I have just changed the alternator on my 2002 BMW 316i 1.8 and its still not putting power back to battery, the car just coughs after two mile and dies. It wont start,any ideas if not the alternator

Most likely you have either a bad relay, or a short circuit. My 1998 had a relay someone had put in that was the wrong relay such that it caused a current drain. Use amp meter between battery ground cable (disconnected from battery) and battery ground post to see how much drain exists, then pull relays one at a time to see if one stops the drain. If not, do the same with fuses to find the bad circuit and fix drain.