On 14/09/2017 21:28, NY wrote:
"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.

Sorry, but trying to get the "right" straight line from three points is
pretty meaningless. Batteries are intrinsically non-linear. You can
(sort of) rely on an exponential for radioactive decay when you are
starting with maybe 10^20 atoms. Batteries are more complicated because
you have competing processes.