What indicates that a boat's battery is fully charged?

A voltage reading of about 12.6 volts or more indicates that a boat's battery is fully charged. This reading is significant because it represents a state of charge for a fully charged lead-acid battery at rest. When a lead-acid battery reaches this voltage level, it typically means that the battery has been charged and is holding its charge effectively.

If a battery reads below 12.6 volts, it suggests that the battery may be partially discharged or in need of charging. For instance, a reading of around 12.0 volts would imply that the battery is about 50% charged, indicating that it requires attention to maintain optimal performance. A reading lower than this, such as 11.5 volts, would show a significantly depleted battery, potentially affecting the boat's ability to function properly.

On the other hand, a voltage reading of 13.0 volts often indicates that the battery is either being charged or is at the beginning stages of charging, which is not indicative of a fully charged state but rather a sign of an external charging source, like an alternator or battery charger, actively supplying power to the battery.