Originally Posted By: SubLGT
Originally Posted By: Rand
..Agm batteries want slightly higher voltage. If your car is designed for AGM its better...
Some AGM batteries require less voltage:
Quote:
...John Munsell, battery product specialist for General Motor Corp.’s ACDelco brand, says the key for shops will be training to diagnose and repair start-stop vehicles…...Munsell says AGM batteries require special care when charged offline. “This special care is to make sure that the charger has an AGM switch that will supply lower voltage to the battery during the charging process.
This lower voltage is required due to the charge receptivity of AGM batteries. The maximum charging voltage for an AGM battery at 80 degrees Fahrenheit is between 14.4 volts and 14.6 volts. If the ambient temperature is higher than that, charging voltage goes down,” says Munsell…
...Munsell says that for vehicles manufactured by General Motors, “we neither recommend, nor endorse replacing an AGM battery with an EFB [enhanced flooded battery] battery. An EFB battery will have a different charge profile than an AGM battery, which will lead to premature failure of the battery.”
http://www.moderntiredealer.com/article/722503/prepare-now-for-start-stop-systems
Going too high can do two things, either drive excess current (don't needs huge voltage over potential to drive current into low impedance), and also can drive electrolysis, beyond some point the AGM can't recombine fast enough.