My first MAC Attack

After about 8 months, my macbook gives me my first scare. There's something wrong with my battery. It says it's fully charged and all the indicator lights are lit to confirm it but my macbook will not run unless the ac adapter is plugged in. Every time I unplug it, the mac goes to sleep. I'm going to Apple tomorrow. I've tried everything I could see in the manual and the ones I've googled online and at the apple support page but to no avail. I've read about some people encountering the exact same problem and say a new battery is all it took. Thank god I'm still within my one year warranty. I will seriously kick someone's a$$ if I'm asked to pay for anything.