BMW Repair: bmw, battery power, alternator

Question
My Bmw had been chugging a lot on a journey and suddenly cut out with the battery light coming on. It will not restart but do get the cranking noise when trying to fire it up and electrics are fine. Wonder if you could help. There is fuel in it and there is a spark as i'm turning it over!

Answer
You say electronics are fine, but the battery light is on.

Could be battery is bad:  A bad cell will deliver low voltage but possibly enough to crank.

More likely, it sounds like your alternator is bad.  Could be turning over on just the battery power.