There are in the market some battery adapters (for example - at Star
Batteriws) which suppose to give you the possibility to replace C and
D size batteries with regular AA batteries. When using rechargeable
batteries,this should save expenses since AA rechargeable batteries
are much cheeper then C or D.
The question: Is this a good subtitue, and disadvantages in performance or others?

dealeng-ga:

The answer to your question depends on the application. To explain
this, let us look at rechargeable NiMH batteries.

When the designer of an electronic device has to select a battery, the
choice is influenced by several key factors:

- the desired size and weight of the device
- the power requirements of the device (voltage and current)
- the intended user

For a device that is meant to be used by the average consumer, the
designer will attempt to select a battery that is readily available in
the retail market (batteries such as the AAA, AA, C, D, and 9V sizes
fit this), yet has enough power capacity to allow the device to be
used as intended for a reasonble amount of time.

The power capacity of a battery is rated in Ampere-hours. That is a
measure of how much current, and for how long, the battery can deliver
before it is drained. The rated voltage of a battery depends upon the
chemical composition of the battery, but the power capacity depends
mostly on the size of the battery (a certain amount also depends on
the purity of the chemical composition). Therefore, the more power is
needed, the larger a battery is needed.

The battery adapters allow a smaller battery to be used where a larger
battery is required. This however does not change the laws of physics.
The smaller battery can only deliver as much power as it can hold,
which will always be less than the larger battery.

In the example of NiMH batteries, you can refer to the following pages
at Star Batteries for an idea of the difference in capacity ratings.

http://www.starbatteries.com/cniba.html
http://www.starbatteries.com/nimhbatteries.html

On the first page, yo can see that the 'C' size NiMH batteries have a
capacity rating of 4500 mAh (milliamp-hours), and the 'D' size is
rated at 8500 mAh. When you refer to the second page, you will see
that the best-rated AA is only 2200 mAh. As they are all nominally
1.2VDC batteries, all else being equal the AA battery will only last
just under 1/2 as long as the 'C' cell, and about 1/4 as long as the
'D' cell.

This brings us back to the need to understand the application. If, for
example, you are using AA batteries in D-size adapters in a
flashlight, the flashlight will only last 1/4 of the length of time
you could achieve with real 'D' batteries. In the case of a flashlight
that is only used for a couple of minutes at a time every week or so,
that may not be a problem. However, if you are depending on the
flashlight for search and rescue activities, that wouldn't be very
acceptable.

There are other differences as well, to do with self-drainage
characteristics, maximum discharge rates, and other properties.
However, for your question, the most important consideration is the
difference in capacity. The real driver for the decision in whether or
not you should purchase and use the adapters is an economical one. Can
you afford to have your device run for only half or a quarter as long
as it would using the actual battery size it was designed for? If you
don't mind having to change and charge the batteries more often, then
go right ahead and use the adapters. It will save you the expense of
getting a 'C'/'D' cell charger (assuming you don't already have one),
and it will allow you to maximize the use of your rechargeable 'AA'
batteries (assuming you already have other devices that use 'AA'
batteries).

Please note that this explanation only applies to comparing
rechargeable batteries. Alkaline batteries have different performance
characteristics. The reason you rarely see a capacity rating on an
alkaline battery is because the life of these batteries is highly
dependent on the discharge rate. Alkaline batteries die very quickly
under high discharge conditions, whereas the performance of NiCd,
NiMH, and LiIon batteries tends to be linear.

I hope that this explanation helps you in evaluating whether or not to
use the battery adapters. Please let me know if you require additional
clarification.

Regards,

aht-ga
Google Answers Researcher

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Request for Answer Clarification by dealeng-ga on 20 Dec 2003 01:58 PST

Thanks for this comprehensive answer.
Just one clarification: When you indicate a capacity of 4500 of C size
battery, does this refer to rechargeable or regular alkaline battery,
or is it the same. In other words: a machine/toy usually designed to
use alkaline battery. if the capacity of C alkaline battery is smaller
then 4500, the difference with the 2200 capacity of the AA
rechargeable battery will be smaller.

