Hey, I've got an 06, and just put a new battery in about 2 months ago. Everything's fine, except in the last 2 weeks, my battery light came on. I doubt that the alternator is the cause (even though it's stock), because no lights are dim, nor are there any "browning out" of any electrical. Any idea why the light would come on?

 

The alternators in our S197's are a weak link as well as the garbage MotorCrap battery. A weak alternator will eventually cause the battery to discharge even if the battery is good, Also, a weak battery will cause the OE alternators to degrade. A great engine with a barely marginal charging system. Who'd have thunk...

BTW, you posted this in the 3.7 section...

 

New batteries can fail. I would get it tested but more than likely your alternator is on it's way out.

I actually got very good service, 5 years, out of my original Motorcraft battery then replaced it with another Motorcraft, MAX BXT-96-R, that was a couple of steps up from the original. My alternator did fail last year at over 100,000 miles and as stated they are sort of a weak link especially if you add a bunch more loads to them like big amps and so on. I personally have had nothing but trouble with Optima batteries in my other hot rod and will never purchase another one. A total waste of money IMO.

 

Its true that brand new batteries can fail, but more likely if its the original alternator from 2006 you are lucky you haven't replaced it yet. Very common issue with the Ford Alternators.