Battery does not stay charged!!!

Any thing from a short to a bad radio or clock could also cause this problem along with the Diode in the alt. like Dotmann metioned. I would start by testing the alt. If that checks out then I would disconnect the radio and clock (if you have one) If you don't then I would start to look at a short somewhere in the system. I would pull the battery cable and put a test light between the post and the cable and see if it lights up with the clock/ radio disconnected. If it lights up there is a power draw. Then it is time to start searching for the problem. I have seen an ignition switch cause this problem once before also.