On 19/09/2017 19:02, T i m wrote:
Reference 'C' gives:
Current, Time to fully discharge:
21A, 1h (i.e. C/1)
11.9A, 2h (i.e. C/2)
5.40A, 5h (i.e. C/5)
3.10A, 10h (i.e. C/10)
1.60A, 20h (i.e. C/20)
0.36A, 100h (i.e. C/100)

The trick is to work out what varies most smoothly with load.

If you work out the battery capacity in Ah and then fit that against
current I you get a reasonable quadratic approximation of capacity

Capacity (Ah) = 35.3 - 1.56*I + 0.04*I^2

It is always going to be a bit rough and ready since individual
batteries will vary from the abuse they have suffered in service.

A linear model would underestimate the low current lifetime

Capacity (Ah) = 33.2 - 0.66I

Datasheet batteries always seem to last longer than real world ones!

You should be able to reproduce these models by putting the numbers into
Excel and plotting I*t against I.

--
Regards,
Martin Brown