Merging Three Systems to Feed an Inverter

Discussion about the FM100, FM80, and FM60 Charge Controllers

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Richard Fairbanks
Forum Member
Posts: 6
Joined: Wed Jul 17, 2013 1:49 pm
My RE system: Three independent systems:
• The original (2007) system, consisting of 800W of Sanyo (now Panasonic) solar panels, going to a (dearly cherished!) MX60, feeding eight, lead-acid Trojan T-105s wired in series/parallel for 12V, and
• Two new (as of 2020.4), identical systems, each consisting of 1.3KW of Panasonic solar panels, going to a new FM80, feeding eight Fullriver AGM DC250-6s wired in series for 48V.
Location: Camped out in the remote mountain wilderness of Utah
Contact:

Merging Three Systems to Feed an Inverter

Post by Richard Fairbanks » Fri Apr 24, 2020 12:32 pm

Greetings, folks!

As noted in my profile, I now have three separate battery banks, each fed by their own set of solar panels and charged by their own OutBack charge controller (one MX60 and two FM80s).

Given the significant disparity between the first system and the two new (identical) systems, I have them set up such that each battery bank is charged and monitored separately by their corresponding OutBack charge controllers (each also has an additional Bogart Engineering TM-2030 meter and the obligatory, accompanying shunt).

My concern is that I also need to be able to use the three banks together to feed an AIMS 3KW, pure sine wave DC —> AC inverter, so I have 2/0 conductors going from the positive pole of each bank of batteries to the positive lead into the inverter, and 2/0 conductors going from the negative pole of each bank of batteries to the negative lead into the inverter.

By wiring the three battery banks in parallel (thus each battery bank functions as a single battery) to go to the inverter, is there any concern that the three OutBack charge controllers might perceive the three battery banks as one battery and not be able to properly measure and charge them independently?

One challenge that has arisen is that now when the MX60 is done with the “Absorb” stage of the charging, the screen stops at the “Bat Full” reading and never goes into “Float” mode.

Your wisdom and advice is earnestly sought; please enlighten my ignorance!

Blessings, and thank you!

Richard Fairbanks

pss
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My RE system: 8330 watts in three strings, Flexmax 60 x 3, Radian 8048A, GSLC load center, Mate 3S, Hub 10.3, FN-DC and 900 Amp, 48V Trojan T105-RE battery bank.

Re: Merging Three Systems to Feed an Inverter

Post by pss » Fri Apr 24, 2020 1:08 pm

Well, I have 3 arrays and 3 charge controllers. I have a 4 string battery bank, each string is 48 volts. The charge controllers are programmed to limit the amperage into the batteries, the absorb and float voltages and times for each charge cycle. They know the battery voltage of the bank. They do not know how large the bank is, they do not know how or what kind of inverters are attached to the batteries and they do not know the load being drawn from the battery bank, they only know the voltage of the bank and they use times to proceed thorough their charge cycles.

So in your case, you could simply connect all three charge controllers to the battery bank strings. You would combine the strings in parallel via a common copper buss bar into one large amperage bank. Then the charge controller's output would be coordinated to function like a single controller of 220 amp output. Then, you would connect each inverter to the battery bank by tapping off of the buss bar to each inverter. This way all is happy. If you need links to copper buss bars, please let me know. I also use a terminal fuse on each battery string and use a DC switch to manually turn off each battery string for maintenance, etc. Some people use DC breakers instead, but I find a fuse is always working (as is a breaker), but a big ON/OFF red colored switch is pretty easy for people to figure out even if I am not there. My charge controllers are installed via Outback GSLC, so they have an 80 amp DC breaker between them and the batteries.

raysun
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My RE system: Flexpower One: FX3048T, (2) FM80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Merging Three Systems to Feed an Inverter

Post by raysun » Fri Apr 24, 2020 2:46 pm

The three battery banks feeding a single inverter by combining the banks in parallel is fraught with issues of imbalance between the banks. When connected in parallel, the banks will not be individual pools of charge, independent of each other, but an interactive amalgam.

The Charge Controllers will not have an issue charging their individual banks, however, the banks may not necessarily achieve the same state of charge. The problem will be discharging the banks in parallel.

It's all but guaranteed each bank will be of a different capacity. The AGM banks, a bit of difference, the L16 bank a good deal of difference. Minor imbalances between cells cause issues with battery longevity. Large differences can cause catastrophic cell failure, with them being driven to negative voltages in certain cases.

If the AGMs are less than 6 months old, and have been scrupulously maintained, they could theoretically be combined and charged in common, but in practice would end up creating too many parallel strings to keep in balance. IME, the current banks of 4 parallel strings are too much to keep in balance, but its what's in place.

Were it my system, I'd replace the Aims with a 48 volt inverter, put the AGMs into two 48V strings, and hook both FM80 solar array/chargers in parallel to charge the battery.

The L16s are odd man out. Hook them up to the Aims. Charge them with the MX60/array, and use them to power a separate load.
Last edited by raysun on Sat Apr 25, 2020 8:19 am, edited 1 time in total.

pss
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My RE system: 8330 watts in three strings, Flexmax 60 x 3, Radian 8048A, GSLC load center, Mate 3S, Hub 10.3, FN-DC and 900 Amp, 48V Trojan T105-RE battery bank.

