In LFP, temperature influences voltage more than SOC in the region of 20% to 80% SOC. The voltage changes just a tiny amount between 20% and 80% SOC.

Basically, if the voltage changes it is more likely to be due to a change in temperature, than a change in SOC.


Indeed, you could measure the temperature of each cell, along with the voltage, to tease out the SOC, but you need the average internal temperature of the cell, rather than the external temperature of the cell, and you need a very accurate measure of voltage. It is way simpler and cheaper to count amps.


On 8/7/2017 2:31 PM, 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



       From: Willie via EV <ev@lists.evdl.org>
  To: Matthew Quitter via EV <ev@lists.evdl.org>
  Sent: Sunday, August 6, 2017 9:13 PM
  Subject: Re: [EVDL] Curtis 1238-6501 State of Charge

On 08/06/2017 07:38 PM, Matthew Quitter via EV wrote:
Hi Guys,

My 1238-6501 Curtis controller gives very different SOC % for similar
voltages.

For example I've recorded a pack voltage of 82.2 and had the Curtis show
22%, 31%, 43%.

The pack is made up of 25 Thundersky 160Ah batteries.
With LFP cells, you can't accurately infer SOC from voltages in the
middle of the range.  The voltage/SOC curve is too flat.
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