. (3) In electrical vehicles (EVs) applications, the limited driving range and the needed charging time of the lithium-ion (Li-Ion) battery pack is one of the major reasons slowing down the adoption of EVs. (4) The transmission efficiency of wireless power transfer (WPT) systems is decreased as the distance and misalignment between transmitter (Tx) and receiver (Rx) increase. (5) In order to realize the wireless power transfer in BESS, additional components such as DC-AC inverter Tx coil, Rx coil and AC-DC rectifier are required, which increase the cost and size of the system. This dissertation work focuses on investigating the challenges mentioned above to further improve the overall performance of battery system, reduce the number of components and converters, increase the system efficiency and realize a robust and cost-effective battery energy storage system. Chapter 2 and Chapter 3 focus on the challengers related to SOC balancing and large number of components in battery systems. In Chapter 4, one unique aspect of the WEDES system is used in order to add flexibility and improve safety. Chapter 5 and Chapter 6 address the challenge related to decreased transmission efficiency in wireless power transfer when charging a battery. In Chapter 7, in order to deal with the challenge that additional components are required to realize wireless power transfer, a single dual-type-output power converter is discussed and analyzed. Chapter 8 provides a summary and conclusion for the work presented in this dissertation and discusses some potential future work.