Can't figure out how to change smoke det...

I don't have the manual so I loosened the unit from the wall to look at the back panel. It is hard wired with a battery back-up. The "plug in" part doesn't pull straight out--the edges fit into tiny slots on the side and there is a little flap that fits over the top part of the battery chamber. I can't get the battery out without getting the plug out of the way. I tried pulling the plug and sliding it up to disengage it, but no luck. The beeping is driving me crazy. Help!