When a car battery is overcharged, the most common effect is the battery rapidly dying, however, a battery that swells and becomes hot to the touch is also a direct result of overcharging. When the battery swells, sulfuric acid can leak from the battery. This is potentially harmful when it comes in contact with the skin and should be avoided if at all possible and if not, needs to be washed off the skin immediately.
An overcharged battery will get hot and could even explode. The explosion will throw battery acid everywhere, endangering those within the vicinity of the explosion. If the voltage of the battery is overcharged, the battery will be severely damaged and have to be replaced.
As car batteries are used, the chemical properties begin to degrade. Over time, they will hold less and less energy, meaning their capacity decreases. This causes the battery to wear out faster each time the battery is charged. Connecting high-voltage chargers will then exhaust the worn out battery, potentially resulting in overcharge scenario.
Always use the correct charger when charging a car battery, and have it checked periodically by a specialist if charging must be done frequently. Pay attention to labels that indicate exactly how the battery should be charged; this helps eliminate overcharging and possible harm and injury.