So I had a wedding this past sat and I was using the extra battery grip for the first time, with brand new batteries. I use it for about an hour and it kept shutting off telling me my battery was already dead. I would shut the camera off turn it on and full battery, 2 sec later battery dead.
Is mine defective?

Are they Nikon-branded batteries? I've had issues with after-market ones.

No, maybe thats the problem. Guess I'll buy some and use it one day and see if it makes a difference. The battery grip should last a whole wedding correct?

Depends on the camera. With my D200, it lasts longer than a wedding. It also does with the D100, but it's not used nearly as much as the D200.

On the d300 with lithium batteries in the grip, it will last for around 2000-3000 frames with no problems. At around 3500-4000 it starts to show low battery and spazes out a bit.

We might be able to help a bit more if we had some more information to go on. like what body are you using, is the grip a nikon grip or off brand grip, are you using AA batteries or a rechargeable battery, if it is rechargeable is it nikon or off brand and how long are you charging the battery for, and how long have you been using that rechargeable battery for before this has happened.

Some simple things that you might want to check are:

are the battery contact points clean?
are the lens contacts clean (dust and dirt can really effect the way the camera functions)?
try a different charger if you have one or knows someone who has one
try a different battery b/c you could have gotten a bad battery(nikon did a recal on the en-el3 batteries a while back)

If all else fails, nikon tech support is great and NPS is amazing from what i hear if you have it.