Battery systems are widely used in many applications including portable electronics, EVs/HEVs, and distributed smart power grids. In addition to battery technologies, the battery management system (BMS) plays a critical role in enabling the widespread adoption of battery-powered applications. This dissertation work focuses on addressing several issues and improving performance of several aspects of battery powered applications. These focused topics include online monitoring of battery impedance, charge balancing between battery cells during both discharging and charging operation, and power electronic topologies and control in order to improve reliability, efficiency, and density of the battery-powered applications. In chapter 2, a practical method is presented in order to achieve accurate online battery impedance measurement while maintaining output voltage regulation of the power converter. The proposed method is based on converter duty cycle control and perturbation. As a result, all the external signal injection circuitries are eliminated. In chapter 3 and 4, the charge balancing issue is addressed from the root by automatically adjusting the discharge/charge rate of each cell based on a new distributed battery system architecture with energy sharing control. The proposed energy sharing controller does not require any charge/energy transfer between the cells, thus eliminating the power losses during energy transfer process. To gain insights into the dynamics of the energy sharing controlled distributed battery system, the state-space averaging small-signal modeling and controller design is performed in Chapter 5. Simulation and experimental results are presented for verification. Single-inductor multiple-output DC-DC converter has gained increased popularity in the portable applications where a battery is used to power multiple loads. However, a common issue facing the SIMO converter design is the cross regulation between the multiple outputs during steady-state and dynamic operations. To address this issue, a power-multiplexed controller is presented in Chapter 6 which eliminates the cross regulation between the outputs by multiplexing the conduction of each output channels. Each output is independently regulated under steady-state and dynamic operations regardless of the operating mode, i.e., continuous or discontinuous conduction mode. Chapter 7 summarizes this work and provides conclusions before discussing some possible future research directions related to this dissertation work.