A battery maintainer is a device designed to keep a battery charged without overcharging it. It provides a low-level 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, including automotive batteries and marine batteries. By preventing battery discharge and sulfation, a battery maintainer can extend the lifespan of the battery, making it a practical investment for anyone looking to preserve their battery's health.