Why is the battery SOC figure so unreliable?

Had another instance last night where the battery SOC figure was completely out of touch with reality. The State Of Charge was still showing 19% when the battery voltage had actually fallen to 44.76V (0%) and a forced charge was initiated. My battery reserve limit and cut-off limit are both set to 4% but were ignored as the SOC was still showing 19% when the battery was actually empty. It seems the only reliable SOC figure is the 0% which is displayed immediately after the battery voltage drops to the predetermined lower voltage limit and initiates a forced recharge. At the upper end of the range my battery continues charging for varying periods after the SOC shows 100% suggesting that the system is again using a predetermined voltage limit (rather than the SOC figure) to decide when the battery is full.

The problem, of course, is that all the user control settings are entered as battery SOC percentages which do not (but perhaps should) have a consistent mapping to battery voltage. What I do not understand is, if the lower and upper battery voltages for 0% and 100% have been pre-determined through calibration and are fixed, why does the mapping of SOC to battery voltage have to float?

Is it not possible to just apply a fixed one to one mapping based upon a standard charge/discharge curve? At least the SOC figures displayed would then be consistent within the predetermined 0% to 100% voltage range. Having the battery SOC figure divorced from voltage reality by a constantly changing amount is making user control of the system difficult to say the least!

How straight forward though is the relationship between battery voltage and SoC? I would have thought it also depends on cell temperature** and whether the battery was under load or charging? (and the cell temperature themselves will be a combination of ambient temperature and discharge or charge rate). I'm no battery chemist, but don't think it's particularly straight forward ?

You may well have far more experience than me of Lithium battery chemistry (I'm more used to lead acid, having used those for years for a small PV system), this was nearest explanation I could find: https://batteryuniversity.com/article/bu-903-how-to-measure-state-of-charge#:~:text=Voltage%20Method,with%20a%20charge%20or%20discharge.

I also run a set of Pylontechs, essentially blind (run off the voltage only to work out SOC) - its a bit of a wild stab in the dark off voltage alone as the curve is fairly flat on Lifepo4 - compounded with the fact that you also get a fair amount droop when you start loading them up before bouncing back at idle.
(I.e I can pull 500W and get more kwh out of them than if i ask for 2000W using voltage as a measure alone)
The BMS in the battery should account for this if it talked with my inverter, so although the batteries are good for further discharge, it tends to throw a wobbly and cut them off a bit too soon on low voltage as it assumes 20% SOC when it's likely nearer 30.
(I knew all this when I got them 2nd hand)
The pylons are stingy on cells so instead of being 16 cells (like GE) they have 15 @ 3.45V so your already about 3V short to most other batteries before you discharge anything at all.

I suspect the givenergy bms employs a coulomb counter alongside voltage. The exact algorithm will I suspect remain a mystery but I'm guessing there is a fair interaction between voltage, energy in/out and also the buffer goalposts (which I assume/hope move to give more capacity as the battery ages) hence why SOC is able to float around like you've experienced?

I suppose the further question is:
When your battery SOC gets stuck, are you still getting the full 8.2kwh or whatever from it or are you getting something less?

Happened to me on Monday (again). Dropped from 20% to zero. Forced recharge off daytime grid (grrr) BUT the recharge stopped at 4% and then waited till 0030 for Go cheap recharge, so not a complete disaster, as I don't think last time it did that, I think the last time (4 weeks ago) it did a full 100% recharge on peak grid. I have the Reserve Limit set to 4% and Cut Off Limit to 5%. The Reserve Limit is a setting I don't understand.
Just sharing this for others to compare outcomes - I wish (and know from Givs bug report) that Giv can fix this......

hoggy Not stuck on this occasion although I have had that in the past. The displayed state of charge has been going up and down happily for weeks. When the battery voltage hit rock bottom last night the displayed state of charge was on a normal gradual downward overnight trajectory but had only dropped as far as 19%. This was the first time the displayed SOC had gone as low as 19% for quite some time however so I am guessing that this figure represented the amount by which the SOC was over reading when the low voltage forced recharge cut in.

GrahamRead Yes I agree with your thoughts and I would not fancy writing the algorithm to do this.

What is making life particularly difficult is the system is not only displaying an unreliable battery state of charge but it is using the SOC figure to action user input settings and so is not always responding at the actual state of charge requested.

My battery regularly discharges before the start of 'octopus go' at 12.30am and the house resorts to the grid for the last 2-3 hours a day.
However my battery SOC when the battery becomes idle has been creeping up over the last 2 weeks by about 2% a day. In the past it would bottom out at 4% but currently it's at 34% and idle and the system is drawing from the grid.
In 'battery data' the voltage is currently 43.44V which I'm assuming is bottomed out or is there a fault with the battery?
Is there a way to reset the system?

mitman you have to calibrate the battery. Which you should be able to do via the portal and the inverter tab. You should see a gear icon which you click on to get into the settings.
But make sure it is discharged first and it will dump everything to the grid if there is no other load. And then it will fully charge.

mitman the battery SoC value is generated at calibration, the actual battery SoC is monitored via voltage.

Over time the two diverge and it ultimately ends up with the battery running out early and this resets the battery calibrated SoC value as this point.

You might see it drop suddenly from around 20% SoC down to 0% in the space of 5 minutes when this happens. It's not a predictable thing and not everyone has reported it happening.

Thanks for the replies. The battery is calibrating now.

sorry to jump into this discussion, new to the battery world and have the same issue with my 8.2. my question is how to i recalibrate in the remote control section? i cant see anything that mentions recalibration, i completely discharged my battery and still showing 75%...

clinton I think a calibration can only be initiated by GE now, I do believe there was an option at one point to do it yourself. How long have you had the battery? and what BMS version software you running?

Not sure which thread it was, but someone mentioned that the capacity, hence range, of EV’s was pretty reliable.
Well, that simply isn’t true.

This is not isolated. It’s not even consistent between the cars display and the app. Not to mention that the vag servers are down regularly.

anglefire

Not come up once on Briskoda as far as I can see.

There are of course plenty of people moaning that displayed range is less than expected after driving like they stole it then expecting the range to be the same.

Range predicted by guessometer is not the SoC though.

kram this is true, but it’s relevant as most people with cars use that to judge the capacity. Though the later version of firmware do show battery % as well.

Also as I guess you know for about a week the vag servers were down whilst they did something- though scant details as to what, apart from it broke the HA integration last weekend after it went down again.

Must admit I don’t look at briskoda much these days, prefer https://www.enyaqforums.co.uk/