Hi Jinx,

Ok, so you made me go an look it up ;-)

Have a look at:

http://www.shodor.org/unchem/advanced/redox/index.html

This is not a reference about batteries, but about electrochemical
reactions in general, so there could be something specific to batteries
that I don't know about, but I don't see how it could be all that much
different from other redox reactions.

If you look about halfway down the page you come to the Nernst equation,
which tells you how to compute the potential in an electrochemical reaction.

When you look at an electrochemical series (table of half reactions and
their potentials), it is important to notice that some assumptions are
made. The table is given under "standard conditions", which is a specific
temperature and concentration (1 molar). You must use the Nernst equation
to determine what the potential would be in your particular situation.
However, the dependence on concentration is logarithmic so for a wide range
of practical concentrations, you get roughly the same voltage. Note,
though, that the temperature dependence is linear, which is part of why (I
think) it affects batteries so much (although this is absolute temp not
Celsius). The Nernst equation also explains why batteries go down in
voltage as they discharge :-)

It's been a few years since I last took a chem class and there is part of
this that I don't remember, though, and that is what role "n" plays in this
equation. This web does not really explain what it is. In one spot they
call it number of moles of electrons transferred, and in another place they
call it number of moles _being_ transferred, almost as if it were a rate.

Sean

At 09:42 PM 3/13/2003 +1300, you wrote:
> > Also, IIRC, you cannot state that a particular chemistry gives
> > exactly 3V, for example, and you cannot do anything about it
>
>My opinion is that a battery based on the simple electro-
>chemical series does have a fixed voltage. The potential
>produced between two dissimilar metals/conductors, eg
>lead and sulphuric acid, will always be the same. Changing
>the concentrations will not alter the potential of the individual
>electrons but it will alter their number (cf voltage vs current,
>water through small hoses / big hoses)
>
>However, there are now a lot more complex chemistries than
>good old accumulators that may have secondary or even
>tertiary reactions after the initial redox
>
>Quite an interesting read
>
>http://www.wppltd.demon.co.uk/WPP/Batteries/Chemistry/chemistry.html
>
>Back to Alex - 6V isn't terribly terribly out of spec and it's below
>max rating - give it a go
>
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