First the alternator was replaced, the car started fine for like a week then the battery died, bought a new one it lasted about another week and it died while driving so I turned the car off 15 minutes later it starteed again but then died again. Why is the battery not holding its charge?
By the way this is not a maxima its a GMC Van.

 

Turn the car off and remove the positive side of the battery. The car should stay running. If it cuts out then your alternator is bad or one of it's connections is wrong.

 

wait... turn the car off?... and remove postive side.... and the car should be running... hmmm that doesn't make sense...

 

The car should be able to run with out the battery. The main purpose of a battery is to start the car and then the alternator starts doing all the work.

The alternator either doesn't work or wasn't installed properly.

 

DO NOT DISCONNECT THE BATTERY WHILE THE CAR IS RUNNING...
That's a great way to mess up your electrical system on newer cars...

Use a voltmeter to check the voltage at idle and 3K rpm and/or clean the terminals

 

the battery is there to regulate the voltage to 12volts. it provides amperage to you system when your alternator isnt producing enough. when you take the battery off on a computer controled car while its running, the voltage will shoot up to 15+ volts. computers start to fry after 15volts. never disconnect the battery while the car is running.

 

bad alternators make batteries die. I have seen people replace their alternators and a week later they go bad. I would say 50% of the time alternators from stores like autozone wind up being bad (starters are like that too). Take it back and make them replace your alternator and battery. They usually will (first have them test the alternator just to make sure. they do it for free at most places.)

 

Yeah dont disconnect the battery. The sudden voltage drop will make everything confused.

Hook up a simple voltmeter (they're on sale this week at Harbor Freight for 2.99. Everyone get one) to each of the terminals on the 20V scale.

If the voltage is over 14 volts then your alternator is good. If it's 13.8 it's questionable. If it's 13.0 then the output is low. If its 12 then the alternator is not working at all because you need more than battery voltage to transfer little electrons.

GM vans usually have very easily servicable electrical systems.

 

Your first mistake is owning a GMC van. j/k!

Second, take your car to Autozone, have them check your charging system. If the reading is less than 14v, then your alternator is STILL bad.