Days Since Parms met

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Dnnap
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Re: Days Since Parms met

Post by Dnnap »

@raysun, excellent information! thank you!

This forum helped out a ton. I spent months reading on this forum and other online information pertaining to OB systems prior to receiving my Flexpower One.
This is my second system I've setup and I'm glad I've gone with OB this time. The first offgrid system I setup, which was for my brother was a 3.8kw 48v Magnum system and researching that system was nowhere as easy as OB mainly due to this forum.
Thanks again guys, much appreciated!
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Re: Days Since Parms met

Post by gtarolli »

I haven't seen CPM in about 4 days, even though I completed Absorb cycles on those days. So I raised my Return Amps from 32 to 40 (6 batteries, 8 is the recommended value). First I am not convinced that Return Amps measures net, IMO it seems to measure Shunt B (PV output). In my system there's usually a 10-20 amp difference (500w to 1000w) difference as those are my normal loads around noon time. Today when Absorb ended after 6 mins at 56.4v I was still generating a lot, probably 20-40 amps, can't remember exactly. Absorb began when corrected net amps was -18 or so, and ended about 6-8 amps later, so I think the batteries were still a few percent from truly full. Hence , they were still absorbing too many amps for Return Amps to go below the recommended value of 24 (4x6) or even 32.

I started bulk again, and within a minute the voltage was back at 56.4. Net amps dropped from 30 to 28 to 10 to 8 over a few minutes, another few amps were absorbed, so my corrected net amps was probably about -8. I say probably because when net amps reached 0, SOC resets to 100% (float mode), and then corrected net amps resets to 0 also. Now I truly believe my batteries are full ("jamming it to the stops"). So I am raising my Charge Factor from 94% to 96% as the corrected net amps after 2 Absorb cycles was still about -6 amps. I am almost thinking I should raise Absorb time to 12 mins and then I bet the Return Amps would be very close to the recommended value (2% of your battery bank).

The second reason I set Return Amps high is because I don't see any real harm in it, even though it is higher than the recommendation. I do not use auto charge termination control, so it doesn't really matter to me if I hit it or not. If I did have it enabled, and CPM was never reached then you still complete the absorb cycles. If CPM is reached because you set Return Amps too high, then you might exit Absorb mode early. But think about it - if you only raise Return Amps above the recommendation because you keep completing Absorb w/o hitting CPM, what harm does it do increase Return Amps so that you hopefully hit it at the end of Absorb (and exit absorb). Obviously if you hit it too early, then you don't properly go through an Absorb Cycle.

Anyway, that's my 2 cents.

p.s. I do like CPM being triggered after or at the end of Absorb, because then I can tell by looking at it if I really filled by batteries. Else I have to check the minutes of Absorb time. And it just seems nice to trigger CPM when you truly believe your batteries are full.
raysun
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Re: Days Since Parms met

Post by raysun »

I take the Net AH readings with a pound of sea salt i collect from the cliffs by Honuapo. Rarely does Corrected Net Battery align with my CPM values. The FNDC always hits 100% first, and anywhere from 5 to 60 AH can be wiped off the registers. Mostly, thankfully, its closer to 5AH, which represents about 1.25% of C20 capacity. My FNDC seems to track consistently 1% high per day.

I don't know, or really care, exactly what the FNDC is tracking during charge, but expect it to be the AH charge current "discounted" by the battery Charge Efficiency parameter, which in my case is 98%. I could fiddle with Charge Efficiency, but to what end? Just to juke the numbers so the charge value coincides sometime with CPM? It might work, or it might not, but either way, I consider it a low-value exercise.

Meeting CPM is my metric for battery full charge. With my LiFePO4 battery I could set a clock by charge termination, its that concise. (BTW, forgive me for continuing to harp on this, but charging to 56.4V does not include an Absorb phase, rather Absorb
is the spot Charge Termination values are entered. 56.4V Absorb Voltage is actually the Charge Termination voltage. The "Absorb Time" of 0.1H is simply the minimum that can be entered and still have charging work. If 0.0H is entered in Absort Time, the chargers skip Bulk and proceed directly to Float.)

When my battery reaches 56.4V, the charging current starts declining rapidly. If left at 56.4V for the full 0.1H Absorb Time, charge current will drop to 0A. The specified Charged Return Amps for my battery = 9A. If the Charged Time = 1 Minute counter starts at 9A the charge current likewise drops to 0A before time expires. I don't care for that drastic a termination current, so played with the Charged Return Amps numbers. I found that 24A with Charged Time = 1 Minute results in 9A charge current at the end of the minute.

