Batteries are primary sources of electrical power found on most aircraft, delivering direct current. (Secondary sources of power are described in Chapter 6.) There are several types of battery used on aircraft, defined by the types of materials used in their construction; these include lead-acid and nickel-cadmium batteries. The choice of battery type depends mainly on performance and cost. Other types of battery are being considered for primary power on aircraft; these include lithium and nickel-metal hydride. Electrical power delivered by batteries is used for a variety of applications, e.g. lights, radios, instruments, and motors. This chapter reviews the battery types used on aircraft, typical applications and how they are installed and maintained.