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In a B5/5.5 Passat, an AGM battery will never be fully charged in typical use, and in most cases it will be over-charged on long drives.

"Some claim that AGM batteries actually are a drop-in replacement for flooded lead-acid units in car applications, because the optimal charging voltages for the two battery types are not all that different.

There is some truth to this claim: the optimal absorption and float voltages of flooded lead-acid and AGM batteries are indeed within 0.2V or so: typical values are 14.7V/13.6V for AGM and 14.5V/13.4V for flooded.

However, this claim misses an important point: an AGM battery is much more prone to both under- and overcharging than a regular flooded lead-acid battery. Specifically:

AGMs suffer much more from insufficient absorption charge
AGMs are more sensitive to overcharging
So while the crude charging profile of a car alternator & regulator combination is not really optimal for either battery type, AGMs suffer much more from the deficiencies."
 
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