New batteries can also come only partially charged. I would put the battery on a battery charger (on low) for at least overnight and THEN confirm you have a problem. Could also be a bad connection at either side of the battery cables. Make sure terminals are tight. If you see any gunk around the terminal, you need to pull the cable and shine up the post and clean the inside of the cable clamp. A simple brush-type cleaner is very cheap at an Auto Zone or Advanced Auto store and can do the posts AND the insides of the clamps.