Automotive batteries are put under extreme demands, even in everyday driving conditions. Repeated starts, hot conditions and heavy electrical loads all create stresses that batteries must withstand on a regular basis. Despite this, automotive batteries usually require little care or maintenance, and they usually perform well for the duration of their lives. Under unusual circumstances, however, such as when an alternator fails and overcharges the electrical system, batteries can deteriorate quickly, displaying warning signs that a problem exists.
Overcharging in an automotive battery causes a build-up of hydrogen gases, which creates pressure within the battery casing. This pressure forces electrolyte out through any openings, such as vents or the filler caps in the battery's top. This pressure can cause the battery casing to develop cracks or splits, resulting in electrolyte leaking out of these openings.
When a battery becomes overcharged, material sheds from the plates inside and settles to the bottom of the battery casing, causing the plates to lose their ability to hold an electrical charge. This creates resistance to an electrical current, and excess electrical charge is radiated as heat. This causes the battery to feel warm or hot to the touch after the vehicle has been running.
An overcharged battery creates excessive amounts of hydrogen gas quickly. This build-up of gas creates pressure inside the battery casing that results in the battery appearing to swell. The normally flat sides and top of the battery will appear somewhat rounded or bulging. In serious cases, the battery can split open or even explode.
Low electrolyte levels are a symptom signalling that the alternator is overcharging the battery. Electrolyte levels within the battery become low because heat generated from overcharging causes the electrolyte to evaporate. The battery's ability to hold a charge lessens as the electrolyte evaporates, which further accelerates heat build-up and increases the rate of evaporation.