Checking Battery

[bad]

I haven't had any problems with my battery but a garage checked my battery without me asking them to and they told me my battery was "dead" as it "was putting out 100 A when it needed to put out 500" ... I'm pretty sure that is what he said. I decided to say no to whatever he was trying to sell me and came home and took a look.

I watched this 2 min and 27 second video on youtube about checking your battery...

http://www.youtube.com/watch?v=tsJUuLu1cw0

... the guy in the video says if your battery goes below 9.6 V while starting it, that it is a bad battery. My battery was at 12.6 V before starting the car and at 8.4 V while starting it. So according to this video my battery is bad. The car had been off all day, but I started it a few times for about 30 seconds at a time about 20 minutes prior to this test. The dome light was probably on while my dad was starting the car. No other lights were on besides the dome light while occasionally opening the door throughout the day, and the high beams for about 10 seconds twenty minute before the test.

Also, I checked the battery water and it is at at the correct level.

Do you think my battery needs replaced and why?

Thank you.