Re: Merging Three Systems to Feed an Inverter

Post by pss » Sat Apr 25, 2020 7:23 am

In my setup, all 3 charge controllers charge the entire parallel bank, they never see an individual bank. And all of the issues you have raised have simply not happened in 3 years of use. The parallel bank functions as a single 96 cell 48 volt battery of 900 amps. Charging capacity is up to 180 amps. And performance has been consistent without issues. And this is real world use. I don't really see a difference between this configuration and using strings of larger 2 volt batteries and trying to get them to perform evenly. It could be that a lot of what you warn against is do to poor parallel designs and separate string charging rather than aggregate charging. And when you use large copper everywhere, the electrons move freely about and play nice. Since batteries lack intelligence, they do not compete against each other. And all my batteries are from the same time and of high quality so they are pretty consistent.

raysun
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Posts: 2786
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, (2) FM80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Merging Three Systems to Feed an Inverter

Post by raysun » Sat Apr 25, 2020 7:54 am

pss wrote:
Sat Apr 25, 2020 7:23 am
In my setup, all 3 charge controllers charge the entire parallel bank, they never see an individual bank. And all of the issues you have raised have simply not happened in 3 years of use. The parallel bank functions as a single 96 cell 48 volt battery of 900 amps. Charging capacity is up to 180 amps. And performance has been consistent without issues. And this is real world use. I don't really see a difference between this configuration and using strings of larger 2 volt batteries and trying to get them to perform evenly. It could be that a lot of what you warn against is do to poor parallel designs and separate string charging rather than aggregate charging. And when you use large copper everywhere, the electrons move freely about and play nice. Since batteries lack intelligence, they do not compete against each other. And all my batteries are from the same time and of high quality so they are pretty consistent.
Exactly the point. You have a battery comprised of identical cells and with 3 charging sources working in parallel, all cells are electrically identical and are experiencing the same charge/discharge conditions.

That is how a battery should be commissioned.

A battery consisting of separate banks with different cell capacities and individual charging sources is asking for trouble.

Take a look at OP's profile. There are two distinct battery models being used. That's a guaranteed imbalance between banks.

Also, since each bank is being charged separately, its virtually guaranteed the SoC between banks is going to end up imbalanced.

pss
Forum Czar
Posts: 608
Joined: Mon Jul 24, 2017 8:40 am
My RE system: 8330 watts in three strings, Flexmax 60 x 3, Radian 8048A, GSLC load center, Mate 3S, Hub 10.3, FN-DC and 900 Amp, 48V Trojan T105-RE battery bank.

Re: Merging Three Systems to Feed an Inverter

Post by pss » Sat Apr 25, 2020 8:40 am

All good points. So you are saying we give good advice? Then he should be able to know what he needs to do.

raysun
Forum Emperor
Posts: 2786
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, (2) FM80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Merging Three Systems to Feed an Inverter

Post by raysun » Sat Apr 25, 2020 11:07 am

pss wrote:
Sat Apr 25, 2020 8:40 am
All good points. So you are saying we give good advice? Then he should be able to know what he needs to do.
Yep, somewhere in that tsunami of words is a path forward.

Using your example as a template:
• Connect all identical battery blocks into a single battery.
• Connect available charging sources in parallel to the battery.

The challenge is the 16 AGM monoblocks are 6V, and the nominal battery voltage is 12V. That means 8 parallel 12V strings. For any lead-acid battery chemistry, 8 parallel strings will be extremely difficult to keep in balance.

Flooded lead-acid cells at least, can be directly evaluated for state of charge by measuring specific gravity. If the cells are out of tolerance, an equalizing charge can be applied until the cells are brought back in line.

For AGM it will be nearly impossible to know if the strings are in balance or not, there is no direct, accurate test for state of charge. Also, equalization charging is much harder on AGM cells than FLA cells. The sustained high voltage leads to accelerated grid corrosion, also loss of electrolyte as the recombinant catalyst is overloaded and gasses are vented, resulting in permanent loss of capacity.

AGMs have a higher per-cycle cost and the shorter service life than FLA batteries because of this delicate balancing act of maintaining intercell balance while not overcharging. Anything that exacerbates the situation only makes it worse.

My earlier post gave a suggestion with the above in mind. The most expensive part of OP's system is the battery. The cheapest is the Inverter. The current configuration calls for sacrificing battery longevity in service of the inverter's 12V limitation.

By investing in a 48V 3kW Inverter, the battery can be reconfigured to be more effectively maintained, extending its service life.

Richard Fairbanks
Forum Member
Posts: 6
Joined: Wed Jul 17, 2013 1:49 pm
My RE system: Three independent systems:
• The original (2007) system, consisting of 800W of Sanyo (now Panasonic) solar panels, going to a (dearly cherished!) MX60, feeding eight, lead-acid Trojan T-105s wired in series/parallel for 12V, and
• Two new (as of 2020.4), identical systems, each consisting of 1.3KW of Panasonic solar panels, going to a new FM80, feeding eight Fullriver AGM DC250-6s wired in series for 48V.
Location: Camped out in the remote mountain wilderness of Utah
Contact:

Re: Merging Three Systems to Feed an Inverter

Post by Richard Fairbanks » Sun Apr 26, 2020 10:37 pm

Thank you SO MUCH, pss and raysun, for the feedback!

