Shuts down after a few pictures, even with a full battery.

The camera Canon Rebel xs will take a few pictures, and then completely shut down. I have 2 batteries, one is a 3rd party, brand new battery, and one is the original that came with the camera. This problem just started within the last week or so. (no drops or other damage to the camera) I have read that the internal battery may be bad, but I am not sure how to replace this. I have been using a SanDisk Ultra 16gb memory card and I have over 1600 pictures on it. Could it just be overloaded? Any suggestions would be appreciated. Thanks in advance.