Signs of an overcharged car battery include loss of water, battery failure or a hydrogen explosion. In vehicles equipped with an onboard computer, the overcharging battery causes faulty error codes. The most common cause of overcharging is a faulty voltage regulator. If the batteries fail prematurely, check the regulator or have an expert check it.
When a battery overcharges, it leads to the cleaving of water. The electrical charge breaks the bond between hydrogen and oxygen in water, forming hydrogen gas and oxygen. Normally, these gases escape through the holes in the vent cap, doing no harm. However, as the water in the cells breaks down, the level drops. If the gases build up too quickly, an internal spark in the battery ignites the two gases, causing an explosion. The battery case usually breaks, and battery acid sprays over the surrounding surfaces. The acid can cause paint to peel and the metal to rust.
When gases dissipate, the lower levels of water in the battery expose the plates to the atmosphere where they oxidize. Unless the owner refills the water regularly, the oxidation reduces the battery's ability to hold a charge and leads to premature failure.
The onboard computers in vehicles depend on a regulated charge in the battery. If it overcharges, the check engine light often illuminates. Checking the computer with a scan tool often reveals erroneous diagnostic codes due to the overcharging battery.