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The instructions do specify "lead-free solder" with many exclamation points.
Large Motors Class
Intro to Fusion 360: Lamp Design
Mold Making & Casting Class
While the 18650 won't fit most of these cheap lights, I really like using the 18500 size. They have a larger charge capacity, somewhere between the 14500 and 18650, but fit nicely inside. They are commonly available in four-packs for about $10, for use in solar pathway lights.
3D Printing Class