Batteries are portable energy storage devices that convert chemical energy into electrical energy, powering a wide range of electronics including smartphones, laptops, and remote controls. Each battery consists of one or more electrochemical cells with a cathode (positive electrode), anode (negative electrode), and an electrolyte. When connected to a circuit, chemical reactions drive electron flow, producing current. Capacity is measured in ampere‑hours (Ah) or milliampere‑hours (mAh), indicating charge delivery over time. Common types include alkaline, lithium‑ion, and nickel‑metal hydride (NiMH), each offering unique performance characteristics for consumer and industrial applications.