On Fri, 02 Nov 2018 21:55:40 -0000, Theo wrote:

In uk.d-i-y Steven Watkins wrote:
I've got some very good 18650 Panasonics - 3Ah, but there seems to be
nothing worth getting at the 14500 (AA) size. I thought LiIon was meant
to hold more power per volume, which is certainly true of the 18650s,
about double an NiMH. But 14500s are all 850mAh (the ones claiming higher
are Chinese fakes and aren't higher). So you don't actually get more
power out of an AA sized LiIon than NiMH. Can they not make LiIon cells
that small very well?

The problem is cylindrical batteries are based on a roll of cathode, anode
and separator, all tightly rolled up.

The capacity of the battery is proportional to the area of the roll, which
is proportional to the length of the unrolled material. The length you can
fit in a cylinder is roughly proportional to r^2.

The cell wall thickness is about 0.2mm, which makes an 18650 have 17.6mm for
the roll and a 14500 have 14.1mm. By r^2, an 18650 would have 1.56 times
the capacity of a hypothetical 14650. But it's a 14500, which means an
18650 can fit 2.03x as much roll area. That would mean about 1500mAh in a
14500. The rest I suspect is overheads, and not trying so hard (much higher
demand for high power density in a larger form factor).

I see, that explains why Panasonic (the only make I've found that actually has the capacity they state on the label) don't even bother making them.

Also, don't forget lithium batteries are 3.7V nominal compared with 1.2V for
NiMH. So you get 3x the Wh.

Yes, but the AA size LiIon has 1/3 of the Ah, so the Wh is identical.