Discharge rate and capacity, somethin' ain't right

[brembo]

Been banging around in the battery university and poking around in this subforum and I'm still at a loss.

I have a pretty good grasp of what 1C rates are, a 2000mAh cell discharged at 1C will in theory be 2000mAh in one hour. A .5C rate will take two hours, and a 2C rate will take 30 minutes. Easy-peasy there.

Now this is where I get all confused. As I understand it, the higher the cell's rated mAh, the more internal resistance. Li-ion cells in general are not supposed to be discharged above 2C(correct?). So, a 2000mAh cell with (theoretically) less resistance than a 2900 mAh cell is not supposed to deliver 5800mAh (rate) while a 2900 mAh cell is (again, theoretically) okay in that situation? What critical bit of info am I missing here?

Is capacity tied to max discharge rate, with little regard to resistance? Or am I reading too much into this "rule of thumb"? It seems odd that the discharge RATE is linked to the overall capacity.

I ask this as I have a single cell 14500 light that rides the bleeding edge of 2C with a 750mAh cell (little over it in fact). I'm using AW cells in it, so I'm in good shape there, but any steps in the direction of safety are welcome. I could always just get an IMR and forgo a little bit of capacity, still I'd like to know the relationship of capacity to discharge rate in "typical" Li-ion cells.

*edit*

Fixed a few calcs.

 

[Mr Happy]

Rules of thumb really are just that, and are always secondary to the manufacturer's data sheet.

A 2000 mAh cell could be viewed as 2000 x 1 mAh cells in parallel. And 2900 mAh as 2900 x 1 mAh in parallel. So the greater discharge current gets split over more parallel circuit paths, so it all balances out.

If the 2900 mAh cell is physically bigger it could even have a lower internal resistance (and probably would). If it's the same size (e.g. 18650) then the internal parts will be thinner and may add a little more resistance.

Ultimately, high discharge currents are stressful on a cell and will probably wear it out quicker. So it's best to follow the data sheet guidelines and be conservative where possible. On the other hand, when you are pushing the bleeding edge you may be comfortable with a shorter cell life and buying replacements when needed.