Battery OK but warning light is red

yep the alternator is not charging. check the voltage out of the alternator, look for a slipping belt. look for a bad ground. get the Alternator tested.

Wouldn't that be the 13.8v reading with the engine running? Now it may be that the alternator output should read higher like 14+v but I wouln't think that 13.8v would trip the low battery light.

Richard

if the alternator was out, the battery should be about 13.2 or less with the engine running

The fact that the voltage goes up a volt when the car is running means the alternator is working some, as it is putting out some voltage/current.

How long you had the battery?

Well, I guess the warning light doesn't lie. I got stuck yesterday 20 miles from my house. AAA guy came and determined that the alternator was kaput (and in turn the battery was dead). I got a lift to nearest dealer and shelled out $280 for new alternator and another $90 for battery). Lesson learned :-(

Still don't know why the battery kept showing high voltages up until the last day. I assumed (wrongly) that if I was indeed running completeley off the baterry, it would have drained quickly and made starts sluggish.