Could my 18 month old optima battery be shot?

2A slow charge seemed to work. I drove the car several times today, and it started up hard and fast each time.

Took Phreeq out for a ride before doing some work on his car, then taking HIS for a ride.

 

AGM batteries can handle quick charging.. they have a higher "acceptance" of amps during recharging. They can use higher amp charging to recover faster after a discharge, than can a flooded type battery. Besides low self discharge rates it is a feature of this battery type, and an advantage over flooded.
Conversely, they can be more easily damaged by overcharging and deep discharges!

The key to charging them is a having correct regulation. Ther are three stages to charging a battery.. ever since they were invented.

1. Bulk/ Flood charging. 14.4 volts. The size and type of battery determines the amps and voltage it can accept. AGM's can take more/higher.
2. Acceptance/taper down. Resistance has developed and the amps and volts 13.8 V areslowly lowered so as to continue charging without developing heat.
3. Float. Charging is complete. Voltage is maintained at @13.5 V and no amps.

This was done manually in the old days on submarines.. WWI. Sub Mariners would do the initial bulk charge then spend days if not weeks balancing and slowly lowering the charging volts and current against the building resistance/heat. A careful charge could allow them to stay under water longer..
Modern chargers do this automatically, called "smart chargers" or "three stage" integrated circuit chargers. Solid state.
Older chargers, the sears or schumaker portables are "Ferro Resonant" and do not charge efficiently or maintain a battery correctly. They taper back almost immediately when they sense the surface charge then tend to undercharge, taking forever to charge. Then on their maintenance or trickle, they overcharge. Not a bad idea with these types is to have a small current drain occuring ( small light, clock..etc..) so as not to allow overcharging.

Any discharged battery can "accept" about 70% of it's charge back relatively quickly, the first stage.. and AGM's more so. It's the second stage, that is so very critical, to avoid cooking the battery.

One can take a capacitor(AC storage device) and a rectifier(AC to DC converter) and make a charger.. good luck with that.