My Volvo V70 alarm is unstoppable?

My remote key button didnt open my Volvo. On opening the door with the key I set the alarm off but placing the key in the ignition did nothing (normally this cancels alarm). I believe the battery to be flat. The battery is in the boot which remains unlocked as can't unlock it without power. I believe there is a form of battery under the bonnet but couldnt see that and while looking the alarm went off repeatedly. Has stopped for now but I am dreading this going off during the night! Any advice greatly apreciated!