I'm having a slightly similar problem. I'm not under AppleCare, however when I was under the default 1-year warranty I did have my Battery replaced due to overheating issues. I believe it's been a little over a year since receiving the replacement battery and since ~3 weeks ago my battery's life has
dramatically decreased.
I've called Apple before and told them I thought it was defective since it's under 300 cycles, yet they turned me away since 1) I don't fall under the serial number for defective MBP batteries [the 1 year, 300 cycles, bulging battery thing]. 2) I'm under AppleCare (doesn't matter anyways) 3) It's been over a year since I've had the battery.
Is it worth calling again and trying? They probably already have my information in the database from my previous call.
What is the normal life-span for batteries? I haven't done
anything irregular in terms of battery life in the past couple of weeks.
Any help would be greatly appreciated.