Capacities of C size NIMH batteries compared to AA.

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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.


Re: Capacities of C size NIMH batteries compared to AA.
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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.
http://data.energizer.com/PDFs/nh15-2500.pdf
http://data.energizer.com/PDFs/nh35-2500.pdf
The only spec difference is internal resistance and impedance, which
are both lower in the C cell of course.

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Claim of 6000 here:
http://www.batteryjunction.com/tpeh-t6000.html
but no data sheet to back it up.

Dave.

Re: Capacities of C size NIMH batteries compared to AA.

"Mauried"
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** 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.



.....   Phil







Re: Capacities of C size NIMH batteries compared to AA.

"Phil Allison"

** Correction:


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......   Phil



Re: Capacities of C size NIMH batteries compared to AA.

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Is the SA of the electrode(s) more important than the volume - which is the
limiting or critical factor the electrolyte volume or the electrode(s) SA?



Re: Capacities of C size NIMH batteries compared to AA.

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The dimensions of the NIMH batteries that Jaycar sell which are the
ones Ive been comparing capacities of are

AA 13.5 mm by 50 mm
C 26 mm by 50 mm
Thats pretty close to 4:1 , not 3:1


Re: Capacities of C size NIMH batteries compared to AA.
Gday All,

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Ok Volume of a cylinder = Pi*r*r*l

So if L and Pi are the same the difference is r*r (of C size) / r*r (AA)
size

Therefore 13x13 / 6.75x6.75 = ratio of 3.71:1 or just under 4:1 ratio..

Sorry for the school stuff but it was 35 years ago since I did that so I
was interested if I still had it in me.


Cu

Mik

Re: Capacities of C size NIMH batteries compared to AA.
On Thu, 09 Oct 2008 02:12:58 GMT, snipped-for-privacy@tpg.com.au (Mauried) put
finger to keyboard and composed:

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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:
http://www.maxell.co.jp/e/products/industrial/battery/cr/index.html

 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 ...
http://data.energizer.com /

 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.

Re: Capacities of C size NIMH batteries compared to AA.
On Fri, 10 Oct 2008 10:30:54 +1100, Franc Zabkar

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The weight may be the clue.

I've now added weight and surface area (SA) to the table.

 part number    Size    mAh   Diam   Height Weight   SA    Net Wgt
------------------------------------------------------------------
 H12-850mAh     AAA     850   10.5    44.5    13     1641     9.5
 H15-2000mAh     AA    2000   14.5    50.5    30     2631    24.3
 H35-2500mAh      C    2500   26.2    50.0    66     5194    54.8
 H50-2500mAh      D    2500   34.2    61.5    73     8445    54.8

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.

Re: Capacities of C size NIMH batteries compared to AA.
On Fri, 10 Oct 2008 11:46:34 +1100, Franc Zabkar

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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.


Re: Capacities of C size NIMH batteries compared to AA.
On Fri, 10 Oct 2008 04:30:48 GMT, snipped-for-privacy@tpg.com.au (Mauried) put
finger to keyboard and composed:

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You can use an 'AA to C' or 'AA to D' type battery converter:
http://www.batterylogic.co.uk/battery-converters.asp

Or, in the case of a D cell, you could bundle 3 tagged AAs and solder
the 3 tags together at both ends.

BTW, this is what an AA NiMH battery looks like in cross section:
http://data.energizer.com/PDFs/nickelmetalhydride_XSection.pdf
http://web.archive.org/web/20030314120132/http://data.energizer.com/batteryinfo/cross_sections_for_manuals/csnimh.pdf

I haven't been able to find a similar view for other sizes, though.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

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