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      03-12-2010, 11:55 PM   #24
tom @ eas
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Originally Posted by 808MGuy View Post
I have never heard of any charging system in older cars that would overcharge an AGM battery. I had an Odyssey AGM in my last car and there was no such procedure to go through. In fact, most automotive batteries don't like quick charging so it wouldn't make sense for the alternator to try to pump full current into the battery for faster charging and then ramp down once its charged. Its typically better to trickle charge them to avoid overheating.

Isn't the computer interface related to the battery associated with the regenerative braking feature where the alternator charges the battery when the car decelerates? I thought this feature wasn't available in the US. In this scenario, I can see the need for computer intervention since the computer needs to know what the battery level is and when the car is decelerating but otherwise, it just doesn't make any sense.
I've been wanting to post a tech article about this subject for some time, might as well start now:

Absorbent Glass Matt Battery (AGM)

In contrast to conventional lead-calcium batteries, the sulfuric acid in an AGM battery is not held freely in the battery housing. The sulfuric acid is instead entirely bound into the mats of the glass-fiber fleece. For this reason , no acid can escape if the battery housing is damaged. In addition, the AGM battery is a sealed, airtight unit. This is possible because the gasses are converted back into water by the permeability of the separators.

AGM batteries differ from conventional lead-calcium batteries in the following points:

- Larger plates which allow a power density some 25% greater
- Glass-fiber-fleece separators which enable a cycle consistency up to 3x greater improving cold-starting ability, current consumption and service life
- Airtight housing with pressure relief valve
- Inspection plugs sealed and cannot be opened
- Acid bound in glass-fiber fleece

How AGM Batteries Work
AGM batteries differ from conventional lead-calcium batteries in their environmental compatibility and their retention of gases during charging. When a vehicle battery is charged, the electrolysis process emits the gases oxygen and hydrogen . In a conventional lead-calcium battery, these two gases are released into the atmosphere. In an AGM battery, the two gases are converted back into water: The oxygen created at the positive electrode during charging moves through the permeable glass-fiber fleece to the negative electrode, where it reacts with the hydrogen ions that are brought in with the electrolyte, to create water (oxygen cycle). In this manner, the gases, and thus the electrolyte, is not lost.

Only when the gas production is excessive, that is when too much pressure is generated (20 to 200 mbar), does the pressure-relief valve open, thereby allowing gas to escape while also preventing entry of atmospheric oxygen. 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).

Service Instructions for AGM Batteries
When handling AGM batteries, certain special factors must be taken into consideration with regard to battery renewal and installation location:

Do not charge AGM batteries with 14.8 volts or use use rapid charging programs
When charging batteries in the "stand alone" mode, the maximum charge voltage of 14.8 volts must not be exceeded. Even briefly charging an AGM battery with a charge voltage of more than 14.8 volts (voltages usually used in rapid charge programs) will damage the battery

Do not install AGM batteries in the engine compartment
Because of large temperature variations, AGM batteries must not be installed in the engine compartment. This would result in a significant reduction in the service life of the battery

Do not open AGM batteries
By no account should AGM batteries ever be opened, as oxygen from the atmosphere would cause the battery to lose its chemical balance and cause it to fail.

Battery Replacement
An AGM battery, when installed as original equipment, must always be replaced with an AGM battery. In special cases, where a customer's driving profile (e.g. short distance driving), results in a discharged battery, the AGM battery is a recommended replacement.

Note that replacement batteries resulting from a customer's driving profile cannot be claimed under warranty.

Battery Replacement Registration

This Service Function informs the PM that the battery has been replaced. It completes the following operations:

- Battery capacity is set to 80%
- Current Odometer reading are stored. The odometer readings at which the last seven battery replacements took place can be read off from the Diagnosis Requests of Control Unit Functions
- Stored battery statistics (current, voltage , battery charge level) are deleted
- Stored temperature statistics are deleted

Hope this helps.
Tom G. | european auto source (eas)
email: web: tel 866.669.0705 ca: 714.369.8524 x22