Sorry for the delay in responding; I’ve spent the last two days outside building two portable frames for the eight new Panasonic N325 solar panels. Please allow me to offer some more details:

The eight Trojan T-105s are now over six years old. My first set of eight T-105s lasted seven years (they’re rated for three-to-five years), and that included them spending three weeks on a mountaintop, on their sides, slowly draining battery acid when they were just one year old. (Trojan technical support was most impressed that I got them to last seven years!) I have been planning on replacing them in the next year (or two, whenever they finally expire) with the same Fullriver AGM DC250-6s I just installed a week ago.

I never though of combining the two AGM banks (separate battery lockers, adjacent to each other) into one bank and then combining the output of the two FM80s in parallel to charge what would then be just one single bank. Thank you both for that!

I am somewhat baffled as to the dangers of multiple strings. Are eight parallel strings really that much more unreliable than four parallel strings, as I have been using for thirteen years, moving around to different (very remote) campsites on various mountains? (Yes, I have been measuring the specific gravity of the cells every month, and have been averaging a four-hour, equalizing charge to the T-105s about twice per year. I won’t miss doing that, or the inevitable clean-up!)

I very much appreciate your detailed reasoning, raysun, as to not being able to know when AGM batteries are getting out of balance with each other, but how does putting sixteen (eventually twenty-four) batteries in series, instead of a 12V series/parallel configuration, relieve that concern?

Please excuse my ignorance!

(When I asked some of the more-renowned, off-grid, solar panel kit suppliers if there was any advantage to a 24V battery bank configuration, as opposed to a 12V configuration, they all said no, the only advantage would be just being able to use smaller gauge cables. <sigh> )

I do know that mixing disparate battery banks can be fraught with challenges, but I can easily monitor how the T-105s are holding up, and just disconnect them from the inverter as needed with a Blue Sea 350A battery switch. In the two AGM banks, the lengths of the cables between batteries, and the lengths of the cables going to the inverter, were all cut to the respective matching lengths, knowing the risks in having mismatched resistance between multiple sources of power (in this case, the three battery banks).

Your thoughts?

Blessings, and thank you!

raysun
Forum Emperor
Posts: 2786
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, (2) FM80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Merging Three Systems to Feed an Inverter

Post by raysun » Mon Apr 27, 2020 1:29 am

No need for apologies. Ignorance means lacking in knowledge, eminently correctable.

Battery physics is a suprisingly complex electrochemical process. The simplest battery cell is lead acid. Even at that, the chemical process of storing and liberating charge has pHD dissertations wrapped around it.

Combine battery chemistry with electrical circuits consisting of multiple current sources (the battery strings), and even a simple load; and the complexity of charge flow can be headache-inducing. Even these many years out of school, this French guy named Thévenin still shows up in my nightmares. I believe he was the intellectual cousin of the Marquis de Sade.

Enough diatribe, what's the bottom line in practical terms?

A battery is made up of individual cells. Each lead acid cell is 2V nominal, so a 12V battery is 6 cells in series. Under ideal circumstances, the cells in a battery would all be chemically and electrically identical. In practice that is hard to maintain. Small variations in the mechanical and chemical makeup of the individual cells cause them to have slightly different capacities, impedance, charge and discharge characteristics. If the differences get to be too great, the cells will degrade at different rates. Eventually the weak cells become so chemically or mechanically degraded they fail. Batteries seldom die gracefully of old age, rather one or more cells are put to death because of weaknesses relative to the other cells.

A single series string of cells is the simplest battery configuration. Minor differences in cells tends to be compensated by the fact the same amount of current flows through each. The cells may have slightly different voltages and slightly different storage capacity, but the equal current flow tends to help keep the differences in check for as long as the battery functions to an acceptable level of performance. (Note that performance will be degraded by weaker cells, but overall the battery will function.) When charging current is applied, the stronger cells accept more of the charge. The weaker cells accept less of their portion, and the remainder of their fraction is usually dissipated as heat, further degrading the cell. On discharge, a portion of the current flowing through the battery is in turn dissipated as heat in the weak cell. The bottom line is the weak cells cause the battery to have reduced usable capacity and shorter service life.

The above is the simplest case of cell imbalance and battery performance.

To increase overall battery charge storage capacity, either individual cells can be made physically larger, or more cells of a given size can be connected in parallel. In a 12V battery, the individual 12V strings (consisting of 6 cells each) are connected in parallel increase capacity.

The parallel strings also increase the complexity of the charge flow through the battery. Now, weaker cells in each string behave as above, but can add a 2nd dimension, where charge flow through the parallel branches can be different, resulting in uneven charge and discharge between the strings. Minor variations in cell impedance can now have a substantial impact overall on the individual branches. The weaker cells can degrade at a faster rate compared to the overall due to even minor variations in current flow between branches. A degraded cell can cause degradation of the entire string in which it is contained by compromising the flow of charge relative to the other strings. The greater the number of branches, the greater the likelihood there will be weak ones.

Got a headache yet?

The general rule of thumb is no more than three strings connected in parallel in order to keep the imbalance in current flow through the branches at a manageable level.

