I then cut the barrel connector off of the end of the power cable and connected it to a 12v battery (which was actually from a cordless drill), making sure to match the correct polarities.
How did you determine that you did have the correct polarity ?, as that would probably be what the supplier might ask if you return the camera.
There are quite a few ads for the MC900A-12 camera. Many show different voltage inputs, even saying 9v-12v, then mentioning 5v in the spec. ??????
There is always the possibility it was a faulty device. Just a pity you didn't test it first using the supplied power supply, just to prove it worked before cutting a cable.