Last week I tried starting our 2003 325xit (130k miles, regularly serviced) and it just clicked like the battery was dead. All the interior lights, radio and daytime running lights were on but I suspected weak battery. After leaving it on a trickle charger for 7 days I came home and found the same issue, just click-click-click when I tried to start it. I found a loose negative battery cable, retightened it and finally got it to turn over by holding the key on but then it ran very rough, like it was running on 5 cylinders. The SES light was on when I backed out of the driveway and never went off; I pulled back in when the engine kept running rough. Any ideas as what is going on? Could a weak battery cause this? Looking at Bentley it looks like an emissions fault, how can this be cleared?