Like most stories, there are two sides to this one. Although AGM batteries offer a host of advantages over the flooded type, they are extremely sensitive to overcharging. AGMs should never be recharged with a “conventional” battery charger as they may put out 16 to 18 V, which was often necessary to charge flooded batteries with higher resistance. AGM batteries have much lower internal resistance and as a result should never be charged at more than 14.7 V.
When flooded batteries are overcharged, hydrogen gas is created, which escapes from the vented cells. The cure is to simply add distilled water to restore the level of electrolytes. When an AGM battery is overcharged heat is created and while they do not have vents, the battery will expel gas through the safety release valve (this is normally accompanied by a hissing sound). Once that happens there is no practical way to replace the water and premature battery failure results.
Another peculiarity of AGMs is the difficulty recharging them when the voltage drops below 10.5 V. Most conventional battery chargers have the capability to sense low resistance that could be a result of a shorted, or bad cell—these chargers interpret the low resistance of a discharged AGM as a fault and as a result, won’t charge.
While it is true a conventional battery charger can be “fooled” by connecting a discharged AGM in parallel with one that’s charged (the charger reads the resistance of the good battery), the process has to be monitored closely to prevent overcharging.
Considering the fact that AGM batteries are becoming common, there’s not much in the way of an argument for not having the proper charger. In a perfect world batteries would always stay charged, but the truth is if your street rod sits for an extended period of time the battery is losing juice. The proper battery maintainer/charger can keep an AGM battery in shape to provide all the performance it’s capable of when you need it. Think of them as the best in re-volting developments.