Purchasing new batteries: How do you determine what one 'cycle' is?

[max2k]

Probably unnecessary but the 1.002 sampling time is to counter aliasing with the 100/120 Hz current draw from any mains inverter. I use the Linux time.clock call which gives me current processor time down to 1 uSec.

come to think about this I actually missed that: inverter loading battery bank at say 10A current would produce 'rectified' AC wave on the shunt with corresponding amplitude. Of course inverter has some big capacitors to compensate for that so it would probably remind waveform from common rectifier bridge after its capacitor filter. Measuring that at 0.998 Hz sampling rate would transfer that waveform into much lower frequency (around 1/8Hz for 60Hz AC) preserving its amplitude. This would also require the sampling rate to be very stable and as I said Linux or any other non- RT OS is not capable of that. Basically for you it's a dead end if you want to trust your numbers. I hope you connected scope to the shunt and verified the waveform there is only DC or this whole setup would be subject to this effect.

Classically speaking aliasing can only be removed by analog filter at the ADC input with cutoff frequency at least Fs / 10.This would mean you'd run ADC at say 1000 samples/sec and have LPF with cutoff frequency of 100Hz in front of it. That would produce digitized samples of whatever LFP passed which you'd average over say 1 sec to get average DC value or you could calculate RMS from that if you want to be more accurate.

Max, I procrastinated for a while as to whether I should bother answering your post. Maybe I am thin skinned or there are cultural differences between us but I am getting a little tired of what I perceive are the nitpicking and the constant attacks from you. I am happy to answer questions about my setup and my experience and debate the pros and cons of my system. I am not happy when I think you make statements based on incorrect assumptions about my offgrid system and how I operate it and then go on the attack when I point out the errors in your assumptions.

I don't know your background but you seem to be knowledgeable about measuring systems and other things electronic. I would appreciate it if you would credit me and others with some level of competence until proven otherwise.

I'm sorry if I'm coming across as attacking, I never meant that. I simply question methods you used to obtain your results, trying to improve quality of this board. If you outlined those methods in more details I'd have much less questions. I always assume the worst until proven otherwise . You claimed at least 10mV resolution on your graphs and I feel free to challenge that as from my experience that is not easy to get. You'd get 'some' numbers but they won't necessarily mean anything. Then you build some alarm logic around this and things can go wrong.

[karrak]

that is odd sample time to have, wonder why not exactly 1s? Actually Linux or Windows cannot reliably measure time interval below 10mS resolution basically your last digit and the one after that are bogus, you could just as happily write there 1.01 or 0.99 with the same relevance to reality. Unless of course you utilized hi resolution timer present in practically all x86 family processors which is simply 64 bit hardware counter with 100nS LSB and measured your time taking readings off that timer. OTOH this timing error would introduce +-1% error to your SOC, which is probably acceptable.

Probably unnecessary but the 1.002 sampling time is to counter aliasing with the 100/120 Hz current draw from any mains inverter. I use the Linux time.clock call which gives me current processor time down to 1 uSec.

I might be splitting hairs here but each of what you're calling 'calibration point' are actually data inputs requiring 2 individual calibration coefficients each- since you're using individual resistor dividers for each of them and unless you used 0.1% resistors and didn't make mistakes in the circuit in terms of accounting for input resistance I'd run calibration for each of them. I'm paranoid that way- more than once such 'obvious' things turned out to be less than obvious and led to discovery of a problem I was not aware of. What I was referring to is having 2 points in each of 16 values (and current) far enough apart (50 - 80% scale) and measure that value using your circuit and some well calibrated DMM to calculate those linear approximation coefficients, the common approach.

If you read my design notes you will find that I do use 0.1% resistors and use the same divider resistor values for all the individual cell voltage inputs. The cell voltage range of interest is 2.8V-3.6V which corresponds to less than ~22% of the total voltage span. There are no non linear components in the circuitry between the battery and the A to D, just three resistors and a capacitor so any linearity errors will be generated within the A to D. These are well documented in the A to D datasheet.

I thought we established the voltage is poor indicator of SOC for LFP?

It is a reasonably good indicator when the SOC is below 8% which is below the bottom knee.

From my point of view you don't really know your actual capacity, all you know (minus possible calibration problems) is how much Ah max it can store by taking difference between max SOC and min SOC measured in Ah. These 2 points should belong to the same cycle and not the lowest possible SOC off your graph- your whole system can drift down over multiple cycles so unless you counting Ah in the same cycle I wouldn't assume I'm getting actual capacity looking at the lowest SOC. What I'm saying one day you might see your SOC indicator at 40% and your bottom voltage alarm going off indicating you have only 60% of rated capacity left.

