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Technical Forum: Difference between a conventional lead-acid battery, AGM battery?


8/24/2010

Counterman magazine presents 11 technical and sales topics in an easy-to-read question-and-answer format.
 

What’s the difference between a conventional lead-acid battery and an AGM battery?
A.  AGM stands for “Absorbent Glass Mat.”  An AGM battery contains no liquid electrolyte (acid) so it can’t spill. The acid is held in boron silicate (glass) mats between the lead cell plates.  The mats are like highly absorbent paper towels and are saturated with acid. This allows the cell plates to be spaced closer together to increase the battery’s cold start capacity. The mats also help cushion the cell plates, making the battery more resistant to vibration damage. Cell plates may be flat like those in a conventional battery case, or wrapped into cylindrical “spiral wound” cells.

The manufacturers of AGM batteries say their batteries are more durable and typically last five to seven years, which is considerably longer than most conventional liquid lead-acid batteries. Because of this, AGM batteries are used in many late model European luxury cars. They are also marketed as premium replacement batteries.

AGM batteries use “recombinant” chemistry.  The oxygen and hydrogen that is given off when the battery is recharged recombines to form water instead of evaporating out of the top of the battery case. This reduces gassing and loss of electrolyte to almost nothing.

By comparison, a conventional wet lead-acid cell battery (also called a “flooded” battery because the cells are covered with water and sulfuric acid) typically uses water over time, even sealed-top maintenance-free batteries. If the water level gets low enough to expose the tops of the cell plates, they quickly sulfate and lose their ability to accept or hold a charge. That’s why typical battery life in a hot climate for a wet cell lead-acid battery is only about three years.

Another difference is that AGM batteries will usually hold a charge much longer than a wet lead-acid cell battery. This makes AGM batteries a good choice for vehicles that may sit for long periods between use.

AGM batteries are sometimes confused with “gel” batteries. This type of battery also contains no liquid because the water and acid have been mixed with silica to form a thick gel between the plates. But there are no absorbent mats between the plates. Like AGM batteries, gel cell batteries are also spill-proof and longer lived due to reduced gassing.
One thing all types of automotive batteries share in common is the need to be maintained at or near full charge. Fully discharging a car battery more than two or three times can severely shorten its service life. AGM and gel batteries may require a “smart” charger or a charger that does not produce more than 14.4 volts because higher charging voltages may damage such batteries.   

Q.  Are there any special precautions that need to be observed when replacing a battery on a late model vehicle?
A.  Yes. The battery on many late model cars or trucks should not be disconnected without first attaching some type of backup power supply to the vehicle’s electrical system. This could be another 12-volt battery, a 12-volt battery charger, or a 9-volt “memory saver” that plugs into a 12-volt power receptacle or cigarette lighter (which must have power when the key is off to feed voltage to the electrical system). This is necessary to power the Keep Alive Memory (KAM) settings in the PCM and other onboard modules while the battery is being replaced. Failing to do so may cause some systems to stop functioning. A scan tool or special relearn procedure may then be required to restore normal operation.

A new battery should also be put on a charger before it is installed. Most batteries are shipped “dry charged” from the factory, but may require some additional charging after sitting for months on a store shelf. Also, both battery cables should be cleaned and inspected to make sure they are in good condition — or replaced if damaged or corroded.

Q. What is Cold Cranking Amp (CCA) Rating?
A.  This industry rating measures the cranking power a battery has available to start a car’s engine at 0 degrees F. Battery Council International defines it as the number of amperes a lead acid battery at 0 degrees F can deliver for 30 seconds and maintain at least 1.2 volts per cell.

Q.  What is Cranking Amp (CA) Rating?
A.  Similar to CCA; Cranking amps is a measure of the number of amperes a lead acid battery at 32 degrees F can deliver for 30 seconds and maintain at least 1.2 volts per cell. (CA ratings are more commonly used in climates where temperatures rarely drop to 0 F.)

Q.  What is Reserve Capacity (RC)?
A.  A battery’s Reserve Capacity represents the length of time the battery can maintain the vehicle’s electrical needs in the event the alternator fails. Battery Council International defines Reserve Capacity as a measure of the time (in minutes) a lead-acid battery can deliver 25 amps at 80 degrees F and maintain terminal voltage of at least 1.75 volts/cell.

Q.  How can I determine what is the correct battery for my vehicle?
A.  Consult your vehicle’s owner’s manual. It will provide the vehicle manufacturer’s group size and CCA rating requirements for your car. Or, ask your battery retailer to refer to his battery application materials for recommended fitment. Remember: Never use a battery with a CCA lower than the manufacturer’s recommendation. Also, whenever available, a battery with a higher CCA is more capable of providing for the electrical needs of older vehicles, and will not adversely affect the vehicle’s electrical system.

Q.  Why is battery power not always proportional to its size?
A.  A battery’s group size is simply a measure of the physical dimensions of the battery. This measure has no relation to the battery’s electrical capacity. Regardless of group size (physical dimensions), two batteries are equal in power if the RC and CCA ratings are the same.

Q.  What effect does extreme cold have on my battery?
A.  Cold temperatures dramatically reduce the effectiveness of chemical reactions within the battery, while increasing the battery’s internal resistance. Both of these cause a reduction in cranking power as temperatures drop. Batteries left in a discharged state are also susceptible to freezing, which damages internal components and containers. Cars require an increased amount of cranking power in cold weather, due to the fact that motor oil is thicker and makes engines harder to crank.

Q.  What effect does extreme heat have on my battery?
A.  Heat is the number one cause of battery failure. Extreme heat causes the water in the battery’s electrolyte to evaporate. Further, heat causes a battery’s positive plate grids to corrode more rapidly. Both of these conditions are detrimental to the long-term life of a battery.

 















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