What happens when more than three strings are connected? That has to do with the actors outside the battery itself. Those actors are the charging source(s) and the load(s).

Let's start with load. For a given load, the greater the number of parallel branches, the lower the current flow through each one. Kinder, gentler current flow should be a good thing, and it is until the physics of the lead acid cell rears its ugly head. The chemical reaction that takes lead and sulfuric acid, and turns them into lead sulfate, water, and free electrons, is more sluggish in weaker cells at low current flows. In itself, this can result in uneven discharge between branches in the battery, but we don't see the worst of it until we go to reverse the reaction.

That's where charging comes in. When the battery is charged, the available charging current is divided between the branches. Ideally that division would be equal. However, weaker cells may cause a lower current flow in some branches relative to others. The end result is some branches may end up at a lower state of charge than others. We compensate somewhat with the Absorb phase of the charging cycle, overcharging the strings slowly fills all the cells, and incidentally helps the weaker strings to catch up a bit. We further compensate for gross imbalance with a periodic Equalization charge, which applies a much longer overcharge in an attempt to bring all the cells to a full state of charge.

The chronic, slight under charge of weak cells allows lead sulfate to build up on the lead plates, reducing cell capacity, further weakening the cell, and leading to greater imbalance between the strings.

For a given amount of charging power, the more parallel branches the lower the charge flow to each individual branch and the weaker the charging force to "stir things up" in each individual cell. The ideal target charging current for a lead acid battery is 10-15% of its capacity at the C20 discharge rate. For example, let's assume the C20 capacity is 200AH and we want to charge at 10%. The ideal total charging rate would be 20A for one string, 40A for two strings, 160A for 8 strings. Charging at less than the 10% rate leads to undercharging the battery, and insufficient current flow to keep the cell chemistry in line. Like a slow moving river, a low charging flow allows muck to settle in the bottoms of the cells (in the form of lead sulfate) and allows stratification of the electrolyte where it is weaker at the top of the cell than at the bottom.

This settlement/stratification scenario is especially applicable to flooded lead acid cells. AGM cells are less susceptible to it because the electrolyte is bound in the glass mat. However AGM is not entirely immune to the effects of under charging. Lead sulfate can build up on the plates, and also on the conductive grid that is part of the cell structure. When the grid gets too sulfated, the cell fails, generally catastrophically. AGM cells don't go gently into the night, they tend to turn off like a light switch.

Two FM80s and enough panels can produce sufficient charging current for eight strings on a good day, but at 12V they are limited to 12 x 80 = 960W of panel power. It has to be a very good day for the panel/charger arrays to sustain 80A each,160A total as is needed through the full bulk charge cycle. IME, it is an impossibility, so the proper charging current will not be maintained on a regular basis.

Compare that to the same battery blocks connected as two strings at 48V. Now the charge current demand is 20A x 2 = 40A total. The charge controllers can now handle 48V X 80A = 3840W of maximum panel power per controller, 7680W maximum compared to 1920W total at 12V. At higher battery voltages, more potential charging power can be made available, and lower overall current demand is required to properly charge. The higher the battery voltage, the easier it is to properly maintain the battery. That's a bigger reason to go with a higher voltage, than the suggested smaller gauge battery cable is. The vendors don't know, or don't care. I might be perverse enough to suggest they would prefer the battery have a shorter service life so that they can sell you another one sooner, but I won't suggest that.

Two strings at 48V will also have less inter-string variability than eight strings 12V. Weak cells will take much longer to fail.

The Fullriver AGM DC250-6s are even a bit more demanding in practice. The company implies a minimum charge current of 12% C, and recommends 20% C, where C = 250AH. That means the ideal charging current for a single 12V string is 250 x 0.20 = 50A. Eight strings in parallel = 400A current at manufacturer's recommended rate. They can be charged at a lower rate, but all the above applies. Under charging an AGM battery materially shortens its service life.

A common trap with a very large capacity battery is it's likely to be shallowly discharged and weakly charged. The numbers look OK, but internally the cells are not getting the excersize they are designed for. Like athletes who aren't working to their potential, the cells have a tendency to go "soft". They retire before their time.

That's a whole lot of battery theory and practice to digest in one sitting. I'm not going to.subject you to the indigestion of other issues that crop up with a greater number of strings.

I will leave you with one further caution however. Mixing the new AGMs with the old FLAs - with their different charge/discharge profiles and different capacities, is, IMO, a very, very bad idea. Please think long and hard before doing this. It won't be too many cycles, less than 100 would be my bet and I'd be willing to cover the spread, before one type, or the other, or both, fail.

Even isolating them with a manual switch is fraught. Since it must be manually switched, its almost guaranteed times will crop up when the switching isn't carried out in a timely manner, resulting in a battery being overly discharged before the deficit is noticed. At least that is what would happen in my world. The more operational steps that must be maintained, the more chances for a misstep.

Aloha.

