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SoC% false accuracy? State of Charge
#21
(03-15-2019, 12:17 AM)completelycharged Wrote: Issue with typical joule / coulomb approach is it's your battery cycle efficiency that limits the accuracy unless you reset the counter each charge cycle [...]

Li-ion has very high coulombic efficiency (typically over 99.5%) so that's not really the worst problem with coulomb counting fuel gauges. Rather, it's correcting for losses not measurable by the gauge, e.g. self-discharge (esp. if it's idle for long periods) and also losses due to loads too small to be measured by the gauge (e.g. BMS quiescent current), which may require a full learning cycle to recalibrate (in the simplest gauges), and such full cycles may occur only rarely (which can be alleviated to some degree by using various methods of voltage modelling). See this Barsukov book for much more on fuel gauges.
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#22
Yes but i would rather do it when fullt charged ir 100%
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#23
^^^ Not clear what the prior post is replying to, possibly post #20?
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#24
Agree the most practical method to keep the SoC meter close is for it to reset at full charge like daromer says.
The problem of knowing when your battery bank dropped capacity a bit from use/aging, etc has to be found with a full cycle & adjusting the coulomb counters AHr value.
Running off solar, DIY & electronics fan :-)
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#25
Honestly - I don't pay any attention to SOC but instead, focus on inverter on/off voltages and the power/voltage-range produced each day.    The current battery voltage is perfectly adequate (for me) to have a spit-ball notion of where things are in the charge/discharge curve.
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#26
But the voltage during load is hard to counter how much you actually have left. Proper SOC is alot easier Smile Especially on LiFe where its impossible to read charge-level based on voltage during run-time

I never look at voltage during normal behaviour. Only when doing a full run below 10% soc because thats where my system start to drift.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#27
(08-25-2020, 03:10 PM)daromer Wrote: But the voltage during load is hard to counter how much you actually have left. 
Ah - but remember that I'm nearing 2 years of detailed monitoring and things are stable - so voltage is a perfectly accurate 'enough' predictor for me.    One of the things I'm tracking is ah/v - hoping that this will be an early indicator of battery bank degradation.

But yes, when you don't have a long history or stable system or are just interested in SOC - I can see where its an interesting metric.
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#28
Yes but the voltage differ due to current. You need to have an equation based on current/volt to be able to do it but yes on LiIon that you test you can basically do it but its not as accurate as Colloumb counter is.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#29
So it sounds like OffGrid has a "lesser tech" but functioning method, a sort of lookup table requiring a detailed understanding of your batteries & load currents, etc.
If that works for you, great!
The coulomb counter methods are more hands off & automatic + sound easier to use (mine is!)
Whatever works, but know your SoC!
Running off solar, DIY & electronics fan :-)
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#30
Yea - I think I'm missing the focus on the usefulness of SOC - honestly interested in understanding more about this if you'll try to help me.  Maybe I am but I don't 'name it' that way? Smile

My PV is 12.85kw and battery bank is 80kwh.  To operate, I twiddle with the charge/discharge voltage range to 1) consume all PV power produced and 2) to extend battery life.    Right now - battery cut-off is 49.5v (3.54v/cell)  and float is 56.6v(4.04v).    On average, I only reach 3.90v/cell as the daytime consumption is high enough I don't hit 4.0v/cell max very often.   

This cycle repeats each day - and I track stats daily such as total PV power produced,  total run time of inverters, and kwh(s) of output from the inverters, ah/v, and some others.   This gives me daily, monthly, yearly stats such as:

For the year so far....
* Have produced 13,023 kwh of PV 
* Have consumed 10,741 kwh of power from the inverters.
* This is 82.5% efficiency ( inverter-kwh / pv-kwh )        **Needs tweaking to reach 85% which is doable
* Average hi battery voltage = 3.90    (low is always 49.5v)
* Average DOD is ~ 46% ...                                         **This is an SOC affiliated number**

I'm interested in DOD but SOC as a number doesn't figure into daily operational focus...     What I focus on is max pack voltage differences (are they healthy),  low/hi battery voltage range for 100% PV consumption and long battery life.

What am I missing about SOC focus?
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