Wednesday, 3 February 2016

AGM Battery (Absorbent Glass Mat)

The AGM battery type application in vehicles is becoming increasingly common because manufacturers equip their vehicles with technological systems which require huge power supply for their equipment.

It should be noted that AGM batteries have become widely used as a result of the introduction of vehicles fitted with the Start & Stop system. As of today, most vehicles are equipped with this kind of batteries due to their great performance.

What is an AGM battery?

The AGM-type batteries belong to the VRLA battery family which stands for Valve Regulated Lead Acid, that is to say, they are lead acid batteries regulated by valves.

Their constitution is similar to conventional batteries; they are wet lead calcium batteries. The AGM battery has the same number of positive plates (lead peroxide) and negative plates (sponge lead) and they provide a 12-Volt nominal voltage.

The new aspect of the AGM-type batteries is that they are based on a fibreglass grid placed between the plates that absorb the electrolyte and the distilled water.

What are its characteristics?

- The electrolyte gases produced during the discharge and charge cycle are transformed into water.

- The electrodes of the plates fix the fibreglass using pressure, which means that the fibreglass is always in watertight and uniform contact with the electrodes.  In this way, the loss of active mass caused by vibrations is minimized.

- In the case of water vapour generation, at high temperature (between 20 and 200 milibars), the system uses an overpressure valve which releases the gas to the atmosphere but prevents the entry of the atmospheric oxygen into the battery.

- The plates are larger in size than the conventional plates. 

- The intensity of the energy interchange cycle (discharge and charge) may be up to three times greater due to the fiberglass separators.

- The battery acid is impregnated into the fibreglass.

12 V
6V and 12V
6V and 12V
Battery Capacity (Ah)
from 40 to 180
from 16 to 210
from 33 to 225
Cold Start Current EN (A)
from 330 to 920
from 180 to 1030
from 680 to 950
Mounting Angle
No inclination
Any position
Any position
Charge Requirement
DC, 10% of the capacity
DC, from 25% to 50% of the capacity
DC, any amperage
Discharge time
After 8 months, 35% of the charge is maintained
After 2 years, 85% of the charge is maintained
After 2 years, 90% of the charge is maintained
Service Life (charge/discharge cycle)

Between 350 and 400

Between 550 and 600

Between 950 and 1000
Maximum Discharge Power
Around 55% -60%
Around 75%
Possibility of Electrolyte Loss
Possible loss of liquid
Possible loss of gel
Indicative Data

How does a car battery work?
Battery Discharging

If a consumer is connected to the battery, an external electric circuit is established between the battery terminals due to the potential difference between both plates and electrodes. The different nature between them produces the discharge.

The discharge consists in transforming the chemical energy to electrical energy. In this way, an electron current is produced and it circulates from the negative to the positive plate of the battery at the exterior of the accumulator, and from positive to negative at the interior (electrolyte).

Remember: The direction of the current is from (+) to (-). This direction is the conventional one, but the real direction is reverse, it is from (-) to (+).

The chemical decomposition occurs as follows: the sulphuric acid breaks up in sulphate ions  and hydrogen ions , the sulphuric acid of which combines with the plates.

In the positive plate the lead peroxide is decomposed and transformed into lead sulphate, and oxygen is released as a result.

In the negative plate, the sulphate reacts with the lead, so lead sulphate is also formed. The oxygen and hydrogen ions released in both reactions combine to form water, which is accumulated inside the mixture as a part of the battery electrolyte. The electrolyte decomposition (water + sulphuric acid) decreases water density.

These chemical transformations take place inside the battery, and generate electrical current due to the alteration of the chemical elements between the two plates.

Battery Charging

Charging a battery reverses the chemical process that occurs during the discharge. This process consists in transforming the electrical energy from a generator (alternator) into chemical energy. The electrons circulate from the positive to the negative plate, so a process reverse to the discharge is produced.

The electrolyte composition (water + sulphuric acid) increases water density.
The electrolyte water generated during battery discharge is decomposed by electrolysis in ions (2H+) and (O2). The oxygen of the water of these ions reacts with the lead of the positive plate. Thus, the lead peroxide is formed again (PbO2). The sulphate (SO4=) of both plates is released which makes the hydrogen react with the sulphate, and sulphuric acid is formed again (H2SO4).

The electrolyte composition (water + sulphuric acid) increases water density.

The discharge and charge cycles which are produced inside the battery cause the deterioration of the chemical elements that form the battery, limiting its service life as reflected in the number of cycles.


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