displayed SOC around 50%

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Aug 9, 2022
Hello. I have a 2018 VW e-golf 100 KW/35.8 KWh. I am very satisfied, the car is perfect in terms of driving characteristics, comfort and quietness in the cabin. As a techie, I can't sleep on one thing. When I come to work and I have 75% left in the battery after the morning arrival, when I come to the car in the afternoon, i.e. after 9 hours, the car has still 75% and I can see a maximum loss of 100 wh in capacity in OBD11. However, if I charge less, and i hit work in the morning with about SOC 56%, then in the afternoon after getting into the car I see a capacity loss of up to 6%, so SOC is 50%, and in energy it is even almost 1700 wh. The temperature is not much different morning and afternoon. I'm not sure why such a loss, I was wondering if the car is doing some balancing of the cell around 50%?. When I looked at what the cells voltages are at moments after the morning arrival, OBD11 says SOC 56%, the cells voltages are usually lower than when I come in after 8-9 hours, when OBD11 says SOC 50%. Look at attached picture form OBD11. However, if I come to the car after next 4 hours, it will still SOC 50% and the same battery power, voltage of cells, etc. Do you know where the problem might be? Is it some calibration curve, where the car uses a different curve for SOC 60-100% and SOC 0-60% to determine SOC from voltage? Thank you
Strange. I have left my car sitting around 50% for several weeks and saw no change in SoC or pack voltage.

Also, it is weird that the cell voltage goes up when the SoC goes down, and the pack voltage is unchanged.

I don't understand how the SoC can change when the pack voltage is identical at 326.0 V as it is my understanding that the indicated SoC is merely a calculated value based on the pack voltage.

I also believe the energy available is calculated from pack voltage. I do not think there is a direct way the BMS can measure energy content - each cell group's voltage can measured, and the car's efficiency while driving can be measured (BMS can measure energy use), and thus determine pack energy content based on a given pack voltage.

Maybe you need to charge to 100% to calibrate the pack and adjust the SoC calibration? I do not believe any balancing happens outside of a top charge.
First of all, thank you for response. I was a bit worried if this forum is already in a lull and if someone will answer me about such a curious problem.

Yes, It's strange. On the other hand, it bothers me just from the standpoint that it's about numbers. Otherwise, the battery seems to be in good shape, both from driving and from the SOH_P and SOH_Q readings (OBD11), which are 94% which is pretty good for 67k km i guess.

Otherwise, the fact that the voltage went up slightly doesn't surprise me, as those first readings are after the arrival, so the battery was measured immediately after the load. After 9 hours the voltage is slightly higher. Yes I also expect that the car can calculate the SOC from the battery voltage and that's why my head is not bothered why the SOC is lower...

Or couldn't the SOC reduction be due to charging the 12V battery while standing? But why would it be working right around the 50% SOC where I noticed it... and I would expect it to happen more when charging from the wallbox.

Completely independent of the subject of the post, today I charge to 100% SOC, ultra slow, using a charger that charges at 1 kW. That's just in terms of sort of regeneration, balancing the cells.
I think the SOH numbers are for the 12 V battery, not the HV battery.

The best way to determine the HV battery pack energy content is by performing a driving test.

FYI my car now has 44,000 miles on the odometer and on a long highway drive, I calculated a usable 29 kWh from the pack (29/31.5=92% SoH) assuming 31.5 kWh initial usable energy capacity. Also, highway driving tends to extract less energy out of the pack due to heat losses at high speed than does low speed driving.

Unless car is turned on, 12 V battery does not get charged.

Now that you've charged to 100%, you'll see if anything changes.
Based on my data recording of the pack voltage and SoC over the years, I've come to the conclusion that the pack voltage vs SoC curve is non-linear, with a second order polynomial fitting the data pretty well. I've seen that the data at the high and low SoC ends fits the curve better than the data around 50% (since I have more data at the pack SoC extremes), but I believe pack temp influences the curve fit and have data taken at a variety of pack temperatures, accounting for some of the less than perfect fit. Currently, my data suggests pack voltage of 326 V equals 47.5% SoC, so your observed 50% SoC makes a lot more sense than 56% SoC.
Since I tried to determine the current capacity of the battery based on driving, it corresponds to about 29.5-29.7 kwh, which is also the number that shows me OBD11 and if I use your capacity of 31.5 kwh (how did you estimate it?), as a starting point. If I do the math, that's equivalent to 94% SOH. The fact that it's to a 12v battery surprises me, but of course, I don't know, and given your post counts and the fact that you've had those Golfs significantly longer than me, I believe you :).

