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What's the 0 for in 18650?

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On the smaller cells, you need the digits.

First two are dia, the next 2 or 3 are length or thickness.

A 2032 (you've seen these in fobs and other things) is 20mm dia, and 3.2 mm thick.

So a 18650 is approx 18mm x 65mm (like many things, this is not exact)

A 21700 is correct but so is 2170 since there is a not a 21mm x 7.0mm cell yet.

They should probably stick to the 21700 designation. There is a 2354 which is pretty close to what a true 2170 would be.
 
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