This chapter explains how batteries and capacitors are similar, in some respects. Both are charge and energy storage devices and consist of positive and negative terminals, called electrodes. In this chapter, the reader learns how batteries and capacitors can be constructed from flat or cylindrical electrode surfaces. In this chapter, the reader is shown how the negatively and positively charged electrodes, in batteries and capacitors, serve as interface points between these devices and the outside world – the outside world consisting of electrical loads and electrical power sources. This chapter describes various types of batteries; their pros, cons, strengths, and weaknesses; and lithium–ion battery is explored in depth. The charging and discharging profiles of various batteries are discussed. The concept of “C-rating” of batteries is discussed. This chapter explains Peukert’s law and its practical and mathematical applications. Furthermore, in this chapter, the concepts of specific energy, specific power, and energy density – as they apply to mainstream secondary batteries – are explained and illustrated through graphs and numerical examples. This chapter also introduces the reader to the electrochemistry behind the construction and operation of secondary batteries.