I think your point of view is wrong. I have already said that I compared the lowest SOC measured with my BMS with the SOC calculated using battery voltage and they agree within a few %. On top of this I know that my SOC counter accuracy only drifts by a few %. The reason I know is by looking at the SOC reading each time the SOC counter is reset. If the counter accuracy didn't drift the counter would always read 100% when the counter was reset. A reading other than 100% will be caused by errors in the current measurements and the coulomb inefficiency of the battery. I use this error to dynamically calculate and update an error coefficient that is used to adjust the SOC reading to take into account the coulomb inefficiency of the battery.

Max, I procrastinated for a while as to whether I should bother answering your post. Maybe I am thin skinned or there are cultural differences between us but I am getting a little tired of what I perceive are the nitpicking and the constant attacks from you. I am happy to answer questions about my setup and my experience and debate the pros and cons of my system. I am not happy when I think you make statements based on incorrect assumptions about my offgrid system and how I operate it and then go on the attack when I point out the errors in your assumptions.

I don't know your background but you seem to be knowledgeable about measuring systems and other things electronic. I would appreciate it if you would credit me and others with some level of competence until proven otherwise.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

No, sample time is 1.002 sec and specified in the config file. I use the measured time between samples to calculate the SOC.

that is odd sample time to have, wonder why not exactly 1s? Actually Linux or Windows cannot reliably measure time interval below 10mS resolution basically your last digit and the one after that are bogus, you could just as happily write there 1.01 or 0.99 with the same relevance to reality. Unless of course you utilized hi resolution timer present in practically all x86 family processors which is simply 64 bit hardware counter with 100nS LSB and measured your time taking readings off that timer. OTOH this timing error would introduce +-1% error to your SOC, which is probably acceptable.

No, for voltage I have as many calibration points as there are cells in the battery. For current I rely on the accuracy of the shunt and the linearity of the TI 16 bit AtoD for the span accuracy and measure the offset.

I might be splitting hairs here but each of what you're calling 'calibration point' are actually data inputs requiring 2 individual calibration coefficients each- since you're using individual resistor dividers for each of them and unless you used 0.1% resistors and didn't make mistakes in the circuit in terms of accounting for input resistance I'd run calibration for each of them. I'm paranoid that way- more than once such 'obvious' things turned out to be less than obvious and led to discovery of a problem I was not aware of. What I was referring to is having 2 points in each of 16 values (and current) far enough apart (50 - 80% scale) and measure that value using your circuit and some well calibrated DMM to calculate those linear approximation coefficients, the common approach.

I calculated the SOC of the lowest point on that graph using cell voltage and got a discrepancy of ~3% between the logged SOC reading and the calculated reading using voltage. I am happy with that.

I thought we established the voltage is poor indicator of SOC for LFP?

You are right on how I reset my SOC counter. I calculate and take into account the coulomb inefficiency of the battery, I also know within a few % the actual capacity of the battery so how could my SOC reading be so far out to read 95% when it is in fact 60%?

From my point of view you don't really know your actual capacity, all you know (minus possible calibration problems) is how much Ah max it can store by taking difference between max SOC and min SOC measured in Ah. These 2 points should belong to the same cycle and not the lowest possible SOC off your graph- your whole system can drift down over multiple cycles so unless you counting Ah in the same cycle I wouldn't assume I'm getting actual capacity looking at the lowest SOC. What I'm saying one day you might see your SOC indicator at 40% and your bottom voltage alarm going off indicating you have only 60% of rated capacity left.

[karrak]

if I understand correctly your logger takes current readings off the shunt every 10 sec. I haven't checked the python code but I assume you simply multiply current reading by 10s to get Ah for SOC purposes.

No, sample time is 1.002 sec and specified in the config file. I use the measured time between samples to calculate the SOC.

Your calibration procedure looks a little off but probably acceptable depending how close to 0 one runs it: instead of taking 2 pairs of points and then solving linear approximation coefficients you calculate slope using single pair (assuming 0 offset) and then offset using another. Near 0 offset could be comparable to the value and this would produce incorrect coefficients rendering the rest of the data invalid.

No, for voltage I have as many calibration points as there are cells in the battery. For current I rely on the accuracy of the shunt and the linearity of the TI 16 bit AtoD for the span accuracy and measure the offset.

