"newshound" wrote in message
o.uk...
On 14/09/2017 20:14, T i m wrote:
Now, it's obviously not a straight line (Peukert's law), but can you
extrapolate a graph (or create a formula that would be more useful for
my project) from just 3 points please?

p.s. The nearest *I* could get to an answer would be some graph paper
and a Flexicurve. ;-)


Probably no simple answer because it all depends on the shape of the
curve.

I'm not familiar with that particular "law" (I use the term advisedly) but
exponentials are buggers to deal with (especially when, as in this case,
it is obviously only an approximation: exponentials are fine for
radioactive decay, but it will certainly have limits in this case).

I think my approach would be to try to collect some data for your specific
battery, and try to work with that. Quite possibly with a flexicurve, or
with some sort of polynomial fit if there was more data available.

I'd say the best approach would be to find a transformation (eg y=log(x) or
y=sqrt(x)) which gives a good, well-correlated straight line. Then
extrapolate that and do an inverse transformation (eg antilog or x-squared)
on the predicted value. Obviously the more data points you have, the better
prediction you can make and the better you can construct a least-squares
regression line for extrapolation and then back-transformation.