which has more power, a 6000mah NiMh or 2900mah LiCo?

I'm thinking of building myself a cusom light. I thought it was a no brainer to use a 6000mah NiMh C cell but....

If my calculations are correct:

LiCo capacity = 3.6v * 2.9ah = 10.44wh
NiMh capacity = 1.2v * 6.0ah = 7.2wh

Power to the LED will be about 5.4 watts.

If I use a 1.5 amp driver and a 2900 cell, I calculate:
1.5amps x 3.6vlots = 5.4 watts

To get the same 6.3 watts of power from a 1.5 volt NiMh C cell I need a 4.2amp driver:
1.2 volts * 4.5amps = 5.4 watts

If I use a 85% efficient driver

5.4w / .85 = 6.35w

Run time for my 2900 Redilast would be: 12.18wh / 6.35w = 1.92 hours
Run time for my 6000mah NiMh cell is: 9wh / 6.35w = 1.41 hours

so it looks like a 2900mah LiCo cell will run my light (most lights?) longer than a 6000mah NiMh C cell. Does this seem correct?

no.
nimh is not 1.5v but 1.2v
li co is 4,2v only hot off the charger,
some li co cells are different from others, depending on manufacturer some might work ok, others might just drop V to 3,6-3.7 under load fast

There are various points of confusion in your calculations there but if we just look at the typical energy storage of the batteries as you did we find the following:

LiCo: 3.6 V x 2.9 Ah = 10.4 Wh
NiMH: 1.2 V x 6.0 Ah = 7.2 Wh

Therefore the LiCo cell has more energy and probably will run most devices longer if they have the same power consumption.

A second point to note is that the single NiMH cell delivers its power at a relatively low voltage, so that makes it less useful for high power devices. For instance, this is why a single AA light usually has a lower turbo mode than a 2AA light. You need the extra voltage of 2AA cells to get maximum power to the LED.

ok, I went back and re-did the calculations to account for the voltage difference. Still, the results are the same. LiCo is much better for a single cell application. I guess that is why single C cell lights don't exist.

By the same calculations, why not use 3 C size NiMh batteries for a larger light. They are not much larger than 2 18650 cells and are much safer in series.

3 NiMh C cells: (1.2V * 6000Ah) * 3 = 21,600Wh
2 LiCo cells: (3.6V * 2900Ah) * 2 = 20,880Wh

to answer your question, not always.
like i said before if you use crappy li ion cells, don't expect anything close to what they claim, plus voltage might drop so much that you wont get half of theoretical output\runtime.
use AW cells those are of good quality, and rated capacity is what they really have. they are also protected.
so assuming you use good quality cells, yes li ion will have more energy. thus run longer, they are also lighter than nimh

tre said:

ok, I went back and re-did the calculations to account for the voltage difference. Still, the results are the same. LiCo is much better for a single cell application. I guess that is why single C cell lights don't exist.

By the same calculations, why not use 3 C size NiMh batteries for a larger light. They are not much larger than 2 18650 cells and are much safer in series.

3 NiMh C cells: (1.2V * 6000Ah) * 3 = 21,600Wh
2 LiCo cells: (3.6V * 2900Ah) * 2 = 20,880Wh

Click to expand...

Just going by the numbers, the energy storage of the two batteries is indeed similar, but design is more complicated than that.

Your calculations in the first post are a bit confused and muddled, but I don't have time to go into the details right now. Maybe someone else will pop up with more info.