Have you had a chance to measure capacity of your bank in Ah after 4 years of use? I mean logger would record 'relative' Ah but you don't really know what percent of the initial capacity the bank has until you get it through entire voltage swing from low knee to the top and counting Ah along the way.

I have never measured the capacity of my battery. In the SOC graph I used the manufacturer's rated cell capacity of 90Ah to calculate the SOC. I am more interested in the trend in capacity loss over time rather than absolute figures. Remember my battery is in use 100% of the time. I calculated the SOC of the lowest point on that graph using cell voltage and got a discrepancy of ~3% between the logged SOC reading and the calculated reading using voltage. I am happy with that.

I can see the max SOC is pegged at 100% and I assume this was done by detecting upper voltage knee, stopping charge at that point and 'resetting' SOC to 100% at that point. If this was not done such systems have tendency to accumulate errors over time as with each cycle some part of energy is lost. Assuming this was done correctly resetting SOC to 100% provides only 1 point of reference- when battery discharges it is impossible to tell % of SOC knowing just Ah passed without knowing the actual capacity at that point so your 60% could mean 95% of the actual remaining capacity. Reaching low voltage knee would allow setting SOC to 0% and total Ah would be equal to the actual remaining capacity at that moment.

You are right on how I reset my SOC counter. I calculate and take into account the coulomb inefficiency of the battery, I also know within a few % the actual capacity of the battery so how could my SOC reading be so far out to read 95% when it is in fact 60%?

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

Have a look here https://github.com/simat/BatteryMonitor/wiki to see how the data is logged

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

if I understand correctly your logger takes current readings off the shunt every 10 sec. I haven't checked the python code but I assume you simply multiply current reading by 10s to get Ah for SOC purposes.

Your calibration procedure looks a little off but probably acceptable depending how close to 0 one runs it: instead of taking 2 pairs of points and then solving linear approximation coefficients you calculate slope using single pair (assuming 0 offset) and then offset using another. Near 0 offset could be comparable to the value and this would produce incorrect coefficients rendering the rest of the data invalid.

Have you had a chance to measure capacity of your bank in Ah after 4 years of use? I mean logger would record 'relative' Ah but you don't really know what percent of the initial capacity the bank has until you get it through entire voltage swing from low knee to the top and counting Ah along the way.

On your graph SOC is expressed in % - is it relatively to the initial rated capacity? I can see the max SOC is pegged at 100% and I assume this was done by detecting upper voltage knee, stopping charge at that point and 'resetting' SOC to 100% at that point. If this was not done such systems have tendency to accumulate errors over time as with each cycle some part of energy is lost. Assuming this was done correctly resetting SOC to 100% provides only 1 point of reference- when battery discharges it is impossible to tell % of SOC knowing just Ah passed without knowing the actual capacity at that point so your 60% could mean 95% of the actual remaining capacity. Reaching low voltage knee would allow setting SOC to 0% and total Ah would be equal to the actual remaining capacity at that moment.

[karrak]

OK, can you provide more details on how these data were measured? I was under impression you didn't have SOC 'counter' in your system but such graph would require one.

Have a look here https://github.com/simat/BatteryMonitor/wiki to see how the data is logged

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

Thought I would do a graph of the daily maximum and minimum SOC of my LFP battery over the past year
...

OK, can you provide more details on how these data were measured? I was under impression you didn't have SOC 'counter' in your system but such graph would require one.

I also thought, apparently wrong, that you don't discharge your bank deeply at least not as often as these graphs demonstrate.

[karrak]

thank you for the linked thread and the link to the research paper in that thread as it actually contains the data which are close to what I was after- cycle life of LFP batteries depending on various conditions. There's still problem though as the paper used 26650 cylindrical cells and not prismatic ones usually used in off grid systems. So just for the reference purposes their graph of cycle life dependency from DOD indicates that the cells they used could be expected to have 80% capacity after 2000 cycles at 30% DOD and C/2 rates: LFP_DOD_cycle_life.png

This is far cry from what is usually claimed about LFP cycle life of prismatic cells something along the lines of '6 years with 20% DOD and no capacity loss' or Winston showing 5000 cycles at 70% DOD: http://en.winston-battery.com/index....ategory_id=176

I agree it would be nice to have some independent experimental reports on prismatics made by Winston, GBS, and CALB but I haven't managed to find anything. The results in this report I would think would still be indicative of any results obtained on any prismatics.

