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 component description
An AGM battery can be recognized by its black housing and the lack of a so-called "Magic Eye".
| Item | Description |
|---|---|
| 1 | Positive grille with silver alloy |
| 2 | Positive plate |
| 3 | Negative grille |
| 4 | Negative plate |
| 5 | Separator made of glass-fibre fleece |
| 6 | Set of positive plates |
| 7 | Set of negative plates |
| 8 | Block of plates |
| 9 | Block box with base strips |
Construction
The AGM battery differs from the conventional lead calcium battery as follows:
- larger plates: Larger plates allow a 25% larger power density.
- Separators made of glass-fibre fleece: These can cause an up to 3-times higher cycle stability to be reached. This improves the cold starting capability, the power consumption and service life.
- Airtight housing with pressure relief valve:
The inspection plugs are sealed and can not be opened.
- Battery acid bound in glass-fibre fleece: Battery acid is not found free in the housing as before, but is rather bound 100% in the glass-fibre fleece. This gives increased security against the acid escaping and thus reduces the environmental risk.
How it works
The AGM battery differs from conventional batteries in its non-polluting and substance-retaining behavior during charging.
When a vehicle battery is charged, the electrolysis process emits the gases oxygen and hydrogen.
- 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 manner, the gas, and thus the electrolyte, is not lost.
Only in the event of an excessively heavy build-up of gas, i.e. excessively high pressure build-up (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 the pressure in the battery is regulated by a valve, the AGM battery is also known as the VRLA battery (valve-regulated lead acid).
Notes for Service department
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, for charging via positive battery connection point the maximum charging voltage of 15.2 volt at room temperature must not be exceeded. The batter can be damaged even with short charging of the AGM battery with a charging voltage higher than 15.2 volts. 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
On no account may AGM batteries be opened, as the ingress of oxygen from the atmosphere would cause the battery to lose its chemical balance, rendering it unserviceable.
Battery renewal
On principle any conventional lead-calcium battery can be replaced with an AGM battery. No changes result for the vehicle electrical system by using an AGM battery.
"Problem customers" have a very high energy throughput through the battery. 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.