Batteries like any other consumable device are susceptible to defects, but over time as they are consumed the interactions and gases inside batteries, particular lithium batteries expand, hence APPLE going to a more gel style internal battery.. This is normal expected behavior ..gases as they heat expand. The same is true of Apple batteries as they are consumed they well can expand. If this does happen to you as it has happened to me, just replace your battery! Apple in most cases or APPLECARE protection does not cover battery replacement. In some cases if this does happen over a very short time lets say 3 months then your battery replacement request may have more to stand on.
So if your battery is old like 4 or 5 years and expands to the point of stopping your unit from working correctly "remove it" if your that concerned. It's the same as recognizing that something is failing in your car. Would you expect Ford to replace your alternator if it failed to charge your battery after it was out of warranty.....?