I don't think you can compare lifespan tests done at a temperature of 60^oC to lifespan tests done at the "normal temperature" that the Winston tests were done at. The following graph from the same paper shows there only being a ~7% drop in capacity after ~6500Ah being cycled through the 2.2Ah batteries which is ~4200 cycles at 70% at a temperature of 15^oC.
CapacityFadeTemp.jpg

This inconsistency and lack of equivalent data for prismatic cells from independent sources doesn't make the case for LFP any better.

IMO what does make a good case for LFP batteries in off-grid systems is all the real world data from numerous people running prismatic LFP batteries over a period of up to seven years that implies that in standard off-grid system they should last well in excess of ten years assuming there is no unforeseen mechanical, electrical or chemical breakdown inside the batteries.

There's also mismatch between this graph and the test matrix numbers at the beginning of the article by a factor of at least 2- for 60 C and 20% DOD graph stops at 5000 with 65% C remaining while matrix shows 9800 cycles and still going. They defined End of Life for their experiment as: "failing to deliver the required capacity before reaching a cell voltage of 2.0 V" but never stated what is 'required capacity' is as it is clearly not 80% of initial C and such definition would actually prevent them from plotting fading capacity graph in the first place:

My interpretation Is that they are only using the results from the first 5000 cycles of the ~9800 cycles that they performed. I agree that the 'failing to deliver the required capacity' definition is somewhat vague, but I don't think it makes any difference to the results.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

Right but I couldn't find any cycle life field data when someone used LFP cells in that mode for several years (>2). Karrak & RV folks never exercised their banks that deep and IMO didn't even see true SOC values of their banks due to flat curves and shallow cycles. They were pretty much hovering around upper knee.

Thought I would do a graph of the daily maximum and minimum SOC of my LFP battery over the past year
SOCoverYear.jpg

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

For off-grid systems, both stationary and in RVs etc. you have to have reserve storage if you do not have a generator or do not want to use it excessively to tide you over cloudy days. For LFP batteries a good rule of thumb is having the battery capacity equal to two to three times daily consumption (Sunking's guideline is three days). This means that on average you will be only cycling less that 50% of the batteries capacity on a daily basis. You also want to keep the battery as full as possible to have as much reserve for cloudy weather. That is what "matching to task" is for off-grid systems.
...

good point, you're correct any battery in off grid system will not be very deep cycled on daily basis due to autonomy requirement.

thank you for the linked thread and the link to the research paper in that thread as it actually contains the data which are close to what I was after- cycle life of LFP batteries depending on various conditions. There's still problem though as the paper used 26650 cylindrical cells and not prismatic ones usually used in off grid systems. So just for the reference purposes their graph of cycle life dependency from DOD indicates that the cells they used could be expected to have 80% capacity after 2000 cycles at 30% DOD and C/2 rates: LFP_DOD_cycle_life.png




This is far cry from what is usually claimed about LFP cycle life of prismatic cells something along the lines of '6 years with 20% DOD and no capacity loss' or Winston showing 5000 cycles at 70% DOD: http://en.winston-battery.com/index....ategory_id=176

This inconsistency and lack of equivalent data for prismatic cells from independent sources doesn't make the case for LFP any better. There's also mismatch between this graph and the test matrix numbers at the beginning of the article by a factor of at least 2- for 60 C and 20% DOD graph stops at 5000 with 65% C remaining while matrix shows 9800 cycles and still going. They defined End of Life for their experiment as: "failing to deliver the required capacity before reaching a cell voltage of 2.0 V" but never stated what is 'required capacity' is as it is clearly not 80% of initial C and such definition would actually prevent them from plotting fading capacity graph in the first place:
LFP_Test_Matrix.png

[karrak]

There were few members from RV crowd passing by here who were claiming 4 years cycle life without losing capacity. On closer consideration it turned out they had greatly oversized banks and were not using LFPs to the full potential due to limitations in their BMS/charging systems so they were cycling them in 95%->75% SOC range over those years and only few times over that period down to 20%-30% or so. BTW, this seems to be working for them and I have no problem with that.

Karrak as you might remember uses his house bank in the same fashion and it also seems to be working for him. My only problem with all this is they somehow forget to mention their use pattern as pre- requisite to what they're actually doing and fog discussion with BMS details, balancing, etc while in reality none of that matters as the use pattern would forgive any screw ups in there or cells shortcomings for that matter.

