Editing Battery Refurbishment (section)

=== Cylindrical lithium cells: the number is the size, not the chemistry ===

[[File:Battery guide - 18650 and 21700 cells.jpg|thumb|300px|An 18650 cell (left) and a 21700 (right). The numbers are dimensions in millimetres, nothing more.]]
[[File:Battery guide - 18650 AA AAA size comparison.jpg|thumb|300px|An 18650 against AA and AAA cells with a coin for scale. An 18650 is visibly fatter and longer than an AA and will not fit an AA holder.]]

The familiar five-digit numbers are purely dimensional and follow the scheme given in '''IEC 60086-1''' for cylindrical cells: '''the first two digits are the nominal diameter in millimetres, the next two the height in millimetres, and the fifth digit indicates the cylindrical shape.''' (Some manufacturers instead read the last three digits as the height in tenths of a millimetre, which gives the same answer for the common sizes, and some use their own designations entirely.)<ref name="iec">Wikipedia, [https://en.wikipedia.org/wiki/List_of_battery_sizes "List of battery sizes"] and [https://en.wikipedia.org/wiki/18650_battery "18650 battery"]. Source for the IEC 60086-1 dimensional designation scheme and its alternative tenths-of-a-millimetre reading, the specific cell dimensions listed below, the point that the designation encodes size and not chemistry, the range of chemistries built in the 18650 format including sodium-ion and potassium-ion, the added length of protected cells, and the 1991 Sony introduction date (Panasonic claims 1994).</ref>

'''So 18650 means "18 mm diameter, 65.0 mm long, cylindrical" and tells you nothing whatsoever about what is inside it.''' The same cell is sometimes written as a '''1865''' with the trailing shape digit dropped; it is the same thing. Cells with lithium cobalt oxide, NMC, NCA and lithium iron phosphate chemistries have all been built in this size, and by the 2020s so had sodium-ion and even potassium-ion cells — with different nominal voltages and different charging requirements.<ref name="iec" /> '''Never assume chemistry from the size code.'''

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Cylindrical lithium cell sizes you will meet in vintage equipment'''
! Designation !! Diameter !! Length !! Where it turns up
|-
| '''18650''' || 18 mm || 65.0 mm || The overwhelmingly common laptop and camcorder cell. Introduced by Sony in 1991.
|-
| '''18500''' || 18 mm || 50.0 mm || Slimmer packs, some camcorders
|-
| '''17670''' || 17 mm || 67.0 mm || Some early camcorder and portable packs
|-
| '''16340''' (RCR123) || 16 mm || 34.0 mm || Cameras, small devices
|-
| '''14500''' || 14 mm || 50.0 mm || AA-sized lithium. '''Not an AA''' — it is 3.7 V, not 1.2 V or 1.5 V, and putting one in an AA device destroys the device.
|-
| '''21700''' || 21 mm || 70.0 mm || Modern; useful only if the pack has room
|-
| '''26650''' || 26 mm || 65.0 mm || Modern high-capacity; rarely fits a vintage pack
|}

Cell length is quoted for the '''bare''' cell. '''Protected''' cells carry a small protection PCB and a strip conductor under the wrapper: a protected 18650 typically measures around '''68 mm''' rather than 65 mm, and some designs increase the diameter instead. Either way the cell may no longer fit a compartment designed for an unprotected one, so measure before ordering.<ref name="iec" />