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In the hot climate of the Southern United States, average battery life is 37 months. When a battery lasts considerably less time, a problem is indicated. Diagnosing repeat battery failure is easier with an understanding of how they operate.

Why do car batteries continue to die

A battery is usually considered dead when it fails to crank a vehicle. Cranking the vehicle is an important part of the job batteries do. Other task include, subsidizing the alternator at low engine speed, preserving memory for the onboard computers and keeping system voltage closely regulated.

Most vehicles use lead acid batteries. The lead acid battery operates by a chemical reaction between the sulfuric acid in the electrolyte, and the metal plates.

fully charge, partially charged and discharged batteries

A fully charged battery has positive plates, made of lead peroxide. Negative plates are made of sponge lead. As the battery discharges, oxygen (O2) molecules in the lead peroxide, are displaced by the 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.

Recharging the battery immediately after discharge reverses the process. Allowing a battery to remain discharged, even for a short period will shorten the life considerably. Sulfates on the lead plates become extremely difficult to remove if allowed to remain in place. If not removed, they insulate the lead from the acid and the battery loses capacity.

A similar situation occurs if the electrolyte becomes low. Most often this is the result of overcharging, as with a bad alternator or exposure to extreme heat.

temperature influences the capacity of a lead acid battery

Temperature has a dramatic affect on a battery. Most chemical reactions are sped up as temperature increases. A battery produces optimum power at around 80 degrees Fahrenheit. Higher temperatures will increase output, but also shorten battery life.

Colder temperatures slow the chemical reaction and reduce battery output. A battery reduced to 80 percent capacity by summer heat, may fall below 50 percent on the first cold day. As the temperature drops so does battery efficiency. Even a battery with 100 percent capacity at 80 degrees, will only produce around 65 percent of its capacity at 32 degrees Fahrenheit. Hot weather does the damage and cold weather reveals it.

Parasitic draw robs life from a battery

a parasitic draw will rob life from a battery

Parasitic draw is current that continues to flow, even when the vehicle is switched off. Everyone is familiar with the result of leaving headlights on or leaving lights on inside the vehicle. These are obvious current draws and easily corrected.

Modern vehicles have a huge number of far less obvious draws on the battery. Vehicles may have as many as 50 onboard computers tied together in a network. When the vehicle is switched off, these begin to shut down or "go-to-sleep." This reduces their draw to less than .05 of an amp.

If one or more computers remain on, due to a malfunction, the current draw can easily increase ten fold. This current draw will cause the battery to discharge over night. If the discharge is not too severe, the alternator may restore much of the power when the vehicle is driven. This is known as deep cycling the battery, discharging deeply and then recharging.

A very good battery may be able to tolerate this treatment for several weeks/months, but will eventually die. Replace the battery and it may last several weeks again, but will also die. Unless the draw is discovered and repaired, the cycle will continue. This is also extremely hard on the charging system and shortens alternator life.

Parasitic draws of this nature may give no outward signs other than repeat battery failure. Unlike the interior lights left on, an energized module cannot be seen. Other common sources of parasitic draws include bad instrument panels, alternators, even seat heater modules.

using a digital multimeter to detect parasitic draw on a battery

A digital multimeter capable of measuring milliamps is used.  The meter is placed between the battery terminal and the vehicle’s input power lead to detect a parasitic draw. This can be very time consuming. Once the meter is in place, the vehicle must sit until all modules power down. This may take over an hour. Once all modules have gone to sleep, current flow is measured. If flow remains above .05 amp, a parasitic draw exists in the system.

To locate the draw, circuits must be disabled, one at a time. After each circuit is disabled the process must start again, waiting for everything to power down, before another reading can be taken. With the huge number of possibilities, the process may take several hours.

A much more difficult problem is an intermittent draw. This is a draw that only occurs at times. Testing for such a draw is impossible, unless it is occurring at the time the vehicle is tested.

Professionals use judgement, experience and a great knowledge of the system to locate parasitic draws. It can be one of the most difficult task in auto repair.

Batteries that last considerably less than three years indicate a problem. Whenever a battery dies, have the system checked. You may prevent a great deal of future expense.

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