How does a battery charger function on a boat?

A battery charger on a boat primarily functions by replenishing battery capacity. It achieves this by converting alternating current (AC) from the shore power or generator into direct current (DC) suitable for charging the battery. This conversion is essential because most boat batteries require DC voltage for charging. During this process, the charger regulates the voltage and current output to ensure that the batteries are charged safely and efficiently, preventing overcharging and potential damage.

The correct understanding of this process highlights the importance of the charger’s ability to transform power forms to suit the needs of the battery. By doing so, it ensures that the battery can store energy for use when the boat's engine is off or during periods of low power generation.

In contrast, other functions mentioned in the options do not accurately describe the role of a battery charger. For example, supplying AC power to devices pertains to the electrical distribution system rather than battery charging. Discharging excess power typically relates to load management, not to the function of a charger. Measuring battery performance, while important, is likely a feature of a battery management system or gauge rather than a direct function of the charger itself.