Exactly. They have fiberglass mats between the lead plates, and just enough electrolyte to fully wet the mats, but no extra. This means there is no free liquid, so they can't spill acid even if they get broken. When charged per the manufacturer's specs, gasses are reabsorbed as fast as they form, but if the charging voltage gets even a little too high, they will vent the excess gasses out a safety valve. As they vent, electrolyte is slowly lost, and since there's none to spare, they lose capacity. I'm not familiar with all the smart chargers on the market, but if you choose one for your AGM batteries, it should be designed for charging AGM batteries. I own a solar charge controller that lets me select the battery type, either flooded, AGM, or gel. Its output voltages for bulk, equalize, and float are different for each battery type. Of course flooded batteries are relatively insensitive to overcharging, as long as you keep the electrolyte level where it belongs.