Short Battery Life

I have a MacBook (2 GHz Intel Core Duo) that's about 3 years old. Lately I've had absurdly short battery life. Today I took it to a cafe. It was fully charged up, and I started it up fresh. I worked for about 45 minutes with two applications open (Safari and MS Word), and it suddenly did a hard shutdown. It didn't even give me any warning or go to sleep -- just the shutdown. The battery percentage was at about 75 or 80%.

I've already taken it to the Genius Bar and they did a quick diagnostic and said my battery is normal. (They gave me coconutbattery.) I've tried the battery calibration, and it usually results in the same hard shutdown. The fan has not been running very often, and the system doesn't seem too taxed. I have no idea what the problem could be. Any ideas would be appreciated. (Hopefully I'm not looking at a new battery...)

Thanks!

i would do two easy things first.

reset the PMU - remove the battery and power lead and hold the power button about 5-10 sec. then reinsert the battery and power lead and start up.

then remove the power lead and run on battery. in your situation, you'll want to keep an eye on it til it dies, since it seems to be giving you bad % readings. Once it dies, plug the power lead back in and charge it to 100% in one go. check the stats in either the system profiler or coconut and see how healthy the battery is then. also, give it another test to see if the battery drain and % seem more aligned.

my guess is you have a battery that is on it's way out due to normal to heavy use (lots of medium to deep discharge cycles and maybe a few other things that normally affect this). and maybe an issue where the estimated and actual % aren't synced properly.
you may just need a new battery, hopefully. the alternative is likely a new power unit in the macbook, which is a pricier proposition.