Quick battery question...

Hey, I just tried to start my 88 300e but it won't turn. I checked the voltage on the battery and it was hovering around 5.5 volts. I then trickle charged it for several hours until the charger said it was ready and disconnected the charger. My voltmeter read 11.2 volts across the posts, but I watched it quickly fall to around 8 volts. Could this be anything other than a dead battery? It would be hard to imagine that big of a load with the engine off...

Any help would be appreciated.

Thanks.
Peter
88 300e
88 420sel