Originally posted by Sunking
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9 kW solar, 42kWh LFP storage. EV owner since 2012 -
Originally posted by Sunking View PostHate to keep beating you up, but that is complete nonsense.
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Last edited by Ampster; 11-21-2018, 06:11 PM.9 kW solar, 42kWh LFP storage. EV owner since 2012Comment
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Originally posted by Ampster View Post
Yes, you are correct the range is wider but I was referring to the flat part of the Lithium charge curve before the knee. If I recall correctly the Lead Acid discharge curve is more gradual toward the end and the Lithium is steeper. Isn't that why voltage is not as good an indicator of SOC in Lithium vs. Lead Acid? I am just comparing my miniscule experience with LFPs vs FLAs.
MSEE, PEComment
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Originally posted by Sunking View Post
You cannot use voltage as SOC on a working system. To use voltage as SOC requires the battery to be open circuit and rested for 24 hours. Two conditions that you cannot do on a functional system. Who on earth is going to disconnect their batteries and wait 24 hours? See any problem with that?9 kW solar, 42kWh LFP storage. EV owner since 2012Comment
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Originally posted by Sunking View PostHate to keep beating you up, but that is complete nonsense.
The flattest Lithium Ion is LFP (LiFeP04), and in a 4S operation for 12 volts is 10 to 13.8 volts or a spread of 3.8 volts from 100% to 0% SOC.
At 10% SOC, LFP OCV after discharge is roughly 3.20Vpc. At 90% SOC, 3.33Vpc. The slope of the voltage curve over the usable interval is (3.33 - 3.20) / (90 - 10) = ~1.6mV per one percent SOC. In a 4s 12.8V nominal string, quadruple that to 6.4mV per percent SOC.
A 12 volt Pb FLA battery is 12.6 to 11.9 volts or a spread of 0.7 volts.
In the real world, most of us try not to use more than 50% of lead acid, and thus the usable SOC is {50%, 100%}. The corresponding voltages are {1.98, 2.12} per cell. The average slope over that range, then, is (2.12 - 1.98) / (100 - 50) = 2.8mV per percent SOC. In a 6s 12V nominal string, we get 16.8mV per percent SOC.
Thus, the LFP slope is 2.6 times flatter than the lead acid slope over the usable range.
(In the narrower middle range, between say 30 and 70% SOC, the difference is even more.)
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Originally posted by Sunking View Post
You cannot use voltage as SOC on a working system. To use voltage as SOC requires the battery to be open circuit and rested for 24 hours. Two conditions that you cannot do on a functional system. Who on earth is going to disconnect their batteries and wait 24 hours? See any problem with that?
In a similar way, when my LFP pack is in reasonably steady-state discharge (stable at e.g. 500W, 2kW, 4kW, or 6kW), I can look at the voltage and by experience tell you the SOC within 5%. That a computer with a learning algorithm can do a bit better is no surprise.Last edited by nebster; 11-21-2018, 11:26 PM.Comment
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@Nebster,
Thank you. Both of your responses were very helpfully and articulated the information in a very useful manner.
Of course that is what one would hope to get out of a forum like this.
It is unfortunate that some posters feel the need to to be less than civil in their responses.
Happy Thanksgiving to all.Last edited by Ampster; 11-21-2018, 10:39 PM.9 kW solar, 42kWh LFP storage. EV owner since 2012Comment
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Originally posted by nebster View PostThis is also no longer true. There are now state estimators that model recent historical current plus instantaneous voltage to calculate SOC. They take into account the recent load/charge dynamics that affect the voltage now.
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Originally posted by jflorey2 View PostYep. In 2004 I designed a 2S battery pack for a satellite phone that used state estimation. It took voltage, current, current history, temperature and age as inputs and provided SOC as an output. It was accurate to about 10%.
Here is the problem most folks do not understand and how you are nebster are spinning numbers. That 2S pack for the phone discharges at very low rates on the order of C72 to C/100 which for all practical purposes is Open Circuit Voltage. So you you can get a SOC on a battery with extremely light loads, but 10% is horrible. You an Nebster also have not defined SOC. I understand why you do that because you are advocates and need to spin numbers so consumers do not understand.
There is one good way that is within 5% using lithium batteries, and that is Coulomb Counting (amp hours in and out) which is what all EV manufactures use along with resting voltages. You are only telling half the story. I can take any brand new battery, fully charged healthy battery and make it measure 0% SOC. Or I can take a dead battery and make it read 100% fully charged. Both are false readings at 100% error.
Why do you think so many come here and ask why their expensive battery monitors lie to them and give them false readings?
MSEE, PEComment
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Originally posted by nebster View PostNote to readers: this poster cherry-picked his data here. He chose absolute 0% SOC voltage for LFP, but a much more reasonable 40% SOC bottom voltage for a lead acid battery. Lead acid at 0% SOC will measure roughly 10.5V.
Karrak I got news for you. EV's DO NOT USE LFP CELLS.MSEE, PEComment
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Originally posted by Sunking View PostYou are full of chit KARRAK. SOC FYI is OCV. 10.5 volts is under HEAVY LOAD.
Karrak I got news for you. EV's DO NOT USE LFP CELLS.
Note to self, ignore this poster.9 kW solar, 42kWh LFP storage. EV owner since 2012Comment
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Originally posted by Sunking View PostThank you just agreed with me and backed me up. 10% is horrible accuracy. Anything over 5% is unusable.
I do not have one yet, so I cannot personally attest to their performance. But I believe it is possible.
Here is the problem most folks do not understand and how you are nebster are spinning numbers. That 2S pack for the phone discharges at very low rates on the order of C72 to C/100 which for all practical purposes is Open Circuit Voltage.
What matters for voltage-based estimation is that the load (current) is stable enough for long enough that the voltage can begin to approach its stable, asymptotic point at that current.
The new estimators account for this, using the current-based data to refine the coulomb-counted data when the current is reliable enough, but disregarding it when it is not.
There is one good way that is within 5% using lithium batteries, and that is Coulomb Counting (amp hours in and out) which is what all EV manufactures use along with resting voltages.
Particularly for those of us operating large packs at partial SOC for extended periods, the current-integrating approach becomes limited in its usefulness.
You seem to have a lot of experience with small EV and golf cart packs, but not much with residential-scale ESS. The problems overlap, but not entirely.
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Originally posted by nebster View PostYou seem to have a lot of experience with small EV and golf cart packs, but not much with residential-scale ESS. The problems overlap, but not entirely.
MSEE, PEComment
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Originally posted by Sunking View PostHere is the problem most folks do not understand and how you are nebster are spinning numbers. That 2S pack for the phone discharges at very low rates on the order of C72 to C/100 which for all practical purposes is Open Circuit Voltage.
There is one good way that is within 5% using lithium batteries, and that is Coulomb Counting (amp hours in and out) which is what all EV manufactures use along with resting voltages.
Looks like you put your foot in your mouth again.Comment
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Originally posted by Sunking View PostAt last count over 4,000,000 watts and counting in commercial and residential.
But, I strongly suspect that you don't own or manage a lithium chemistry-based residential or mobile ESS. It sounds to me like you've experimented with a few small-scale golf carts. I'd love for you to start a new thread, though, and tell us about your personal setup. That would be a lot more productive than acting like my seven-year-old in this one.
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