Bill,
I have done this study as well. The temperature of LiFePO4 cells is very stable
under load. It barely rises at all until the cell is 95% drained then the
temperature rises rapidly.
I have this data as well.
From: Bill Dube via EV <ev@lists.evdl.org>
To: paul dove via EV <ev@lists.evdl.org>
Sent: Tuesday, August 8, 2017 3:00 AM
Subject: Re: [EVDL] Curtis 1238-6501 State of Charge
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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