The battery becomes unusually warm during a recharge cycle.
The battery becomes unusually warm during phone use.
The battery case may become swollen. This is detectable by feeling and viewing the inside/phone-side of the battery case. Also, when the battery is placed inside/phone-side down on a flat, smooth surface, it will rock and will sustain a spin motion. The case of a healthy battery is flat, and the battery will not a spin easily.
The battery develops a hard spot. This is detectable on the inside/phone-side surface of the battery by gently pinching about the surface between one's fingers.
Know how to avoid battery failure:


The usable time after recharging is shortened.
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