2005 Pontiac Grand Prix • 125,000 miles

When I turn the key to start the car, the engine tries to turn over and start, but then all of the car lights and electronics go very dim and the engine stops trying to start. I have had a number of recent error codes that indicate engine misfiring. I just got new spark plugs and wire installed, and while the shop was checking everything out they told me that my battery only has 30% capacity, so a new battery will be needed in the near future. My boss thought my alternator may be going bad, but when the car is running, I am getting between 14.5 and 15 volts.

In your opinion, is my engine starting issue due to the battery, a fuel/air intake issue, or something else? What can I do to test it so that I don't send hundreds of dollars to diagnose the issue?
Thank you for your help.
November 27, 2012.

Start by having the battery changed and go from there. It sounds like a bad battery?

Nov 27, 2012.