I haven't seen one coming by who would use smaller bank (matching to the task) and use the cells in 95%-20% SOC range daily taking advantage of claimed LFP properties. That might change their cycle life considerably. OTOH on EV forums where this pattern is prevalent as they are constrained in space later posts don't seem to be against LFP but still I haven't seen hard evidence of long cycle life (>500) under 'EV pattern' of use.

For off-grid systems, both stationary and in RVs etc. you have to have reserve storage if you do not have a generator or do not want to use it excessively to tide you over cloudy days. For LFP batteries a good rule of thumb is having the battery capacity equal to two to three times daily consumption (Sunking's guideline is three days). This means that on average you will be only cycling less that 50% of the batteries capacity on a daily basis. You also want to keep the battery as full as possible to have as much reserve for cloudy weather. That is what "matching to task" is for off-grid systems.

At low charge and discharge rates as found in off-grid systems the lifespan of LFP batteries is predominantly dependent on the amount of energy cycled through them. Cycle at 20% and the battery will last five times as many cycles as cycling at 100%. See this post and this thread for more information about this.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

I think you will find that most if not all serious users of LFP batteries are well aware that you cannot use voltage to determine SOC.

Due to the fact that LFP batteries have such a good and consistent overall coulomb/current efficiency (around 99.5%) you can very accurately measure the SOC of an LFP battery with a shunt. Again most serious LFP battery users will use shunt based SOC counters so they can plan their energy and generator usage given the prevailing weather conditions.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

Well first I apologise if I offended you, was not my intention because I know you are smarter than that.

No offense taken, I know where are you coming from and I really don't see anything offensive in what you posted in response.

... So the DIY EV guys including myself look for salvaged Chevy and Nissan Leaf battery sin salvage yards to harvest the battery. My litle racing golf cart uses a Nissan Leaf battery of 144 volts @ 66 AH. Ever been in a golf cart that does 0 - 70 mph in 4 seconds?

that is truly impressive- assuming constant acceleration it would require only 62 yards distance to reach 70mph.

I hope you're not doing this in a parking lot as some young lad who tried to prove his Porsche could reach 100 km/h in a parking lot. Well, it did but he missed small detail that Porsche also needs distance to stop after that .

Anyway back to LFP SOC. As you said it is difficult to determine SOC if SOC is greater than 10% and less than 90%. However it is no where near flat below 10 or above 90%. Those knees you see at the both ends are extremely easy to detect.

Right but I couldn't find any cycle life field data when someone used LFP cells in that mode for several years (>2). Karrak & RV folks never exercised their banks that deep and IMO didn't even see true SOC values of their banks due to flat curves and shallow cycles. They were pretty much hovering around upper knee.

They are called Delta Voltage Bumps. Working voltage on a LFP is 3.2 to 3.4 volts which is very compatible with Pb battery equipment so you can use off the shelf equipment without a lot of modifications. Most all you have to change is your mindset and battery management. Example LVD setting would be 12, 24, or 48 volt cut-off eliminates over discharge.

A DIY can mimic what commercial EV do. Never ever fully charge your batteries, rarely ever balance the cells., and never ever let the voltage get near fully discharged. Fully discharged is 8 volts on a 12 volt battery, and your LVD is set to 12 volts a heck of a long way from 8, 16, and 42 volts. To chargee is even easier with LFP. A that is required is a Float Charger. No stages, just a simple regulated Float Charger.

I see, basically if one foregoes interest in knowing 'how much exactly left' all that is needed are 2 points: top and bottom. If the house system is sized properly knowing exact SOC is mostly for entertainment purposes. EV of course is different story.

[Sunking]

Karrak as you might remember

Yep I remember, but I quit drinking and smoking dope decades ago. Will not touch that chit today. Make you stupid and stupid's name is Karrak.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

