Why does my car run bad when air conditioner is running?

I have a 2004 Kia spectra. I have had very little trouble out of it. It runs good except when I turn on the air conditioner when idling and the higher I turn on the air the worse it runs. It acts like it wants to die, like it is cutting out. Could it just need a new battery? The battery is as old as the car. It does a little when driving, but really bad when sitting idle.