What is draining my battery?

The vehicle battery is at full charge. As soon as I hook up the positive cable it goes from 12.7 volts down to 4.5 volts, I take the positive cable back off and the battery voltage starts going back up. The vehicle will only try to start if I have my battery charger on it and switch it to 200 cranking amps and spraying starter fluid however the vehicle never does start, I believe the entire system is being drained and will not allow it to receive any spark whatsoever without the battery charger it will not even turn over. However, it is not blowing any fuses or relays it I can tell and there's no smoking wires or anything like that I can't tell what this could be. Thank you I appreciate any help I can get.