About 6 months ago I replaced the battery in my 08 tahoe then 3 months after that my battery dies again. I take it to autozone and they say the battery is fine so we argue for a little bit and they finally agree to give me a new one. So we get it in the car and get it started and everything's fine for a couple of months and then that one dies so we open the hood and the cables are loose so we tighten them and jump it off and it's fine for a couple a weeks and then dies again so we go have the alternator tested and it's fine. So it has to be something draining my battery right? I know this battery is probably bad since we had to charge it so many times. All the lights and everything is off as far as I know. Please help!!!

 

Welcome to the forum....
There can be many reasons the battery is running down ( and you are probably right about having to replace this one because of multiple run downs)

*Check the battery cables for corrosion at all points
*Check that no interior lights are staying on, esp vanity lights on the visor
*Check to make sure nothing is being left plugged in to the 12v outlets, even chargers not connected to a device, use power.
* Check that the brake lights aren't staying on
*Check any trailer wire harnesses for shorts.
* remove any CD's or DVD's from all drives to ensure none are running even after the vehicle is turned off
* Check to see if your AC blower stays on after the vehicle is shut off ( After-blow is a feature used to help dry out the AC after the vehicle is turned off to prevent the musty smell)

Consider having a current draw test done to see how much is being drawn and for how long after RAP is turned off ( RAP is retained accessory power)
Removed various fuses 1 or 2 at a time to see if you can isolate the circuit that is causing the run down...

Keep us posted on what you find

 

A very comprehensive check list from in2pro...
I'll add to remove clean and tighten securely ALL grounds.