Hi Matt,

Voltage is not arbitrary - it is determined by the cell chemistry.
Electrochemical reactions can be described by the Nernst equation.
This basically says that the voltage is equal to a constant plus a
constant times the log of the ratio of the concentration of the
reactants to the products. There are limits on the concentration - too
low and the cell has no capacity - too high and you can no longer make
a solution in water or the metallic parts would wear away before most
of the reactants were consumed. The end result is that there is a
small fully-charged voltage range for each cell chemistry. The range
is so small because of the fact that the voltage is not linearly
dependent on the concentration but only logarithmically dependent on
it. For example, it may be that a factor of 10 change in the
concentration results in only 0.1 volt change in voltage.

Zinc manganese dioxide (aka Alkaline batteries) are about 1.5V and
Nickel Metal Hydride are about 1.2V.

Sean


On Thu, Oct 7, 2010 at 10:25 AM, Matt Rhys-Roberts <matt@nu-ins.com> wrote:
> =A0As per subject. Just wondering if anyone's got more clues than I've
> managed to find so far.
>
> Thanks
> Matt
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