Otherwise, what is happening to me around 50% I personally attribute to some internal recalculation, when for the SOC parking interval one overpotential curve is used and the other part of the other..., it leads me to it even when the cell voltages do not drop... but it's strange that it happens only to me...

Otherwise, you have 326V voltage and SOC 47.5% ... so it's probably fine... I'll probably also look through the pictures of different SOC, but I don't have the voltage of the whole pack, but the minimum and maximum voltage of one cell... but basically, if I do x 88, I'll have the upper and lower voltage limit of the whole pack...

Do you think you could try this sometime? You know, like, you'd get about 15-20% of the battery out and then you'd stand for like 3-4 hours? so you charge it to 70% so you have about 50% at the point of range?

Thank you so much
Hello, i was ploted data, which i had. Of course, it could be better to have even more data for low end...

SOC vs. energy is quite nice diagonal plot, but even so, there is a slight shift.

But for Cell voltage vs SOC, there are two trends with break in zone SOC 50-60%. This corresponds to my thoughs, that at "afternoon" the SOC is calculated according second curve. But this could be in case, that they dont use some polynomic model for whole SOC range and they use two linear trends. The breakpoint is logical even from charge/discharge curve of NMC electrode material, which have more "flat" discharge characteristic after 25 Ah.

But still is surpricing me, that this dont see anybody other.
Thank you. Ok, thank for suggestion, you mean this protocol?

otherwise it's really for a 12v battery, because today I saw 100%. The traction battery will not be in such perfect condition :).
Today i have quite long trip, not so long i was thinking before start. But i was start from 98.5%, ended at 44.5%, so consumed 54% of battery charge. efficiency was 10.5 kwh/100 km, trip was 158 km. So my calculation is (10.5*158/100)/0.54= 30 722 wh. So from this consuption SOH is 97.5% which i quite optimistic, so i will have to evaluate on the longer trip to be more precise. eGolf have 68k km.
Today i was measuring during driving, just because sometimes it crash i have not long as i would like. It seem that voltage level in relation to SOH is wide spread according actual current load, what i understand. So i start to think, that such drop off SOH is just some correction of SOH in interval around 50%, where maybe calibration curve have some change of the slope.

When i look at the data, from 53% down it is almost not statistically significant trend (probably there is starting typical NMC discharging plato), so hard to estimate exact SOH from voltage. i just have to measure it from 75 down to 35 to see shape of the data.
I suspect you may want to measure voltage when car is not driving. When I use Car Scanner, I do not see this much noise in the data.
Yes, in case that i would like voltage at OCV i will measure it when car stay. But i was interested about discharging curve under load... And there is breakpoint visible to...
My friend just bought egolf 2019 40 kkm. We were interested about in which shape is given piece. I was testing, make diagnose over obd11.
Given egolf2019 is in good shape is my conclusion (egolf of my friend) for mechanical part, battery, etc.

I was curious, if there is phenomena around SOC 50% as at mine egolf. I just made test. I charged to 80% SOC and go to work, with little bit km more to hit discharge to SOC 53. After 3 hours i go to car and SOC was 50 % (voltage of battery at 50 SOC was +few mV higher than at 53 SOC). So behaviour the same like with my egolf (2018). I expect, that it is some typical behaviour which corespond with some change of calibration curves around 50% where NMC have breakpoint. How i wrote before, if i charge to 80% and end my trip at 62ˇ%, when i come back to car i see again 62%. Described phenomena i see at longer drive when i finish my trip below SOC 60%. Experiment with my friends egolf 2019 prove it.
I have seen this too. I don't exactly understand why this happens, but I agree it is related to the BMS calibration, and slight changes in temperature of the pack while the car is sitting. I don't worry about it - my battery pack works just fine.
I did some research and found that there is hysteresis in the SoC vs Voltage curve. Additionally, you need to be sure you are looking at the resting voltage of the pack, not a pack that is actively charging or discharging. Finally, it is very likely that the e-Golf SoC estimation is accomplished by Coulomb counting algorithm, and not based on pack or cell voltage.
Thank you for your research. Now, we have agreed for 3 egolf, that it is normal phenomena. I have not any problem with my battery pack too, only this i was observe. I am wathing at this values when car is staying, without any charging/discharging state.

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