My two year old MacBook Pro 17” has not (knock on wood) experienced this swollen battery problem.  I leave the computer plugged in almost all the time although I do swap the battery with another one about every six months. The only things connected are an Apple Studio Display 17” (as a 2nd display), iPhone 3GS, powered external speakers, Gigabit ethernet, and occasionally a Firewire 800 hard disk, a DV camcorder via Firewire 400, and a midi keyboard.

It would be interesting to see if there’s any correlation between this failure and system temperature. For example systems that run in warmer environments have a higher failure rate, or systems with lap stands that raise them up to aid cooling had a lower failure rate.

When the battery of my Core Duo MBP 15” died (apparently, I fried it by always leaving it plugged in and never cycling it) and I bought a replacement, I started leaving my bad battery always plugged in; swapping it for my new one only when I wanted to take my computer somewhere.  That has worked very well for about 2 years now.

I, too, wonder about any correlation between battery failure and system temperature and how much does faithful regular cycling protect charge life,

A few years ago I needed to replace the battery in my cell phone. The new one wouldn’t fit, it was swollen. No problem they gave me a different one that did fit properly. From what I understand, I may be incorrect, is these types of batteries can swell from being overcharged.

On my previous cell phone I bought a backup battery and left it in the charger.  It was thin battery not unlike a CompactFlash card.  After about two months it had swollen to almost twice it’s original width.  Needless to say, it was never going to fit in the cell phone I bought it fore.

I had an issue with my MBP where the trackpad began failing to register clicks.  After going thru Apple support and heading to the closest store 50 miles away, it occured to me that I had a heck of a time getting the battery cover back in place after an Apple rep had me pop the battery to do a PRAM reset (or whatever its called on Intel Macs).  While sitting at the Genius Bar I told them about it and they checked and low and behold the battery rocked on the counter top instead of sitting flat.  They poped a new battery in and trackpad was like new.  Live and learn!

my macbook pro went thru 3 swollen batteries in 4 years.  Apple replaced the 1st 2.  I bought the last replacement, about a month ago.  The 1st swollen one had lots to do with defective design or manufacturing, according to Apple.  When the last one got swollen up, I suspected it might have something to do with me leaving my macbook pro ON all the time in my tight, well-cushioned laptop bag after work.  I rarely shut down my macbook pro, maybe once every few weeks, even months.  Perhaps, it might get real hot in there. 

might have something to do with me leaving my macbook pro ON all the time in my tight, well-cushioned laptop bag after work.

Which might support the theory that it has to do with overheating.

I’m very frustrated. I’m in a very similar case. A 1.6 years old 13’’ MacBook, with a 30+ recharge cycles battery. But the Brazilian Apple WON’T replace it. They say the official replacement program was closed, and I will have to buy a new one. Don’t know what to do. Those are very expensive here.