A power bank is a portable device that stores electrical energy, allowing users to charge their electronic devices, such as smartphones and tablets, on the go. They come in various sizes and capacities, measured in milliampere-hours (mAh), which indicates how much charge they can hold. Most power banks have multiple USB ports, enabling users to charge more than one device simultaneously. They are especially useful during travel, outdoor activities, or emergencies when access to a power outlet is limited. Many power banks also feature built-in safety mechanisms to prevent overcharging and overheating.