Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Brief description of components
An AGM battery can be recognized by its black housing and the lack of a so-called "Magic Eye".
Design
The AGM battery differs from the conventional lead calcium battery as follows:
- Larger plates:
Larger plates provide for a 25 % higher power density.
- Separators made of glass fibre fleece:
These provide for up to 3 times higher cycle reliability.
This in turn improves cold starting performance, current consumption and service life.
- Gas-tight housing with pressure relief valve (see also mode of operation):
The cell plugs are welded and cannot be opened.
- Acid bound up in the glass fibre fleece:
The acid is not as previously self-contained in the housing, but 100 % bound up in the glass fibre fleece. This provides increased protection against acid leakage and thus represents a reduced risk to the environment.
Mode of operation
The AGM battery differs from conventional batteries in its non-polluting and substance-retaining behavior during charging.
When vehicle batteries are charged, the two gases oxygen and hydrogen are released by electrolysis.
- In a conventional wet lead calcium battery, the two gases hydrogen and oxygen are dissipated into the atmosphere.
- In an AGM battery, the two gases are converted back into water: The oxygen which is created at the positive electrode during charging passes through the permeable glass fibre fleece to the negative electrode. At the negative electrode the oxygen reacts with the arriving hydrogen ions in the electrolyte to form water (oxygen cycle).
In this way, the gas and thus also the electrolyte are not lost.
Only in the event of an excessively heavy buildup of gas, i.e. excessively high pressure buildup (20 to 200 mbar), does the pressure relief valve discharge the gas. In this process, the pressure relief valve does not allow any oxygen in the air to enter.
Because a valve regulates the pressure in the battery, the AGM battery is also known as a VRLA battery (Valve Regulated Lead Acid).
Notes and instructions for service personnel
It is necessary when handling an AGM battery to observe some particular points pertaining to battery changing and installation location.
Charging
When charging removed batteries (so-called stand-alone batteries), do not exceed the maximum charging voltage of 15.2 V at room temperature. Also when charging via the jump start terminal, do not exceed the maximum charging voltage of 15.2 V at room temperature.
The AGM battery will be damaged even when it is only briefly charged with a charging voltage of more than 15.2 V. A charging voltage of more than 15.2 V is usually used in quick-charging routines.
Installation location
The AGM battery must not be installed in the engine compartment on account of the high spatial temperature differences, otherwise its service life will be significantly shortened.
Housing
The AGM battery must not be opened under any circumstances as the introduction of oxygen from the air will cause the battery to lose its chemical equilibrium and be rendered non-operational.
Battery changing
Any conventional lead calcium battery can always be replaced by an AGM battery.
Using an AGM battery does not require any alterations to be made to the vehicle electrical system.
"Problem customers" encounter a high energy throughput through their batteries. This high energy throughput is caused by stationary loads/consumers (TV, independent heating, etc.) and a bad use profile for the battery ("chauffeur operation", short-distance driving, "stop-and-go"). The use of an AGM battery is recommended for these problem customers.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.