on battery power - turns itself off

Just yesterday my Macbook Pro (almost 3 years old now) started to turn itself off, somewhat consistently, when running off battery power.

I know the battery is charged. I even left it on the charger overnight last night. Today, I turn it on... it boots up fine but within about 5 minutes, it turns itself off. When I restart it, sometimes I can get 30 seconds of use before it turns off. Sometimes I can only get 5 seconds of use.

If I attach the charger, it runs perfectly all day long.

Sounds like a defective battery? Anything else I can do to test or troubleshoot?

Get this and post a screenshot of what it says about your battery.
Download coconutBattery for Mac - Displays info about your laptop battery, widget available. MacUpdate.com

Doesn't seem to be anything out of the ordinary:

Hmmm.... looks ok, and your Mac isn't even 2 yrs old yet.

Ok, I would do two things.
Calibrate your battery:
Apple Portables: Calibrating your computer's battery for best performance
Reset SMC:
Intel-based Macs: Resetting the System Management Controller (SMC)

I'd also be interested to see the information in system profiler.
>about this mac>more info>power>battery information.

I can't calibrate the battery, as I cannot run it down completely (the computer won't stay turned on!)

The power info in System Profiler also looks quite normal.

I had already done a reset of the PMU and PRAM.

Thanks!