On Fri, 15 Sep 2017 10:46:28 +0100, "NY" wrote:

"newshound" wrote in message
...
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.

Yes, I never suggested that the data would be linear. I'd be highly
surprised if it was anything like linear. But if you can find a way of
transforming the non-linear data so it becomes linear, then you can work out
the best-fit line and predict future (transformed) values, from which you
can back-transform to get the corresponding real value. Easier to do it that
way that to try to fit a flexicurve to a curved line so you can predict an
out-of-range value (extrapolating) as opposed to predicting an in-range
value (interpolating).

So, as you sound like you know what you are talking about, can you
plug some real values in and see what we get please?

I'm happy to answer any questions re numbers and points etc.

I know the off load starting voltage is 13.10 volts.
I know the voltage that represents 50% (and other percentages) for 3
different currents.

We have the formula that should make of it all but I don't have the
understanding of how to take what we have and make use of it ITRW (if
it's possible)?


Cheers, T i m