I know there are probably other threads about this topic but I do not have time to find, check, and read all of them. So here is my problem:
My 95 Disco 1 will not start. I left her in the garage for a week since I've been on vacation. She started just fine before I left and she was placed in the garage in the first place because I needed to replace the rear brake calipers. Now, when I start her up the lights dim and then I get a clicking sound. I checked the battery voltage and it's 12V even. Shouldn't I be getting 12.6V at least? It is not freezing cold over here, but chilly (46 Degrees F). I've taken that into consideration. Where should I start diagnosing? My guess would be the alternator but I want some second opinions.

 

I would check all your electrical conections battery, alternater, starter, and all the grounds. But if it is running you should have at least 13.5 volts if the alternater is properly working.

 

Go get a new battery!!!
Well, yes, have that one checked first a matter of procedure. But you need a new battery, unless you left something sucking juice from the battery.
My LR was outside freezing for the last two weeks and started right up this morning.

 

Get it jump started and then go down to your local parts store, have them do a free charging system test then replace as necessary.

 

I replaced the battery with one in a car we don't use. The battery is brand new, started right up. I'm heading to Auto Zone after school today and getting it tested, I knew it was the battery but I was freaking out the other day because I needed to get somewhere in a rush.. thanks guys.

 

Ahh, to be able to freak out and not get smacked in the head by the wife for doing so...

 

I'm with Mike go get the system checked to properly diagnose the problem.

 

I did, the battery was good after the recharge. Beats me what happened

 

What voltage do you have with the car running?

What about the serpentine belt, have you replaced it yet?