Today I tested even more of 3.7V 18650 Li-ion batteries from Ebay and explained how to tell the real ones from the fake ones. 18650 Li-ion cells that weigh under 40g tend to have a low capacity. The cells marked over 3500mAh are surely fake and actually are about 500mAh only. Cells with spelling errors also tend to be fake. Button top batteries are way more likely to be fake than the flat top ones. Suspiciously cheap cells are usually fake or recycled from e-waste (rewrapped cells salvaged from old laptops or other devices). Marks from previous spot welds may be covered using metal circles. Recycled cells may range from almost unused to totally worn out. Fake batteries may claim a protection they don't have.

List of all 18650 Li-ion cells I've tested:
http://danyk.cz/test18650_en.html

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