Batteries can swell and deform with time. This is a lesson I learned by chance just yesterday, when taking my laptop in for a repair.
I had been having problems with my Macbook Pro for the past week or so. The touchpad would be clicking on just about everything, even as I was typing; causing the text cursor to jump to wherever the mouse was and while moving the mouse/touchpad it would click on everything in its path. As you might imagine, this made virtually everything impossible to get done, without becoming quite frustrated.
I took it to the Apple store for diagnostics and the first thing the technician checked was the battery. He first noticed that the battery door was not tightly closed. He pulled the battery out and set it on the counter, discovering that it wasn’t flat, as it should be; there was actually a bulge in it.
The deformed battery put pressure on the touchpad causing it to misfire clicks almost constantly. 90% of the macbook pros he sees with touchpad problems are due to battery deformation.
Replacing the battery fixed the problem completely. This can happen on anything with a battery. It has even happened to larger battery backup systems, causing batteries to become stuck in racks.
The technician told me a story about a 911 emergency center with a battery backup array on a rack. Over time these batteries enlarged; when it came time to replace them, they realized that they couldn’t even get them out of the racks anymore! They had to pay some professionals high dollars to safely remove the batteries.
So the lesson from this, is be aware that not only do batteries have a limited lifespan and eventually fail to hold a good charge; but they may also become disfigured and damage or disable equipment. Who knows, if left unchecked for long enough, they might even break open and leak out chemicals.