Well first I apologise if I offended you, was not my intention because I know you are smarter than that. You have a better understanding than most, but still have some catching up to do. You are correct, LFP cells are difficult to judge SOC because as you said the charge/discharge curve is very flat from 10 to 90% SOC. Take notice that only applies to LFP cells, and no commercial EV I know of would use LFP cells. If you stop and think about it becomes very clear why they do not use them. It would be impossible to get any meaningful range out of them. LFP cells have roughly half the Specific Energy (Wh/Kg) and Energy Density (Wh/L). That 900 pound battery Tesla uses would have to weigh 1800 pounds and be twice as large in volume. I know you are smart enough to understand that will not work worth a damn. Some DIY EV guys still use LFP batteries and they have to use Chi-Coms like CALB, Winston, Sinopoly, and GBS because they are cheap of 1/4 the price of quality LFP cells made by A123 or LG. However that trend has come to an end as their are now 10's of thousands of used EV' sitting in dealers and salvage yards returned from lease or wrecked. No one in their right mind would buy a used EV because they know the battery needs replaced. So the DIY EV guys including myself look for salvaged Chevy and Nissan Leaf battery sin salvage yards to harvest the battery. My litle racing golf cart uses a Nissan Leaf battery of 144 volts @ 66 AH. Ever been in a golf cart that does 0 - 70 mph in 4 seconds? My little 25 hp electric motor produces 110 ft-lbs of torque from 0 to 6000 RPM, and from 6000 to 1000 RPM is flat HP of 100 peak HP. With a 6:1 ratio direct drive gives me almost 650 ft-lbs of torque at the drive wheels. Not may commercial gas cars can do that.

Anyway back to LFP SOC. As you said it is difficult to determine SOC if SOC is greater than 10% and less than 90%. However it is no where near flat below 10 or above 90%. Those knees you see at the both ends are extremely easy to detect. They are called Delta Voltage Bumps. Working voltage on a LFP is 3.2 to 3.4 volts which is very compatible with Pb battery equipment so you can use off the shelf equipment without a lot of modifications. Most all you have to change is your mindset and battery management. Example LVD setting would be 12, 24, or 48 volt cut-off eliminates over discharge.

A DIY can mimic what commercial EV do. Never ever fully charge your batteries, rarely ever balance the cells., and never ever let the voltage get near fully discharged. Fully discharged is 8 volts on a 12 volt battery, and your LVD is set to 12 volts a heck of a long way from 8, 16, and 42 volts. To chargee is even easier with LFP. A that is required is a Float Charger. No stages, just a simple regulated Float Charger.

[max2k]

Max, do not fall for those lies. That chart you show for a Chi-Com battery went bankrupt and had to change their name. They went bankrupt from warranty claims. They lasted less than 3 years and the company was called Thundersky aka Thunder Turds.

Thundersky split into 3 companies. CALB, Winston, and Sinopoly. Calb is the best of the bunch and is a 2 to 3 year battery at best. Stay away from Winston and Sinopoly.

FWIW assuming you follow standard design protocol, there are 300 cycles in a calendar year.

I'm just trying to be open minded about LFP. From my reading around (not real experience) it sounds the area was under heavy development over last 5-7 years so any data related to LFP needs to be seen from that perspective as well. It also needs to be closely examined on the type of use pattern and type of failure.

There were few members from RV crowd passing by here who were claiming 4 years cycle life without losing capacity. On closer consideration it turned out they had greatly oversized banks and were not using LFPs to the full potential due to limitations in their BMS/charging systems so they were cycling them in 95%->75% SOC range over those years and only few times over that period down to 20%-30% or so. BTW, this seems to be working for them and I have no problem with that.

Karrak as you might remember uses his house bank in the same fashion and it also seems to be working for him. My only problem with all this is they somehow forget to mention their use pattern as pre- requisite to what they're actually doing and fog discussion with BMS details, balancing, etc while in reality none of that matters as the use pattern would forgive any screw ups in there or cells shortcomings for that matter.

I haven't seen one coming by who would use smaller bank (matching to the task) and use the cells in 95%-20% SOC range daily taking advantage of claimed LFP properties. That might change their cycle life considerably. OTOH on EV forums where this pattern is prevalent as they are constrained in space later posts don't seem to be against LFP but still I haven't seen hard evidence of long cycle life (>500) under 'EV pattern' of use.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

[Sunking]

If you really going to charge/discharge them daily- take a look at LFP cells, much better deal in the long run IMO: 20x cycle life + 1.5 times more energy for the same Ah and other stuff I mentioned in your other thread. After 1000 cycles at 0.5C current it will still have 90% original capacity:

Max, do not fall for those lies. That chart you show for a Chi-Com battery went bankrupt and had to change their name. They went bankrupt from warranty claims. They lasted less than 3 years and the company was called Thundersky aka Thunder Turds.

Thundersky split into 3 companies. CALB, Winston, and Sinopoly. Calb is the best of the bunch and is a 2 to 3 year battery at best. Stay away from Winston and Sinopoly.

FWIW assuming you follow standard design protocol, there are 300 cycles in a calendar year.