Hy Lee,
I've done extensive testing, albeit it's been years ago. I still have some of 
the data.
It doesn't take great accuracy two decimal places will get you to within 5 Ah 
even in the flattest part. 
The hard part is knowing when to take the sample. LiFePO4 cells tend to take 
some time to rise back up to their open circuit voltage.It's also impossible 
under a changing load but with a fixed load they are very accurate at 
reflecting SOC per voltage.
I have some spread sheets I made if you would like to see them.

      From: Lee Hart via EV <ev@lists.evdl.org>
 To: Electric Vehicle Discussion List <ev@lists.evdl.org> 
 Sent: Tuesday, August 8, 2017 12:15 PM
 Subject: Re: [EVDL] Curtis 1238-6501 State of Charge
   
paul dove via EV wrote:
> Sure you can. Voltage is the only way to discern SOC.
> You just can't measure accurately under load. You have to measure open 
> circuit voltage.
> www.mdpi.com/1996-1073/9/11/900/pdf

This paper mainly discusses the higher-voltage Li-Ion cells, which have a 
larger 
voltage change between full and empty.

For the LiFePO3 cells being discussed here, the voltage change is much smaller. 
As Bill says, the change is so small that it can't be reliably measured outside 
of laboratory conditions and precisions instruments. Other factors, such as 
age, 
temperature, manufacturing variations, and the accuracy of the equipment play a 
larger factor.

-- 
Where ignorance is our master, there is no possibility of real peace.
    -- Dalai Lama
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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