Billions of 18650 lithium-ion cells move quietly into daily life each year, powering everything from toothbrushes and power tools to e-bikes and electric vehicles. All these batteries share the same 18 mm by 65 mm can, but their simple cylindrical packaging conceals the internal jelly roll structure that will ultimately decide how a cell behaves over time. In an increasingly electrified world, cell quality is critical. Every rechargeable device carries a small but real hazard, and when multiplie...