25180 LIRs, why not?

[xiorcal]

Hi,

Summary question: Why aren't short, fat Li-ion cells, like "25180"s made?

This is really more a question for manufacturers than users, but as a lot of the users on this forum seem to have intimate knowledge I thought I would post it for discussion as well;

High flux LEDs take from 3V (low driven Cree/SSC) to anywhere around 21V, or something insane for 20W Ostars.
Hotwires also take high voltage.

Li-Ion chemistries range from nominal 3.0V (stepped down for consumer use), ~3.2V (?) LiFePO4, to 3.6V conventional Li-Ion. (More when fully charged)

When running single high V, or multiple low V LEDs*, even incorporating a boost driver, a single Li-ion doesn't make the voltage grade.
This is the point at which multiple 14500/18500, so on are used in series to boost the voltage to a level where it meets the LED string's V requirements.


(*I gather multiple Cree or SSC are best run in Series to avoid unbalanced output from bin-variance)

This is very space-inefficient with long cells, due to all of the space around the cells that is dead-air. The only candidates for short-cells in series seem to be RCR123s.

When comparing fatter cells, the energy capacity seems to be dramatically increased over skinny cells. AW's new C cell is roughly 80% of 17670 length, 150% its diameter, (120% the volume) and just over 200% its capacity. Bottom line ~66% greater energy-by-volume.

This leads me to the conclusion:

People are willing to bore C and D-cell lights to insert 3 or 4 long cells.

If high voltage is wanted, and this is achieved by putting li-ion in series, why does no battery manufacturer (to my knowledge) produce protected, short, large diameter cells, that will go in series in an unmodified C or D sized light?

It would;

• save work
• require no battery holder
• decrease wasted space
• increase real capacity

Simply; why are there no 25180 (1000Mah) Li-ion cells?

 

[SilverFox]

Hello Xiorcal,

Saft makes some 26180 cells. They are a little spendy at roughly $40 each. Also, I don't think they are protected.

Tom

 

[xiorcal]

Hello Xiorcal,
Saft makes some 26180 cells. They are a little spendy at roughly $40 each. Also, I don't think they are protected.
Tom

Hi Silverfox,
If I'm not wrong, those appear to be oddly high voltage primary cells. I was talking about LIRs, but thanks for pointing them out.

 

[SilverFox]

Hello Xiorcal,

Oops, I missed that...

How about a 41 by 145? About the diameter of a D cell, but longer.

It is an impressive cell with a capacity of 7000 mAh and capable of 100 amps continuous and a 10 second burst of 250 amps.

I see your point. It seems that short fat cells do not exist in the round form. When the manufacturers get to those sizes, they go prismatic.

Tom

 

[2xTrinity]

I agree that fatter cells make a lot more sense in general just by looking at simple geometry. Shorter, fatter cells are simply a more efficient way to pack energy into the same space -- especially considering that the thickness of the lining of the cell will have to be roughly the same no matter what, so that's why a skinny cell like a 14500 has the same energy as a shorter, fatter 16340 (CR123 size).

For driving things like hotwire flashlights, which already have long cylindrical bodies, having LiIon cells that are essentially the same diameter as C- or D- cell makes the most sense.

 

[chimo]

If you look at how the cylindrical Li-ion cells are constructed, you will notice that there are some areas at the top and bottom of the cells where the protection devices are located and where the anode and cathode are connected. If the width to height ratio becomes large, then this "wasted" area will form a greater percentage of the cell volume and the cell capacity/volume ratio drops. There is likely an optimum H/L ratio for maximum energy density.

As Tom mentioned above, this sort of cell size is more common as a prismatic.

Former CPFer Newbie/Jarhead has a great write-up on the construction of cylindrical Li-ion cells. There is a thread on CPF and you can find information (and dissambly pics) on his web site here.

Paul