Car battery dead

Sometime in the last week Ylva found that our Volvo didn’t start. In fact, it was totally dead. Even the dashboard clock was blank. Today we tried to start it by pushing the car bakwards from the garage and down the slope. No lifesign at all to my big surprise. (Note how irony fails when written).
Time for Plan Bâ„¢. Called a neighbour (Björn) whom I know has two old (not to say ancient) cars. He showed up within three minutes with a spare battery and cables. Unfortunately the spare got completely drained the moment i turned the key, so he went home to get his 1985 Mazda. We hooked his car up, I turned the key, and bingo! It started. After keeping it running for nearly five minutes, I expected it to start without any cables, but it didn’t. In fact, it was as dead as before, excepting the clock. We measured the battery voltage and found it rapidly falling from 8 something to just above two volts in less than a minute. Removing the + lead, it kept steady at 10 volts. I took this to mean that a cell has been shorted and that a new battery is needed.
Started again with help from Björn’s Mazda, I went to the gas-station. I parked almost outside the entrance and went inside and asked for a car battery. The guy inside seemed confused and said I’d have to wait for his collegue, a girl who helped him look up the correct battery model number and send him to fetch one. Unfortunately, they didn’t have any suitable battery at all. My car was still dead and refused to even wake up the start-engine.
On my suggestion, they brought up a rental car, a new VW Passat and cables. We were both equally surprised when we opened the hood and couldn’t find any battery at all. After a while we gave up, and the gas-station guy (a guy in the 20s) brought up a less sofisticated (though french) car. At least we could find the battery, and managed to expose the + pole. Red cable attached in both ends black only on my car, the guy was to connect the black cable to a screw on the engine while I tried to start and disconnect the moment my engine was running. He didn’t really manage to connect really well though since he jumped two feet off the ground when some sparks started to fly.
When I finally got home I called Björn again and we drove off to IKEA where they have a car parts shop. 10 minutes later the battery was replaced and the Volvo started without help again.
Hooray for helpful neighbours.