reserve power message

My ibook has been running fine for the last 3 years but today, I had it on mains recharge for 4 hours and within 10 minutes of use,after full recharge, I was getting the message that the system was running on reserve battery power.This has happened several more times today.
any ideas??????

You might want to download and install the free cocoanutBattery application and see what it says about the condition of your battery.

Regards.

Thanks for that.
Looks like the battery has gone. Current capacity 295 mAh, normal capacity 4400.mAh
not charging at all.
does anyone know how to replace the battery?