Thanks

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Clarification of Answer by aht-ga on 20 Dec 2003 12:05 PST

The values I mentioned as examples are all for NiMH batteries.
Generally, if you are able to find the capacity value for an alkaline
battery (the manufacturers tend not to provide this value due to the
non-linear discharge reason I mentioned above), you will find that it
is slightly higher than the rating for the like-sized NiMH battery.
For example, you can now buy "high capacity" alkaline batteries that
have been "specially formulated for today's high-drain electronics",
with some AA ones having 2850 mAh capacity.

However, it is not possible to simply compare the performance on this
one value alone, because of the difference in discharge
characteristics between the NiMH and alkaline batteries.

To directly answer your question, though, the capacity of an alkaline
'C' battery will be greater than the capacity of a NiMH 'C' battery.
Whether or not the difference between the performance of an alkaline
'C' and a NiMH 'AA' (using an adapter) will be greater or lesser than
the difference between the NiMH C and NiMH AA, will depend on the
actual device and the rate at which it drains the battery.

Generally speaking, NiMH lasts longer than alkaline (when looking at
the same size) in an electronic device. Therefore, you can assume that
the performance of the device, relative to the battery used, will
increase along the scale:

Alkaline AA (adapter) < NiMH AA (adapter) < Alkaline C < NiMH C

I hope that this addresses your clarification request.

Regards,

aht-ga
Google Answers Researcher

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Clarification of Answer by aht-ga on 20 Dec 2003 12:18 PST

As well, to address the point that ldavinci-ga mentions in the
comments below, it is indeed the case that the chemical formulation of
a rechargeable battery makes it so that its rated output voltage is
less than 1.5 VDC. He is also correct in pointing out the minimum
voltage required by many semiconductors. What's more, most toys or
consumer devices that are designed for a single 'C' or 'D' battery are
NOT based on electronics of any kind, being instead simple electrical
circuits (flashing lights, motors, etc.) which are highly dependent on
the voltage. In those type of devices, alkaline is best.

It is important to note that the majority of modern electronics is
designed to operate at less than the rated voltage of the alkaline
batteries that would be used, primarily to ensure that your user
experience remains satisfactory even as the alkalines begin to
discharge and their output voltage begins to drop. The voltage-drain
curve for alkaline batteries has a gradual but noticeable slope,
whereas NiCd and NiMH batteries tend to have a severe drop-off,
holding their voltage level until they are almost drained.

However, this takes us into a whole new area of details and science,
well beyond the original scope of your question about battery
adapters!

Regards,

aht-ga
Google Answers Researcher

Hi dealeng-ga,
Actually one of the performance differences dealeng-ga did not elaborate
much is the "rated voltage" of the batteries.  There is a little but
definite difference in the rated voltages when substituting the alkalines
with the rechargeable ones. An alkaline cell is capable of producing 
sustained 1.5v whereas a rechargeable could hardly produce 1.2 volts.  
This becomes a big issue when you are using multiple cells to derive
the final voltage and the design voltage of the appliance/toy favors
Alkaline. Almost all the semiconductors used in consumer electronic
devices use silicon(CMOS) technology, which needs atleast 1.25 volts for
reliable operation. So, if you are using a single alkaline cell, it is
more likely you cannot substitute with a rechargeable one.  But in case
of multiple cells, the actual minimum design voltage may? be much lesser
that the total voltage supplied, making rechargeables a better choice
if not best. This means that those appliances/toys that are designed 
only for the alkaline ones (the difference being the above) will not 
get any significant benefit by the use of rechargeable ones eventhough
 there is a excess unusable capacity(mAh).  Also when using Ni-Cd batteries
this unused excess capacity will tend to increase the capacity loss due
to memory effect.  You could analyze this even further by looking at the
differences of internal resistance(their change with the supply voltage),
capacity reduction at full loads, internal cell polarization,
self discharge/charging characteristics etc.
Regards
ldavinci-ga