Richard Fairbanks
Forum Member
Posts: 6
Joined: Wed Jul 17, 2013 1:49 pm
My RE system: Three independent systems:
• The original (2007) system, consisting of 800W of Sanyo (now Panasonic) solar panels, going to a (dearly cherished!) MX60, feeding eight, lead-acid Trojan T-105s wired in series/parallel for 12V, and
• Two new (as of 2020.4), identical systems, each consisting of 1.3KW of Panasonic solar panels, going to a new FM80, feeding eight Fullriver AGM DC250-6s wired in series for 48V.
Location: Camped out in the remote mountain wilderness of Utah
Contact:

Re: Merging Three Systems to Feed an Inverter

Post by Richard Fairbanks » Sat Aug 22, 2020 6:10 pm

Greetings, raysun (and pss, too!)!

I am _very_ sorry for having taken so long to respond to your most-valuable post, raysun! (I just spent four months dealing with excruciating pain due to being denied emergency prostate surgery because it wasn’t considered life-threatening.) I will be sending a link to your post to various solar power equipment distributors, as I found it to be a rare jewel of information.

After reading your deeply appreciated tome, I immediately purchased a 5KW, 48V AIMS pure sine wave inverter and the necessary cable and lugs to convert my two Fullriver DC250-6 AVM battery banks into two 48V strings, wired in parallel at two posts (+ and -), immediately prior to the inverter.

I also took your and pss’s advice and wired the two OutBack FM80s in parallel to feed the now-combined, 500aH 48V battery.

I have been testing the FM80’s “Absorb End Amps” setting’s ability to only charge as much as necessary, instead of charging for the same time duration every day, regardless of the degree to which the battery bank was used the previous day. It is a very nice refinement of the daily charge cycle, and is specifically recommended for the Fullriver DC250-6 AVMs.

One consequence of charging both 48V strings with the combined pair of FM80s and using their “Absorb End Amps” setting (both set to 5A), is that one FM80 will consistently get through the Absorbing cycle (dropping below 5A) a few minutes before the other FM80. It will then drop into the “Charged” state, reducing the battery bank voltage, and thus dropping the other FM80 back into “Bulk” mode, where it will stay for the rest of the day. My “solution” was to simply lower the “Absorb End Amps” setting on the FM80 that would finish charging first to 3A. Both FM80s now go into “Float” mode together.

Your thoughts (both of you!)?

My eight, almost-seven-year-old T-105s will soon be “retired,” as I will soon be getting eight more Fullriver DC250-6 AVMs to replace them. I will then have three 48V strings of identical batteries, wired in parallel to feed the 5KW, 48V inverter. This third string will be charged with the OutBack MX60 (set to 70A), and as the MX60 does not have an “Absorb End Amps” setting, I will set it to charge for two hours and see how it goes . . .

The folks at Fullriver tech. support strongly discourage wiring three strings of 48V batteries together in parallel, forming, in this case, a 750aH 48V battery, as they felt it was too unstable; two strings were acceptable. pss, you stated that you are using four strings with no challenges so far . . . 

I have three, TriMetric TM-2030s to monitor each string separately. If I can measure part of a string of batteries and get an accurate reading of the voltage, can I not measure the voltage through each full string separately to monitor any voltage discrepancies between the strings and then charge them separately, as/if necessary?

Your thoughts?

Blessings, and thank you!!

raysun
Forum Emperor
Posts: 2786
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, (2) FM80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Merging Three Systems to Feed an Inverter

Post by raysun » Sat Aug 22, 2020 9:24 pm

Richard Fairbanks wrote:
Sat Aug 22, 2020 6:10 pm
Greetings, raysun (and pss, too!)!

I am _very_ sorry for having taken so long to respond to your most-valuable post, raysun! (I just spent four months dealing with excruciating pain due to being denied emergency prostate surgery because it wasn’t considered life-threatening.) I will be sending a link to your post to various solar power equipment distributors, as I found it to be a rare jewel of information.

After reading your deeply appreciated tome, I immediately purchased a 5KW, 48V AIMS pure sine wave inverter and the necessary cable and lugs to convert my two Fullriver DC250-6 AVM battery banks into two 48V strings, wired in parallel at two posts (+ and -), immediately prior to the inverter.

I also took your and pss’s advice and wired the two OutBack FM80s in parallel to feed the now-combined, 500aH 48V battery.

I have been testing the FM80’s “Absorb End Amps” setting’s ability to only charge as much as necessary, instead of charging for the same time duration every day, regardless of the degree to which the battery bank was used the previous day. It is a very nice refinement of the daily charge cycle, and is specifically recommended for the Fullriver DC250-6 AVMs.

One consequence of charging both 48V strings with the combined pair of FM80s and using their “Absorb End Amps” setting (both set to 5A), is that one FM80 will consistently get through the Absorbing cycle (dropping below 5A) a few minutes before the other FM80. It will then drop into the “Charged” state, reducing the battery bank voltage, and thus dropping the other FM80 back into “Bulk” mode, where it will stay for the rest of the day. My “solution” was to simply lower the “Absorb End Amps” setting on the FM80 that would finish charging first to 3A. Both FM80s now go into “Float” mode together.

Your thoughts (both of you!)?

My eight, almost-seven-year-old T-105s will soon be “retired,” as I will soon be getting eight more Fullriver DC250-6 AVMs to replace them. I will then have three 48V strings of identical batteries, wired in parallel to feed the 5KW, 48V inverter. This third string will be charged with the OutBack MX60 (set to 70A), and as the MX60 does not have an “Absorb End Amps” setting, I will set it to charge for two hours and see how it goes . . .

