Ok the problem started out Sunday night. My daughter called me and said that her battery light was on the entire way home (50 minute drive)on her 99 Volvo S70 GLT. I figured we're in Arizona and batteries last 2 years only so it must be the battery. Battery was only 1 year old hmmm...

Took it in to test in the morning (Monday)at Auto Zone battery was low/dead, car would not start. Battery wwas charged and tested good (1 hour charge). Took it back to test alternator, Alt, diodes??& Regulator test good... actually perfect. Search begins andquickly find a loose ground connection right on the battery. Cleaned the post and the connection and car is fine through Wednesday night. She calls and says battery light was on for a few seconds until she started driving. This morning (Thurs.) I take the car to work, start the car in the morning, battery light comes on and stays on until I accelerate to start my drive, light goes off and stays off. Got in car to come home tonight... same thing on until I gently revved the engine, light went off and stayed off. What gives?? I looked for something that stayed on, glove compartment light, plugs in cig lighter, etc ...etc.. nothing?? Any help??

 

It is most likely an alternator related problem. Maybe the voltage regulator is going bad.

 

How can a voltage regulator be tested? I've had a similar problem. I noticed the running voltage slowly going down, to near 12.3 volts,
The car was running fine, until I changed the PCV vent system. I put it back together, got it running and noticed the vacuum hose from near the gas line on the intake manifold was cracked. So, thinking I would do some good, changed that hose, which extended back to near the trbo. After I did the change, it wouldn't start at all. It will crank, then sput then crank, until the battery gets too low to turn over.
Could this be the alternator, voltage regulator, a vacuum issue? The battery charges back up to 12.3 volts, and stays there until I drain it down while trying to start it, so I know the battery is good.

 

The car would have to be running to test the regulator. But all you measure is the output from it in volts.

But if it isn't running then there is another problem.