A battery maintainer is a device designed to keep a battery charged without overcharging it. It provides a low, steady charge to maintain the battery's voltage, ensuring it remains ready for use. This is especially useful for vehicles like cars, motorcycles, and boats that are not used frequently. These devices are often used in conjunction with lead-acid batteries, such as those found in cars and motorcycles. By preventing battery discharge and extending battery life, battery maintainers help save money on replacements and ensure reliable performance when the vehicle is needed.