For those who are still using the old MBP battery that you kept after receiving the new one in the Apple exchange BE CAREFUL! Just a warning, either stop using it or pay really close attention to your lappie if you are so adventurous. I kept using my old battery up until a few hours ago when it started to hiss. Before it started causing my MBP to just shut down when it got to around 25% power left in the menu bar. No sleep mode, no warning you are on reserve power. I thought it was the laptop and not the battery. So I charged it back up and used it again. Boom! shut down and laptop came back on and it shut right back down and I heard pretty loud hissing from the bottom. Flip it over and it got louder so I snatched the battery out as quick as I could. I put replaced the battery with the new one..charged it and let it run down and the system acted normally...reminded me I am about to be on reserve and went nicely to sleep. Yeah , I didnt think anything was wrong with my battery either, but I suggest people stop using them. They can really change behavior in a split second and you could be screwed. What if my laptop ignited on fire...Applecare would probably laugh in my face.