Battery efficiency refers to how effectively a battery converts stored energy into usable power. It is typically measured as a percentage, indicating the ratio of energy output to energy input. Higher efficiency means that more of the energy stored in the battery is used for its intended purpose, while lower efficiency results in wasted energy, often as heat. Factors affecting battery efficiency include the type of battery, its age, and operating conditions. For example, lithium-ion batteries, commonly found in devices like smartphones and electric vehicles, generally have higher efficiency compared to older technologies like lead-acid batteries. Proper maintenance and optimal charging practices can also enhance battery efficiency.