Batteries degrade over time. The mAh rating you see on the box is the rated capacity, but after hundreds of charge cycles, the actual capacity drops. If a customer complains their vape doesn't last as long as it used to, it's probably battery aging, not a defect.

Another common failure: using the wrong charger. Charging a high-capacity battery with a low-current charger takes forever; using a high-current charger on a small battery can stress it. Always match the charger to the device's recommended specs.

Physical damage is the quickest way to kill a battery. A dented or swollen battery is a safety hazard—replace it immediately. Don't try to squeeze more life out of it.