The folks at Fullriver tech. support strongly discourage wiring three strings of 48V batteries together in parallel, forming, in this case, a 750aH 48V battery, as they felt it was too unstable; two strings were acceptable. pss, you stated that you are using four strings with no challenges so far . . . 

I have three, TriMetric TM-2030s to monitor each string separately. If I can measure part of a string of batteries and get an accurate reading of the voltage, can I not measure the voltage through each full string separately to monitor any voltage discrepancies between the strings and then charge them separately, as/if necessary?

Your thoughts?

Blessings, and thank you!!
Welcome back! Health issues during a public health crisis are not trivial matters to resolve. Best wishes for a speedy and full recovery.

Having a single battery will definitely help both battery life and system operational consistency.

As long as everything is 'dialed in', that is.

With AGMs (which I use), careful charging with as precise charging parameters as possible are the order of the day. A big part of proper charging is getting Absorb set correctly. Unfortunately, Absorb is not 'set and forget'. If too little charge is delivered to the battery during Absorb, an undercharged condition will result. If chronically undercharged, sulfation and shortened service life is the inevitable result. On the other hand, over charging during Absorb can lead to excessive heat buildup, grid corrosion, and water loss as the dissolution of water in the electrolyte overwhelms the catalysts capacity to recombine the gasses. In a flooded lead acid (FLA) battery, the water loss is an inconvenience requiring additional rewatering. In a valve regulated lead acid (VRLA) battery, the water loss is permanent, resulting in reduction of battery capacity. VRLA, the "Goldilocks" of lead acid, don't like their Absorb too cold, or too hot, and are only satisfied when Absorb is just right.

Two parameters come into play in Absorb: Absorb Voltage, and Absorb Time.

Absorb Voltage needs to come from the manufacturer. Hopefully they have destroyed enough battery monoblocks in experiments to arrive at proper Absorb Voltage, so you don't have to. Fullriver undoubtedly has a spec for Absorb Voltage, and it should be followed. More on that later.

Absorb Time is slippery. Its not really a static value, as much as its the product of a calculation that varies with the conditions of State of Charge (SoC) and Charging Amperage when the charge cycle first reaches Absorb Voltage. (The actual formula can be looked up, but varies a bit with each battery.) Nobody in their right mind runs the calculation every charge cycle, so "average" values are set for the most part. Setting an average Absorb Time is fairly simple, but it has problems vis a vis VRLA as described above. Mostly we set Absorb Time as a "failsafe" not-to-exceed value and expect the chargers to terminate Absorb when the time elapses.

The FM80s actually do a very sophisticated manipulation of Absorb Time, but more on that later.

The more effective metric for Absorb charge termination is derived from lead acid battery physics. As the Absorb charging proceeds by holding the charge voltage constant, the charge current steadily decreases to some minimum amount, at which point the current will decrease no further. This is referred to by various names, e.g.: End Amps. The true minimum is generally not used to terminate the Absorb phase because overcharging to that point causes extra wear and tear on the internals. Usually an End Amps of 2%-3% of the battery C20 Amp Hour capacity is employed. For example, if the battery was rated at 200AH @ 20 hour discharge rate, End Amps would be set between 4A-6A. The battery manufacturer should be able to provide a figure to use.

The FlexMax charge controllers do have a parameter setting for End Amps, and it will terminate the Absorb phase when reached, but it is not as useful as one would hope. The charge controller can track the charge current it is feeding to the battery bus. However, it cannot track the amount of charging current other chargers are feeding to the battery bus (other than indirectly, as it adjusts its own output current to maintain the proper constant Absorb Voltage.) More importantly, the controllers have no way of sensing what portion of the charge current is actually making its way into the battery and what portion is transiting the bus to feed loads. For example, if the battery was at a state of charge that would accept 10A, and at the same time the loads are consuming 20A, the controller would need to provide 30A in order to maintain the Absorb Voltage. If 10A was the proper End Amps to terminate Absorb, the load would keep the charge controller from sensing 10A to the battery was being reached. So in practice, End Amps in the charge controllers has to be set as a "best guess" of true End Amps and how much load current may be drawn at the end of the Absorb phase. Set End Amps too low, and termination will not be reached until the Absorb Time expires. Set End Amps too high, and the Absorb phase terminates prematurely. Due to load variations on most systems, End Amps is not especially accurate, so is set to 0 to disable it.

*PHEW* Why can't Absorb charging be simple and rational? Lead-acid physics preclude simplicity, but there's help. An external battery monitor can be used to sense net charge current flowing into the battery to give a clearer picture of when Absorb charge should be terminated.

Outback has an excellent, if spendy, system for just this purpose. The FLEXnet DC battery monitor paired with the Mate3s system controller can provide precise End Amps measurement and automatic charge termination. The rub is that's $1k worth of devices. I am in no way discouraging the expenditure. In reality, there is no better system for monitoring and controlling charge and discharge of the battery, especially with VRLA. Improper charging and discharging sealed batteries can reduce their service life to as little as 100 cycles. Properly managing these factors is the key to approaching the manufacturer's cycle life specifications.

