I fixed your title spelling error, you might get more responses now.
You just got a battery 4 weeks ago, and now you're having serious voltage problems. You should inspect your battery terminals to be sure those are both tight and clean. A battery terminal cleaning tool is like $1 at parts stores. It's like a little cap with steel wires inside to brush off the round terminal. Hopefully you still have your factory clamps. Those are great, undo the nut on the top a few turns, tap nut and stud down, wiggle, and remove.
If the battery terminals are ok, then you might have a problem with the alternator output. Try this: Key off, press and hold trip odometer reset button, turn key on, continue to hold button until the odometer reads [test]. You're now in odometer test mode. It will remain in test mode until you turn the key off. Each time you press and release the odo reset button, you will scroll through one of 30 test functions. Look for the one that reads [bat 12.1], or if your battery voltage is lower the numbers will be different. Start the car without turning it off, and drive. Now you'll be reading your alternator output. If voltage is not within 13.8-14.9 volt range, then your alternator is bad. Lower voltage means the battery won't charge, higher voltage is dangerous to the entire electrical system. Both mean that you will need a new alternator.
You can also check the other end of the battery terminal wires, but typically those are not loose. The hot will go to the starter and alternator, and the ground will go to the engine. There will also be 2 small ground wires that go to the chassis and the underhood fuse box if I'm not mistaken.
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Moderating everything now, let me know if I can help.
Originally Posted by the user formerly known as ZX3_Chick
You're special aren't you.