If, in the future, I find the charging amps diffee from 9A at the end of 1 Minute at 56.4V then I will take that as a clue to investigate something amiss, or simply degrading as batteries are want to do.
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Re: Days Since Parms met

Post by gtarolli »

raysun wrote: Sat Feb 06, 2021 6:25 pm I don't know, or really care, exactly what the FNDC is tracking during charge, but expect it to be the AH charge current "discounted" by the battery Charge Efficiency parameter, which in my case is 98%. I could fiddle with Charge Efficiency, but to what end? Just to juke the numbers so the charge value coincides sometime with CPM? It might work, or it might not, but either way, I consider it a low-value exercise.
There is one reason you want the Charge Factor to be reasonably accurate for your system - and that is if you look at, or use, SOC for anything and you don't reach CPM for a number of days. SOC will depend on Charge Factor. If Charge Factor is too high, then SOC will be too high, and vica-versa.
gtarolli
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- 1 Honda EM5000 generator + one EU3000
Location: Wainiha, Kauai, HI

Re: Days Since Parms met

Post by gtarolli »

raysun wrote: Sat Feb 06, 2021 6:25 pm Meeting CPM is my metric for battery full charge. With my LiFePO4 battery I could set a clock by charge termination, its that concise. (BTW, forgive me for continuing to harp on this, but charging to 56.4V does not include an Absorb phase, rather Absorb is the spot Charge Termination values are entered. 56.4V Absorb Voltage is actually the Charge Termination voltage. The "Absorb Time" of 0.1H is simply the minimum that can be entered and still have charging work. If 0.0H is entered in Absort Time, the chargers skip Bulk and proceed directly to Float.)

When my battery reaches 56.4V, the charging current starts declining rapidly. If left at 56.4V for the full 0.1H Absorb Time, charge current will drop to 0A. The specified Charged Return Amps for my battery = 9A. If the Charged Time = 1 Minute counter starts at 9A the charge current likewise drops to 0A before time expires. I don't care for that drastic a termination current, so played with the Charged Return Amps numbers. I found that 24A with Charged Time = 1 Minute results in 9A charge current at the end of the minute.
I agree CPM is the best metric for full charge. I am not sure what you mean by what I underlined. The Absorb counter begins at 56.4v so while getting there doesn't include a complete phase, usually a system can hold that for 6 minutes so you complete Absorb and go to Float. Not that Absorb is really an important phase for Li , but rather it is a way of terminating the Bulk/Absorb charging. I think what you mean is the Absorb voltage (or .4v less) satisfies one condition of CPM, is that correct?

My system does quickly drop charging amps after reaching 56.4v, but not as quickly as yours. It takes more than 6 minutes to get below the recommended return amps of 8 (2% of 400 ah). Maybe my problem is I never really jammed my batteries full (recently) and so if I keep terminating the charging after 6 minutes of absorb time and I'm still a few percent short of 100%, and that accumulates over time, then that might explain my 16% loss? I am going to do some experiments with CPM return amps set to 8 and a slightly long absorb time to see if I can "jam them full". I only changed from the 55v absorb profile to 56.4 a week or so ago - maybe that's why my charging amps don't drop as fast as yours? I'm hoping its something like that. Although I would think both systems would reach 56.4v with about the same % short of SOC and if there's enough sun then both would receive the same charging amps (especially since its around noon, and the charging amps drops rapidly on both systems, just not as fast on mine).
raysun
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Re: Days Since Parms met

Post by raysun »

My Charge Factor setting @98% is as precise as it gets under practical terms. Its the Phi specification for a new battery, and if the FNDC hasn't drifted (the Peukerts Constant value "hard wired" in the FNDC matches actual consumption rate) then charging to 100% matches CPM exactly. I tried it once, cycling the battery to 80% SoC from full charge at a rate I calculated to emulated a 1.15 Peukerts curve rather than the 1.05 lithium constant, then immediately recharging. Both numbers concided exactly, 100% SoC and CPM met at the same point in time.

The Charge Factor, AFIK, only impacts SoC during charging.

In my usual case, the pre-set Peukerts Constant in the FNDC skews SoC somewhat during discharge (about 1% / day). Thd Victron BMC700, with programable Peukerts Constant, tracks more precisely, IME. While its a classic trap to assign causation to correlation, it's a likely source of error.

Note that on my new battery, when 56.4V is reached, the current decline starts almost immediately. I've watched the charging hit that voltage with a charge current exceeding 80A, and have it decline to 24A within 2 minutes. At that point, the decline to 9A takes 60 seconds.

