An 18650 is a rechargable li-ion cell that's 18mm in diameter, 65mm in length, and cylindrical (hence the number in its name -- the 0 indicates a cylindrical shape). All 18650 cells, and other cells specified by number like that, are by definition rechargable lithium-cobalt cells with a nominal voltage of 3.7v; that is, there's no such thing as a non-rechargable 18650. Lithium-ion is generally used synonymously with lithium-cobalt.
No, it does
not mean that by definition -- that designation is only part of the proper IEC61960 designation which
does specify chemistry, and
is used for other chemistries; it also seem closely related to (and is certainly capable of being confused with) the designations defined in IEC 60086 for primary cells (e.g. CR17345). At
best, you could argue that it specifies chemistry by convention, but while I'm not aware of any primary cells sold by 5-digit designation without a prefix, other Li-ion chemistries most definitely are, so not even close.
The full IEC61960 designation (as near as I can tell -- unfortunately, I don't have access to the standards) looks like "ICR18650":
I: Lithium form
- I: Lithium ion
- L: Metallic lithium
C: Cathode material
- C: Cobalt-based
- F: Iron-based
- M: Manganese-based
- N: Nickel-based
- V: Vanadium-based
R: Shape (note that this is already specified here, so I'm rather sure the 5th digit does
not designate shape)
- R: Round (cylindrical)
- P: Prismatic (rectangular)
18650: Size code, interpretation depends on shape:
- Round:
18: 2 digits, diameter in mm
65: 2 digits, length in mm if <100mm, or length/10 if over 100mm (e.g. M cell: IFR42120)
0: Not 100% sure -- it's obviously not a shape indicator, since that's already specified, and there's no corresponding lone digit for prismatic cells (see below). It is usually 0, but not always, e.g. I have some RCR2 labeled ICR15266, and have seen references to ICR17335 and ICR17345. I suspect it's 0.1mm digit, but lengths are usually rounded to nearest 1mm?
- Prismatic:
17: 2 digits, thickness in mm if >1mm, or 't' followed by one digit in tenths of mm.
60: 2 digits, width in mm.
65: 2 digits, height in mm.
While using the size specification only is common and useful, it
does not indicate anything except the size (and frequently misses that substantially, as sometimes protection boards, button tops, etc. are included and sometimes not). Specifications such as RCR123 are no more specific, as they are also applied to LiCo cells (3.7V, way too hot for CR123-only applications), LiFePO4 cells (3.2V, lower capacity, still hotter than CR123, but may work in some applications, and sometimes marketed as "3V RCR123"), or LiCo with 3V regulating circuit (low capacity, generates heat, doesn't work well for all applications --
also marketed as "3V RCR123").
Your best bet is to get a light that will accept and run on an 18650 (like the TK11), as that gets you the most energy. If the light's electrically good, but too narrow, you may be able to use a 17670 (less capacity than an 18650, better than 2 16340), but many 2-cell lights will not run well from 1 cell, so you
have to use RCR123 to make it work (or make it work
fully). I'm not sure if the PD30 is that way, but it is designed to run fine with the voltage from two LiCo cells, so 2 16340 will work for sure. Search around to see if it can take one cell. Charging any of these is the same, as long as you go with regular LiCo cells, which will also have the best capacity.
