Power sources like batteries provide the electrical energy for circuits to function. Anything that uses a battery is relying on a DC power source. Cell phones, laptops, cars, and cordless appliances like drills or even wine-bottle openers all use batteries as a source of direct current. If a device uses a battery as its' power. By necessity, all power sources involve three interlinked electrical properties: voltage, current, and power. Although these topics are covered in much greater detail in specific tutorials, it is also useful to cover these topics with. The most commonly recognized DC voltage source is the electric battery– a device that uses chemical reactions to produce and receive. We've seen that batteries are often depicted as a circle with a positive (+) and negative (-) symbol indicating the positive and negative terminals: This symbol indicates a generic DC. Batteries are mobile sources of electric power. We use them to power our phones, computers, and, increasingly, our cars. You don't need to.