car wont start - its not the battery but when i turn the key it's dead....

I had what I think is a similar problem. I to thought it was the battery so I replaced it but it still wouldn't start so I got the codes read for my car and it said that I had a bad cam shaft position sensor which would cause it not to turn over or start at all and if it did then it would die not to long after. the part is fairly cheap it cost me about 90 dollars and I had my friend put it in and he said it was really easy to do. I hope this helps.