Battery and charging system experts please chime in

[generx]

Earlier this year, I replaced my alternator due to the dreaded battery light and posted a DIY here

Everything has been normal until last weekend. The battery light came on while we were about 170 miles from home at the coast over the weekend while driving on the beach. My first thoughts were WTF? Turned off the ignition and re-started the car about 4 more times and the light stayed on each time (constant on, not blinking)

Battery is 2 years old (OEM)
Alternator is pretty new

Made a beeline for a local auto parts place and bought a cheapo meter and got a 11.9v reading while running and 12.4v with the engine off. Drove back to the beach house 10 miles away with everything off, luckily it was still daylight.

Disconnected the battery and contemplated my next step (luckily I always carry a tool kit, garbage bag, and duct tape when driving long distance in any BMW) I re-connected the batter after about a half hour and re-started the X:

NO BATTERY LIGHT!

Checked the voltage while running and it was a healthy 14.3v. Was really tempted to keep the X running and drive straight home but it was getting dark and did not want to get stranded in the NW coastal range at night so decided to stay until morning. Locked it up and crossed my fingers that all would be normal the next day. In the morning, packed up and went out to start up the X, no battery light. Drove straight home and everything seemed normal.

The next day, went to a local Battery Exchange where they tested the battery and charging system under load, everything checked out normal, the OEM battery seemed to have normal voltage but the tech mentioned that the battery's amperage was about half of where it normally should be and that they could charge up the battery overnight. He also thought this was odd considering I just drove long distances the previous weekend. I declined and ran some errands hoping that the problem would re-occur.

Indeed the battery light came on again later (a constant on as usual, not blinking) Turned off the X and re-started it, battery light was off. I bought a new battery (from Batteries Plus) that day and installed it. My thoughts are that the 2-year old OEM battery is somehow defective or has a damaged cell(s) from a previous discharge cycle. Checked all connections/ground when installing, no corrosion. Everything seemed normal.

I'm really stumped at this point and paranoid that it may be the DME. I have no FSR issues, nothing is discharging the battery when it's not running, no warning lights, nothing.

Anybody have any thoughts on what could be causing the problem? It's been a couple of days and no battery light though. Could it have been the bad battery refusing a charge from the alternator? (does the DME cut off the charging system from charging a bad battery?)