Battery Life On First Gen iPhone

The funny thing is that this is my second iPhone my first one i took to the Apple store and they replace it and i followed everything that they told me and now i am having the same issues. It happen again today my battery level when all they way down and then couple of mins later it was back to normal i think i might go to Apple store again and exchange it. That's if i could do a second exchange i don't know if they have a policy of how many times you could exchange your iPhone.