2002 sentra will not start

Hello,

I have a 2002 sentra gxe. I went to go out the other morning and the car wouldn't start. It wouldn't turn over, no lights on the dash, no radio, just completely dead. I figured it was the battery. I got a new battery and installed it and the car started right up. I turned it off and a few hours later I had to go to the store. The car started right up, no problems at all. The next morning I got in it and it was completely dead once again. It makes no noise when I turn the key, no lights on the dash, no interior light, nothing. I had jumped it off my wifes car and drove it to the mechanic that I go to. The radio cut off and then on 3 times durring the 8 mile drive. When I got to the mechanic, I turned the car off to talk to them about what was going on. He said they would look into it. He got into the car to check the milage and the car was dead again. No dash lights, no interior lights and wouldn't start at all. This was late in the afternoon and they called me a few hours later, telling me that they still hadn't figured out what was wrong. They said it wasn't the battery(obviously it was brand new) or the altenator, which they tested. I am waiting for them to call me now, but I am affraid they are going to say that they are unsure of what it is. Does anyone know what could possibly be wrong?

Thanks in advance for any help.

Some Control Modules are connected to the battery, instead of going through the ignition switch.
If there's a short, it will drain battery over night.
Need to disconnect pos side of battery, and hook a volt meter.
1) Is there any amps being used with the ignition key off?
2) If yes, need to isolate fuse's one at a time until no amps being used.

Start with the batt fuses!

it could still be the alternator it can pass a test and still be bad. that happened with my 94 chrysler i brought it in class and had my teacher look into it. i too had also bought a new battery and it still when dead. th technician should have done what my teacher did, and lightly tapped on the top of the alternator it made the contacts for the brushes get coser to the windings and create the a/c that recharges the battery. the other thing it could be is a parasitic load thats somewhere in the system either one would cause your vehicle to act like that