Secondary batteries, often called rechargeable batteries, can be used, discharged, and then restored to their original condition by reversing the current flow (charging). Rechargeable batteries are commonly used to power a personal digital assistant, mobile telephone, or notebook computer as well as to start a car. They have become a part of daily life. This article provides an insight into how these batteries work. The basic elements and principal characteristics of rechargeable batteries are discussed along with a comparison of the energy storage capability of common rechargeable battery systems.