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.