# Chevy Volt: SOC curve?



## Vanquizor (Nov 17, 2009)

Best guess:

Full: 4.15 
Chg-pt: 4.05
Disch-pt:3.45
Empty: 3.0


You can probably pull back the full and empty to be nice to your battery.

Guess based on battery testing here:

http://energy.gov/sites/prod/files/2015/02/f19/batteryVolt4313.pdf


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## halestorm (Apr 28, 2009)

Very nice, thanks for that. I showed that report to Elithion just now and they said that the curve looks like their standard setting for standard Li-Ion, but yes, I could pull numbers from that plot to fine-tune it, with Ah discharged scaled 0-100% SOC and the voltage scaled by the number of cells. And yes, I will come in a bit from the full and empty extremes to be nicer to the batteries. I think tonight I'll use DataThief to pull values from that plot. Thanks for your help!


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## Yabert (Feb 7, 2010)

I've done a test on my Volt battery, but only at slow discharge rate (1/8C).
Still give a good idea.

Personally, I charge 4,10v and never discharge below 3,50v.


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## halestorm (Apr 28, 2009)

How do you get that data, Yabert?


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## Karter2 (Nov 17, 2011)

Those data points (voltages) will change significantly depending on the discharge load.......And the number of cycles on the pack !
.....or are you looking for resting voltage at those different SOC points ?
Would it be difficult to test one of your own cells to get an accurate plot .?
A $50 logging RC charger could do that easily.


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## Yabert (Feb 7, 2010)

halestorm said:


> How do you get that data, Yabert?


One icharger 1010b+ with data logging.


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## halestorm (Apr 28, 2009)

Ok, here is what I ended up doing. I extended the line on the plot that Vanquizor linked to, upwards to 4.15v, called that 100% SOC. Where the other end is at a minimum I called that 0% SOC, then scaled linearly. Then I drew the lines according to the procedure on Elithion's page that I linked to, at least where I think they would go. That produced the attached figure. From that I get:

SOC 100% Vfull = 4.15v
SOC 99% Vchg-pt = 4.07v
SOC 8% Vdisch-pt = 3.44v
SOC 0% Vempty = 3.00v

Incidentally, to be a little more conservative in the protection of the batteries, I set the charge/discharge limits as:

Vcell-max =4.05v
Vcell-high =3.95v
Vcell-low =3.45v
Vcell-min =3.10v

In other words, I never charge to more than 4.05 or let them discharge below 3.10v.

I drove with these setting last night, various terrain, (lots of hills), somewhat spirited, about 1/3 of it on freeway, until I had lost half of the SOC bars on the Elithion gauge. I lost the first bar at 1 mi (that was discouraging). Then I lost the second at 6 mi, then 15, 21, and 30 miles. When I got home at 32 miles I connected a computer and the SOC read 60%, cell voltages were 3.75v (min/max/avg).

My theoretical range would be 19.44kWh/300Wh/mi=65mi, so maybe the gauge is not too far off.


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## Karter2 (Nov 17, 2011)

How are you ...and the gauge....allowing for the difference between resting voltage level and loaded (running) voltage level ?


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## halestorm (Apr 28, 2009)

Karter2 said:


> How are you ...and the gauge....allowing for the difference between resting voltage level and loaded (running) voltage level ?


I'm not doing anything other than allowing the Elithion Pro BMS to do its magic. The gauge doesn't do anything, it is just a display. It connects to the Elithion Pro master via CAN bus, which transmits a few status bits and the calculated SOC. All I know is what is on their web page, which I linked earlier. They state,

_ SOC is calculated only from the battery current in the center of its range, but with voltage translation at the 2 ends: full and empty 

The BMS estimates the SOC based on points taken from the voltage vs. SOC curve for the cells used. 

The BMS estimates the SOC using 2 different methods:
• Voltage translation, along the 2 straight lines at the 2 ends
• Current integration (Coulomb counting), in the middle section_​
So presumably when it is calculating the integrals it is using the present voltage, which is whatever it is, resting or under load. In any case, it's an estimate, and that's all I need, a reasonable estimate, and protection against over/under charging. I would like to know how good the estimate is, but I'm not sure how to do that without running the battery down.


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## Tomdb (Jan 28, 2013)

If its a "good" bms. It will intergrate all the current, in and out. 
However this intergral will be reset once certain voltages are reached, so the steap slopes at full and empty.

With a good current sensor this is the best way to go. 

I do not know of the Elithion also measures temperature and state of health. The state of health usauly gets determined by capacity reduction and increase sag for a given current in the middle section of the discharge curve.


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## axex12 (May 26, 2015)

I have done some single cell tests from 2014 Volt pack.
Cell Spec:
Max Charge = 4.2V 
Max Discharge = 3.0V 

I ended up overcharging a bit and cut of the load before 3V. Here are the graphs, planning to do more stress testing in the future!


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## halestorm (Apr 28, 2009)

15Ah is a long way from the nominal 45Ah. Am I missing something?


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## Vanquizor (Nov 17, 2009)

the 45ah is actually a parallel group of 3 15ah cells. It looks like the poster separated 1 cell from the group.


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## halestorm (Apr 28, 2009)

Yes, that makes sense; and if I'd read his post,



axex12 said:


> I have done some single cell tests


then I would have seen that. Thanks,


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