Battery wont stay charged - 2003 Ford Expedition

First charge battery or jump start. then remove negative battery cable. If the engine dies, you need an alternator. if not, shut off car, hook a test light between the negative cable and the battery negative post (with cable disconnected) if it lights, then you have a short draining the battery. make sure the doors are closed and lights are off while doing this, otherwise you could get a false reading. If the battery is original, it may just be bad. Walmart or auto zone can test the battery for you