Quote:

because I moved the switch on the charger to "12 V"



Well then, the correct answer is you "don't know". Dude, I'm seriously trying to help you, however with answers like that you may not need my help.

If you used a wet-cell "12v" charger most likely it is putting out as much as 15+ volts during part of the charge. Do that for long enough and it will kill even a good AGM.