The lead acid battery has been used to start cars for many years. They are dependable and affordable sources of electric power. They are also lasting less time than in the past, for several reasons. Understanding how they work can help extend their lives.

Batteries die for a number of reasons.  Vibration and heat or two causes, but a main cause is improper/inadequate charging.  The battery is one component in the starting and charging system of the automobile. Its two primary functions are to provide energy to crank the engine and to serve as a buffer or reserve for the system.  When this energy is used, it must be replaced. 

Replacing the energy that has been used in the battery is the job of the alternator.  The alternator, an electro-mechanical device, and its output is proportional to the speed at which it rotates. The starter, lights, ignition, fans, etc. consume electrical power. Modern vehicles use considerably more electrical power than just a few years ago.

The total energy used by the vehicle is sometimes referred to as load. The load of an average vehicle is around 75 amps. Some components, such as the starter use considerably more, around 200 amps. Fortunately this is usually only a short-term load rather than a consistent draw.

The average alternator is capable of producing around 100 amps at 2,000 RPM. This supplies energy consumed by the vehicle and some to recharge the battery. The problem is the alternator has to rotate around 2,000 RPM to achieve its potential. At idle a 100 amp alternator may only produce 50 amps. With a load of 75 amps, there is a shortage of 25 amps. This is a draw on the system, and the battery discharges to make up the difference. This causes chemical changes in the battery.

Lead-acid batteries contain several plates of dissimilar metal. These plates are covered by sulfuric acid. The positive plates are made of lead peroxide, chemical name Pb02. The negative plates consist of sponge lead, Pb. When these metals are exposed to sulfuric acid (H2SO4) in the electrolyte, electricity is produced.

As the battery discharges, the oxygen (O2) molecules in the lead peroxide are displaced by sulfur (SO4) from the sulfuric acid. Sulfur converts the plates to lead sulfate (PbSO4) and the oxygen combines with the acid to form water (H20).  When this occurs the battery is discharged and no longer produces the needed electricity.

If the battery is immediately recharged by the alternator, the Sulfur leaves the lead plates, returns to the acid and the battery is again charged. If the battery remains discharged, the sulfur permanently attaches to the lead and the battery will die.  When this happens the battery is said to be sulfated.

Problems develop when vehicles are not driven enough.  This is because the alternator on an engine at idle may actually discharge. Driving short distances and idling long periods in traffic will not allow the battery to charge. Each time the vehicle is driven, the battery gets weaker, due to sulfating of the plates.

This will drastically shorten the life of the battery and is quite common on vehicles driven primarily in town.  Elevated temperatures, such as hot Summer driving tends to make the problem worse. Vehicles need to be driven at highway speed, around 2,000 RPM at least one hour a week. When this is not possible the battery should be checked on a regular basis and replaced at the first sign of problems. In extreme cases this may be every year.

Under normal conditions, batteries last about three to four years. Replacing the battery at three years may be a good policy to avoid being stranded.

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