I guess we could banty semantics back and forth about whether that represents an Absorb phase, or is simply the mechanics of the cells charging in Bulk until full, but the important takeaway is it is an unambiguous demarcation and indicator that charge parameters have been met.
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Re: Days Since Parms met

Post by gtarolli »

I would argue that this really is an absorb phase, because Bulk is maximum current (to a limit), but Absorb is constant voltage so the current drops as you get closer to full. I'm not saying its like FLA absorb, only that it throttles back the charging to keep the voltage constant (if possible).
raysun
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Re: Days Since Parms met

Post by raysun »

gtarolli wrote: Sun Feb 07, 2021 5:52 pm I would argue that this really is an absorb phase, because Bulk is maximum current (to a limit), but Absorb is constant voltage so the current drops as you get closer to full. I'm not saying its like FLA absorb, only that it throttles back the charging to keep the voltage constant (if possible).
Fair enough. They do specify a constant voltage to an end amp value, so that's Absorb.
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Re: Days Since Parms met

Post by Mike Curran »

Re: Peukert's Law and non-traditional acid batteries (Raysun, you're confusing me)

Chemistry was never my strong subject and this Peukert stuff is way over my head, so I Googled Peukert's Law and among others things found this:
Effect of Battery Chemistry
Peukert's law was developed for Lead-Acid batteries, and works well in that application.

It does not necessarily apply to other battery chemistries, especially Lithium-Ion batteries. Lithium-Ion batteries tend to self-heat during rapid discharge, and the Nernst Equation predicts battery voltage will increase with temperature. Thus, the effect of increased resistance is offset by the self-heating effect. This advantage of Lithium-Ion batteries is a well-known advertised feature, see [1]. In a research paper, a 50Ah lithium-ion battery tested was found to give about the same capacity at 5A and 50A; this was attributed to possible Peukert loss in capacity being countered by the increase in capacity due to the 30◦C temperature rise due to self-heating, with the conclusion that the Peukert equation is not applicable.[5]
FWIW
https://smart.tigoenergy.com/p/pZXn7SZQyO45?date=2021-04-19
https://enlighten.enphaseenergy.com/public/systems/Hctc107221
raysun
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Re: Days Since Parms met

Post by raysun »

In practical terms, Peukerts Law is an equation with a constant that varies with the different battery chemistries.

For flooded lead acid: 1.15
For gel: 1.25
For lithium 1.05

The FNDC uses Peukerts in part to calculate SoC during discharge. The calculation uses 1.15 as the constant term.

I've seen that argument quoted before. I can't prove or disprove the hypothesis but it cites experimental results I've not seen duplicated anywhere else. Most of the discharge curves I've seen for lithium batteries show a marked difference in discharge voltage with a 10X difference on discharge current, and I would assume thier differing termination points show different capacities.

Here's another look at the question: https://batteryuniversity.com/learn/art ... ristics_li
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Re: Days Since Parms met

Post by gtarolli »

I posted my trials and tribulations before, where I had to increase return amps to something really high to get CPM to be met. Going on the theory this is because my batteries simply needed a bit more at that voltage to finally get full, I did some experiments. Note this is all after I reset my charging profile to use the 56.4v absorb setting for my six Simpliphi 3.4 batteries. Like everyone else I noticed the charging amps goes down dramatically during absorb, but not enough to trigger CPM after 6 minutes if I set return amps to 8 (the recommended value). So on a few days, I restarted Bulk once or twice more and then CPM got triggered when the net return amps < 8 for a minute. I used to think it was using gross PV amps, but now I believe it is using net, i.e. PV amps minus Load amps. Anyway, I increased my absorb time to 18 mins, which on most days will cause CPM to trigger, and noticed that after a few days it takes less time to trigger CPM. I enabled auto-charge-termination (I had it disabled before), and so can see how long it takes in absorb to trigger CPM. Sometimes 8-12 minutes sometimes closer to 16. I am going to let this run for a few weeks and see if that helps my perceived loss in storage (I feel like I lost 15% in the last year, my batteries are 3 years old). And if the time to trigger CPM slowly goes down. I figure if you have auto-charge termination enabled, its not that bad to set absorb time to 12 mins (18 is a bit long), so that if it takes say 7-8 mins to trigger CPM that will occur, and then absorb ends. That is if you seldom take the full 12 mins to terminate absorb, it doesn't seem that harmful to set it to 12 mins.

Anyway the sun is getting higher and stronger here, so I should trigger CPM on most days, and in another week or two I will report back.
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