A Victron BMS7xx series battery monitor is more like $250, does a good job tracking battery current and SoC. It does not, however, interact with the charge controllers, so will give information, but control will be up to the operator.

I am not familiar with the monitors being used in your system, but if one can be fitted with shunts to sense current flow into and out of the battery bus, it could serve to track charge current End Amps.

On to the question you actually asked about multiple charge controller behavior during the Absorb phase.

The charge controllers are designed to operate autonomously. Their control triggers are battery voltage. During the charge cycle, the Bulk phase is considered constant current - in essence the charge controllers provide current and the battery voltage rises until it reaches the Absorb Voltage. At this point, the controllers switch to constant voltage, holding the battery at the Absorb Voltage as the charge current declines. Absorb terminates when End Amps are reached, or when the Absorb Time expires. The trick to making this work is each charge controller must read exactly the same battery voltage. If the FM80 status screens are examined, the OUT voltage, which is the battery voltage, should read exactly the voltage measured at the battery terminals. If there is a discrepancy between OUT voltage and measured battery voltage, the controllers should be calibrated. Careful voltage calibration of the two controllers will have them "dancing to the same beat" and will go a long way toward more closely synchronizing charging behavior.

Start by noting the two controllers' OUT voltages, and the measured battery voltage, preferably at a time the battery voltage is steady, like early in the morning before sun up wakes the chargers, and sun up wakes the household.

We can delve into calibration once the figures are captured.

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Re: Merging Three Systems to Feed an Inverter

Post by raysun » Sun Aug 23, 2020 4:43 am

Flexmax charge controllers and adjusting Absorb Time.

In the previous post, it was mentioned that the FM charge controllers have an algorithm for modifying Absorb Time.

As mentioned, the Absorb phase should not be run too long, nor should it be run too short in duration. However, the proper time for the Absorb phase to execute varies with battery conditions.

For example, let's assume a theoretical battery, discharged to 50% of capacity, needs 2 hours of Absorb charging in order to restore it to 100% SoC, therefore the charge controller is set to Absorb Time = 2 Hours. If the battery is subsequently discharged 25% after previously being fully charged, the 2 hour Absorb Time would now be too long for the following charge cycle, resulting in a degree of overcharging during the session and resulting in some battery stress.

The Absorb Time should be varied in proportion to the needed amount of charge. The Flexmax controllers can accomplish this. How? By using an algorithm tied to an unlikely parameter: Re-Bulk Voltage.

As a battery is discharged, its voltage drops. Under a very controlled set of circumstances, the battery voltage can correlate to State of Charge (SoC). However, this correlation only applies when the battery is open circuit (no load, no charging current) for an extended period (24 hours) and at a specific temperature (75°F).

Further, battery voltage depends to a degree on rate of discharge (load). For example, a fully charged 12V lead acid battery may have a resting voltage of 14V. If a small load, say 1A, is applied, the voltage may drop to 13.8. If the load is removed, the battery voltage may again rise to 14V. A small amount of charge has been removed from the battery, but its difficult to tell how much just measuring voltage. Next, take the same battery at 100% SoC, and place a very large load on it, say 100A. The voltage will immediately drop, say to a reading of 12V. After a very brief discharge, the load is removed, and the battery voltage again rises to near 14V. Again, it is difficult to know how much charge has been drawn from the battery by noting voltage.

There's another "wild card" in the deck: Puekert's Law, which states (here, over)simply that the higher discharge rate from the battery, the less usable capacity the battery has. This is why we generally pick a target discharge rate, generally 20 hours, and use the related capacity in our measurements We won''t add this complexity here, but its good to understand the effect.

Re-Bulk Voltage is an attempt to correlate voltage and state of charge in a battery under load, and subsequently calculate how much Absorb Time is needed to bring the battery back to a 100% SoC.

At its simplest, Re-Bulk Voltage is the value to which the battery voltage must drop in order to trigger a new Bulk charge cycle. Once a charge cycle is completed, an internal "charge complete" status is set in the charge controller. The controller then switches to Float charging to maintain the battery SoC. Float charging continues until the controller runs out of sunlight and goes idle. If, however, after the Bulk/Absorb charge cycle is completed, enough charge is drawn from the battery to drop its voltage below the Re-Bulk Voltage setting, a new Bulk/Absorb cycle is immediately triggered.

In practice, Float generally maintains the battery above the Re-Bulk Voltage, and the battery doesn't drop below the Re-Bulk Voltage until later in the evening when the controller is idle. Also, when the Flexmax cintrollers "wake up" after sunrise the next day, they automatically start a new Bulk/Absorb charge cycle, whether the battety voltage has dropped below the Re-Bulk Voltage or not.

So what practical purpose does Re-Bulk voltage serve? A very sophisticated one, as it turns out. After the Bulk/Absorb charge cycle completes, the Flexmax controllers keep track of the amount of time the battery voltage is above and below the Re-Bulk Voltage. For each minute the battery is above Re-Bulk, one minute is subtracted from the Absorb Time counter. For every minute below Re-Bulk Voltage, one minute is added to the Absorb Time counter. In certain circumstances, for every minute the battery voltage is well below Re-Bulk Voltage, four minutes are added to the Absorb Time counter (to accommodate the Peukert effect.)

