My original battery, after about a year and a half, started to develop a bit of a bulge in the middle. It’s fairly subtle, maybe just a half millimeter of bulge; I didn’t notice it, even looking edge-on. However, a battery vendor in China showed me a quick test to determine if a battery is failing in this manner: place the battery on a flat surface, and attempt to spin it. If it spins freely, then the middle is bulged out, and the battery is no good and should be replaced.