You now seem able to buy AA NIMH batteries with 2500 mAH capacities but similar branded C size Nimh batteries have only 4500 mAH. Given that the C battery is 4 times the volume of the AA something seems wrong here. Anyone seen NIMH C batterys with much more than 4500 mAH capacity.
Interestingly, the best Energizer NiMH C cells are only 2500mAh rated, same as their AA cells (@ 500mA CC discharge to 1V). Compare the data sheets of both and the discharge curves are identical up to and including 2C.
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The only spec difference is internal resistance and impedance, which are both lower in the C cell of course.
** Firstly - the volume ratio from AA to C is close to 3:1.
It would be rare to find a C cell with 3 times the capacity of the BEST available AA of the same chemistry class.
So there is nothing " wrong ".
The explanation lies in market forces:
AA cells would have to be easily the highest selling size by FAR - as a result it is also the part of the market where there is the fiercest competition for top performance ( Eg - Duracell have been fighting Energizer over it for decades with saturation TV marketing ).
No similar thing exists with C or D sizes - so cell makers have much less motive to go to the same efforts with them.
Another example is with "sub C" size Ni-Cd cells that way outperform similar C size cells - cos "sub C " is what the RC hobbies use.
On Thu, 09 Oct 2008 02:12:58 GMT, snipped-for-privacy@tpg.com.au (Mauried) put finger to keyboard and composed:
On a slight tangent, I was curious as to how other cell chemistries stack up in mAh per unit volume.
Here are CRxxyy lithium 3V coin cells:
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xx = diameter in mm yy = thickness in 10ths of mm
I suspect that their mAh rating is proportional to their internal thickness, ie after allowing for the metal casing, and assuming that the dimensions of the electrodes can be ignored.
So if we take the biggest and smallest of the 20mm cells, we have ...
CR2032 - 220mAh CR2012 - 50mAh
Let d be the thickness of the casing.
Then (3.2-d)/(1.2-d) = 220/50
... which gives ...
d = 0.612mm
So one would expect that a CR2016 battery would be rated at ...
(1.6-d)/(1.2-d) x 50 = 84mAh (actual = 90mAh)
... and a CR2025 would be rated at ...
(2.5-d)/(1.2-d) x 50 = 161mAh (actual = 170mAh)
As for diameter, comparing a CR2016 (90mAh) with a CR1616 (55mAh), we get ...
rating of CR1616 = rating of CR2016 x (15.4 / 19.4) ^2 = 57mAh
... which tallies quite well.
I then checked out the NiMH rechargeable specs at ...
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part number Size mAh Diam Height
--------------------------------------------- H12-850mAh AAA 850 10.5 44.5 H15-2000mAh AA 2000 14.5 50.5 H35-2500mAh C 2500 26.2 50.0 H50-2500mAh D 2500 34.2 61.5
If we calculate the expected capacity of an AA battery when compared with an AAA, using only external dimensions, we get ...
850 x (50.5/44.5) x (14.5/10.5)^2 = 1840mAh
After allowing for 0.6mm casing thickness, we get ...
850 x (49.9/43.9) x (13.9/9.9)^2 = 1905mAh
This makes sense, but the C and D sizes are a mystery. It seems to me that, with no change to the chemistry at all, the manufacturer could easily fit three AA cells inside a D package, resulting in a 6000mAh capacity. But then the cost would be 3x ...
In fact I see that WES have tagged 6500mAh NiMH batteries (code RB536) at $36.50 each.
Maybe the C and D rechargeables are just AA cells in C and D packages, for cost reasons.
- Franc Zabkar
--
Please remove one \'i\' from my address when replying by email.
Assuming that casing thickness is the same for all cells, and assuming that the only difference between the C and D cells is the casing, and ignoring the weight of the electrodes, then a difference in the surface area of 3251 sq mm corresponds to a weight difference of 7g. This is 0.00215 g/mm2.
Based on this figure, I've deducted the weight of the casing, computed the net weight of electrolyte, and added it to the table.
Assuming that the mAh rating is proportional to the weight of electrolyte, then comparing the AA battery with the AAA we have ...
rating of AA battery = 850 x 24.3/9.5 = 2174 mAh
This still makes sense, but the C and D cells appear to have about twice as much electrolyte as the AA, or maybe some inert substance is adding to the weight to make it feel like the "correct" weight.
- Franc Zabkar
--
Please remove one \'i\' from my address when replying by email.
The bottom line seems to be that AAs are the most cost effective NIMH batterys around, and even for appliances that need C or D its better to somehow fit AAs in them.
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