In theory, a lightly discharged battery may be Absorb charged initially, and never need another Absorb charge again. The charge controller completes a Bulk phase, the Absorb Time counter is set to 0, and the charger skips to Float.

In practice, we discharge our battery sufficiently to trigger an absorb phase with most every charge cycle.

With a properly set Re-Bulk Voltage parameter, the Absorb Time counter should be modified to provide the appropriate duration Absorb phase. How do we know what the proper Re-Bulk Voltage setting is? Therein lies the rub. It can be set for the battery type and nominal voltage, but to "get it right" requires a bit of experimentation. That fiddling is for another post.

Some manufacturers are aware of Outback's (and others) using the Re-Bulk algorithm and publish a value. Most do not, so we are left to determine it for ourselves. One can start with the default suggested by Outback and work from there.

The effects Re-Bulk may have on the Absorb Time counter can be seen on the charge controllers. Navigating to the Misc section of the controller's menu shows ChgT, the Absorb Time counter. It will count from 0 to the default (or modified) Absorb Time.

This whole complex affair is detailed in the Flexmax owner's manual. In the downloadable PDF version, search for ChgT, and the applicable sections will be highlighted.

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• Two new (as of 2020.4), identical systems, each consisting of 1.3KW of Panasonic solar panels, going to a new FM80, feeding eight Fullriver AGM DC250-6s wired in series for 48V.
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Re: Merging Three Systems to Feed an Inverter

Post by Richard Fairbanks » Mon Sep 14, 2020 12:55 am

Hi, raysun!

Thank you, yet again, for sharing your battery wisdom. It has taken me some time to process it all! (And to figure out how to be able to post again to this forum, as it appears to be under some kind of restructuring.   :-)   I have finally learned to just cancel any alerts regarding the site’s certificates.)

To respond to your advice, please allow me to briefly list the most common charging conditions for my 500AH, 48V, Fullriver DC250-6 AGMs (in two parallel 250AH, 48V strings):

One FM80 is set to an End Amps of 5A and the other is set to an End Amps of 3A. The difference between the two results in both chargers going into Float mode at roughly the same time. Both are below the 2-3% you recommend, but they are rarely absorbing for even 15-20 minutes each before going into Float mode, and some days they go straight from Bulk into Float mode. Even with my very limited knowledge, I expect the DC250-6s probably aren’t getting wildly overcharged. I defer to your wisdom.   ;-)

As you suggested:
raysun wrote:
Sat Aug 22, 2020 9:24 pm
The FLEXnet DC battery monitor paired with the Mate3s system controller can provide precise End Amps measurement and automatic charge termination.
At some point, when I can afford it, I will certainly consider it!

You continued:
raysun wrote:
Sat Aug 22, 2020 9:24 pm
Absorb terminates when End Amps are reached, or when the Absorb Time expires. The trick to making this work is each charge controller must read exactly the same battery voltage. If the FM80 status screens are examined, the OUT voltage, which is the battery voltage, should read exactly the voltage measured at the battery terminals.
My two FM80s are currently reporting the same battery voltage. The TM-2030 battery meters are consistently reading 0.3V lower than the FM80s. I have long-assumed that drop to be due to the meter reading the voltage after the shunt that measures the load. In a few days, I will put a hand-held meter on the two 48V strings.
raysun wrote:
Sun Aug 23, 2020 4:43 am
With a properly set Re-Bulk Voltage parameter, the Absorb Time counter should be modified to provide the appropriate duration Absorb phase. How do we know what the proper Re-Bulk Voltage setting is? Therein lies the rub. It can be set for the battery type and nominal voltage, but to "get it right" requires a bit of experimentation. That fiddling is for another post.
I am looking forward to that “fiddling” post!   :-)   Fullriver recommends a Re-Bulk Voltage (the nominal voltage, I expect) of 48.0 V.

In a week or two, I will be getting the final set of eight Fullriver DC250-6 AGMs, that will become the third and final 250AH, 48V string, tied in parallel with the other two 250AH, 48V strings (750AH total). The third string will be fed by my MX60, which does not have an End Amps setting. It can only output via the Absorb Time setting. Until I can afford to get a third FM80, what do you suggest I use for the MX60’s Absorb Time setting?

Blessings, and thank you, yet again!

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Re: Merging Three Systems to Feed an Inverter

Post by EA6LE-ONE » Wed Sep 16, 2020 1:16 pm

Raysun, your wisdom in battery chemistry and operation is eye opening. I do see that some manufacturers have a minimum charge rate for AGM batteries and some don't. outback and enersys (the manufacturer of the high capacity RE models) don't list any minimum charge. enersys has a re-freshening charge option which is a bit higher than absorb charge on outback brochure. In theory in AGM battery should not need a minimum charge as there is no electrolyte that need to be stir.

I gather from what reading from your writings that the charging amps should continue to reduce while keeping the absorb beyond the 2% that is advisable to use as end amps. the older the batteries the stabilizing amps will increase and going over the 2%. so as long that the charging amps will go well beyond 2% should be considered healthy? I have my batteries set at 45 (90 for the double bank) amps for few min as end amps setting, once it goes to float will continue decreasing down to 18 amp which is lowest i registered with the solar charge while in float. will this be a good way to determine the health of the battery? how low the amps will decrease in float or in absorb?

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