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SoC% false accuracy? State of Charge
#11
Vd varies with SoC - which is why good inverters will have a variable (current dependent) LVD
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#12
not2bme and if you read mine you see I confirmed it Wink Though the voltage drop varies dependant on SOC too Wink
completelycharged likes this post
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#13
The below chart is of a Panasonic CGR18650E. You can see between 80% and 20% it's quite linear. It's not exactly linear but it's good enough. The voltage drop doesn't vary much during the entire SoC range as you have mentioned.

So using my formula, I put it into my Grafana and you can see the SoC chart at the bottom is fairly accurate and smooth, while the battery voltage on top swings as the load and PV load changes during the day.

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#14
For sure, yes the curve on Li-ion (typical 18650 cells) is steeper & easier to read SoC from.
So it seems the question is "can SoC be measured by other than coulomb count methods?".
And it seems that if
- you have Li-ion (not LiFePo4)
- you characterize/measure your battery curve over the range of SoC,
- know the load/charge current influence on voltage
- know the temperature effects on voltage
you can calculate reasonable SoC values like not2bme is doing.
Which is easier/better/more accurate?
Seems like the coulomb method is more direct to say SoC = xyz% right now.
But each method requires calculation for any accuracy.
I guess it's up to each persons preferences & how they build their system :-)
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Running off solar, DIY & electronics fan :-)
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#15
My philosophy is SoC your looking for a rough indication and not really bothering about a 220Wh diffencence when you have "about" 10kWh left. For me the rough lookup on the sheet and seeing if there are any more clouds...

I wonder if most SoC coulomb counters on a 15kWh battery pack have an end SoC accracy at 10% the equivalent of boiling the kettle for a cup of tea... Isn't that what really counts, having enough energy for a fresh brew ? :-)


All cells decrease in voltage through discharge, just depends if your meter detects it..  Do many people really know thier real powerwall voltage profile....


The lookup + load / charge offsets might get more complicated as the volt drop in the cells under load is not linear through the discharge process, so full accuracy needs a matrix to get relatively well accuracte... The rest of the wiring, etc. will see the basic IxR volt drop.

This is my pack, old data though and based on a controlled test on one cell, extrapolated for the pack size. I have some separate test data on the whole pack but it's a little rough (manual sampling) but follows the curve well..



This is a different view for the same pack, with actual figures, load was varying a bit, hence the noise.



This then shows how many kWh you have per incremental volt drop in the pack, This is where what you think is linear is really maybe 10% out in the middle, or more...
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#16
Fiddled around the SoC today and looks like it is working, although I don't have a victron unit to be able to benchmark it against. But at least now I know even though my voltage is at 28.4V (my max) it is still only 76% charged. 

I didn't think knowing my SoC was important but it sure is nice to be able to tell how charged my batteries are!

The SingleStat number does fluctuate by 5% sometimes because my readings are not captured in realtime but in 15 second intervals, so the output isn't actually captured all at the same time. Using the graph I used a mean over 1 minute and that smooths out the blips. Also it was easy to figure out the voltage drop as you bump from 0.2V to 0.6V in 0.1V increments you can see the graph just smoothen out.

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#17
I would say SOC is one of the most important numbers. I dont look at Voltage any more. I look at SoC and the computed "time-left" and 3 years later it havent let me down a single time.
The Ultimate DIY Solar and build place
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
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#18
I have a 48v@780ah 18650 Lith-ion battery bank with Midnight Solar Wizhbang Jr Shunt and Batrium system with Batrium shunt.

Operationally I use voltage  -  e.g.  Inverter/auto-transfer-switch on at 52.0v,  max/float at 56.4v (4.03v/pack),  and Inverter off at 47.5 (3.39v/pack). This allows my battery bank to supply up to 24kWh (60%) of my 40.5kWh battery in conjunction with the PV array and home wiring so I can just barely consume 100% of PV array on most intense days in summer. The goal is to consume every watt of power that I can and not waste any to 'battery full'.

SOC is not important in the short run but I'm looking to use SOC for the long run as the battery degenerates to maintain the 24kWh from the battery as a higher and higher SOC is required due to battery degradation. For example, right now a 60% SOC = 24kWh. I imagine that in a couple of years I'll need 70% SOC then 80% SOC etc... to maintain the 24Kwh per day.

So, I'm recording an SOC metric because over time I hope to detect the battery degradation in terms of SOC and adjust the voltage range to maintain 24kWh per day. 

So far, I'm using voltage difference - highest voltage for the day (from PV array charging) minus the invert off voltage (47.5).  SOC = (HiVolt - LowVolt)/0.1483/100.  

I also have Batrium "Discharge AH" info that I'm recording but...  the PV array can go up, down (discharch AH), and up again and down again (discharg AH) in same day - e.g. multiple up/down - so its not clear to me that Discharge AH is a good SOC metric.

I also have Bartrium SOC info - but I've set it and it seems to reset and I don't understand how it works or what its based on. Maybe you guys know?
 
I also have Midnite Classic SOC - but it resets only if the system reaches max Volt setting (e.g. Float voltage) and does not seem that accurate if you don't hit max voltage quite often.
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#19
It resets on top and bottom voltage. Thats how you callibrate it. Under shunt tab
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#20
(03-20-2019, 05:59 AM)daromer Wrote: It resets on top and bottom voltage. Thats how you callibrate it. Under shunt tab

I don't see a direct 'voltage -> reset' on Shunt page.. but looking at the Batrium Shunt page and reading your comment...   and since I know my low voltage = 47.5v / 28% SOC then...

If I use "Re-Calibrate Low SoC" = ON and set the percent to 28%,   then Batrium will rest to 28% at lowest voltage point each day?
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