I recently found an old S II around the house and upon trying the battery in the bag(non-rechargeable) I found out it was dead. So I went to a batteries plus and got a replacement 2CR5 duracell battery. I inserted the film put the battery in and the film was loaded into the camera. However after taking one picture the viewer went black and the display showed that there was no more battery. So I took the battery out and put it back in and the same thing happened. Then I went back to the batteries plus and returned that one and got a new one. The same thing is still happening. Does anyone know why this is happening and if its fixable?
Is the battery display showing a steady outline of an empty battery or is it flashing?
About the only thing you can reasonably do as a user is to use a cotton swab and some isopropyl alcohol to carefully clean both the battery and lens contacts but neither is likely to be the problem. But there is probably a fault with the shutter/mirror assembly and unless you have a really strong sentimental attachment to this camera repair isn't worthwhile. Even the EOS 1 film cameras are relatively cheap now so even if you are heavily into film rather than spend money to repair a lower end body it would make more sense to buy a good condition higher end Canon film SLR in working condition.
"Does anyone know why this is happening and if its fixable?"
Old electronic stuff can and does die on the shelf. A variety of things can happen. Capacitors can dry out over a long period of time and lose capacitance for instance. Is it fixable, of course it is if you have the skill and parts. If it is service by a professional shop, you need to weight it carefully. Is it worth fixing? Can you buy a good used working model for less?