> First place I'd look would be on eBay or the Chinese distributors like De=
alExtreme.

Be aware that small low cost cells may not have internal protection circuit=
ry.

Note ahead of protection comment: In-cell protection circuitry is not
as crucial in tiny cells if all is down well. The prospect exists with
some spectacular results but usually nothing too nasty. With laptops
and other systems with multiple 18650 cells the energy levels can be
substantial. A 2 Ah 18650 cell contains about 7 Watt.hour of energy
(depending on discharge rate ~=3D 20 kW.s or ~~=3D  30 Horsepower seconds.
If that gets out fairly instantaneously or even over a few 10's of
seconds. Add a number of companions getting in on the act and it can
make for a very spectacular result.it can make

Protection circuits made for inclusion inside the cell packaging
usually limit charging voltage and some or all of minimum voltage. max
discharge current , max charge current, minimum voltage at which full
charge will be accepted. minimum voltage at which ANY charge will be
accepted. Gross over current is usually handled by a fuse* or maybe
polyfuse.  *Maybe in the cap / nipple end and not looking much like a
normal fuse.
Minimum voltage at full charge matters as a cell may become unhappy
below this. Pump up from lower voltage may work but there is a voltage
below which a cell may be considered dead (even if actually not).

Most protection circuits will not cover all these features - many
being left to the charger IC proper.

Modern LiPoly are generally better behaved than earlier LiIon tubular
BUT "vent with flame" is still a possibility in the large majority of
cases. Protection circuitry is intended to present this or, rather, to
reduce its incidence. Problems that occur within the battery structure
cannot be prevented by external circuitry protection. Some laptop
batteries made by very reputable battery makers and installed in very
well known brand laptops, occasionally ate their laptops for breakfast
when a mechanical shock was applied "just so" to some units. There was
a mechanical clearance issue and the shock could cause battery short
circuit and away it went. Once it gets to the gaseous Lithium stage
you are good to go under almost any external conditions. Adding water
adds to the fun.

None of this is liable to be an issue with small LiPoly - especially
if your control circuit is suitably benign. But, be aware that an
unprotected cell can be a 'fun' device to manage incorrectly.



                 Russell McMahon
                 Applied Technology ltd.
--=20
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