Battery Dead Everything DEAD in X5

[digitron]

I had my 2001 X5 4.4i battery replaced from BMW service center about a year ago. They put in a 650 CCA battery on. Now (this winter) X5 is was acting cranky to start if I did not start my car for over a day. I spoke to AutoZone, & realized one of the cells was dead & advised me to replace the battery. The manual under there computer suggested 850 CCA batter, & I replaced with this newne. Now I had no problems in starting the car.
New problem starts, when put in the key, the car is initialized. When I try to start the car it is completely dead. Nothing at all works & is completely dead. I jumped another battery & opened the trunk to see if the battery terminal was ok. I touched the positive terminal & I heard a clicking noise & everything started again. The connection seems ok & firm.
This problem has started to happen regularly now.
Any one have any idea with this?
Also, can anyone please suggest how I could open the trunk when the battery is completely dead. I am in a situation & cannot jump, I have to access the battery & am not able to open the trunk. PLEASE HELP.