What Happens When You Overcharge a Car Battery?

While a few variables may be involved with overcharging a car battery, the results are simple to check. A battery can quite simply die from being overcharged. This is the safest side-effect, but not the only thing that can happen. An overcharged battery will boil the sulfuric acid and distilled water mix. The casing of the battery can become hot to the touch, and begin to melt or swell. Flammable hydrogen can build up inside the sealed cells of the battery, causing swelling of the casing under pressure and seepage through small vents. Once the hydrogen is introduced to oxygen, it becomes a sitting time bomb. A small electrical spark can ignite the gas and cause the battery to explode, sending plastic and lead shrapnel flying around, in addition to a caustic sulfuric acid spray. Obviously, this is the most dangerous side-effect of an overcharged battery.

Most modern cars come equipped with an alternator that has an internal voltage regulator. The voltage regulator supplies a controlled electrical charge to the battery while the vehicle is running. If the voltage regulator fails, an uncontrolled electrical charge to the battery can occur which will overcharge the battery every time the vehicle is running. Another way to overcharge the car battery is human error. Using a battery charger without properly knowing how can also overcharge a battery. Incorrect volts or high amp settings on a charger can overheat the battery or charge it too quickly, becoming ineffectual for the intended purpose. Leaving an unattended charger on for too long is another human error.

A brand new battery placed on an awaiting display shelf for purchase begins to slowly die. Lead sulfates begin to deposit on the lead plates of the internal cells. The two cycles a battery needs is charge and discharge for shelf-life longevity. When a battery that sits too long without these two stages, the sulfates begin to crystalize and harden. Charging an idle car battery can decrease the sulfation by converting the sulfate particles attached to the discharged areas of the battery back into useful plate material.

A multimeter can be used to check the volts of the battery while the vehicle is running. A fully charged battery will sit idle at 12.6 volts; 2.1 volts per six internal cells. Once the vehicle is started without the introduction to a load (lights, radio, A/C and fan), the volts will rise to an average of 14 volts. If the multimeter reads higher than 14.8 volts, then the alternator should be checked for overcharging. When using a battery charger to restore a charge to an idle battery, always charge the battery using a low amp setting. This is called a trickle charge, and is the best way to recharge the battery. It provides the slow equalization required to electrolyte solution to diffuse into the less accessible areas of the contaminated cells and converts the lead sulfates back into useful material.