In March of this year, HC Ryan posted a Global Current Limiting White Paper on this site.
Is it safe to ASSUME that this function is still in the latest Firmware, and that each Classic that is on the system needing Limiting still will need its own WBjr?
Believe that it is OK to Attach this document, as should be shown below.
A friend is Commissioning a LARGE Backup system to Grid power with 13.7 kW STC PV and 7 Classics, and really needs this Limiting.
Thanks, Vic
OK, the friend that needs Global Limiting is probably working with MN Tech Support, he is fairly technical, but does not have a lot experience with battery-based PV systems. I have tried to advise him on this recent rehab of the system, but I know nothing of the Classic Global Limiting, so guess that I will be of no help to him.
While I do have systems with more than one Classic, would prefer to not hack into other systems, adding Shunts, and ditzing with the Cl programming, as it is difficult to spend very much time at remote sites in watching system behavior after any of these changes, and so on ... usually like to spend several days with systems that have had significant changes made to them.
Assume that no one here is using this potentially great function in the Classics, and therefore are unable to comment on this important function.
IMO this function is/would be another feather in the Classic's cap, but am reluctant to note this as a cool function elsewhere, without a confirmation (other than that White Paper), of its existence, and what are the observations of its performance ... (although, I did mention it recently on the Wind-Sun Forum, FWIW.
Just prattling. Thanks for any feedback, Vic
Hey Vic,
I should have an off grid system upgrade (I hope) with two classics 7kw/PV next fall where I would need to use it, but right now I'm with you and know really nothing about it except this white paper from Ryan, sorry. I will be very interested by your next comments about it.
Good luck!
Erik
Throwing a complicated one at us Vic! We could use a little more info so that we can get you the correct answer.
What inverter system is your friend using?
I would guess that it is a 48V system, is that correct?
What is the battery capacity?
Hi Sue,
Thanks for the reply;
This chap is already working with Jim in MN Tech, so was not trying to have the very busy MN folks try to support this project, was hoping to make sure that this Limiting still exists, any added caveats about its function, AND if Members here were using it. Some here seemed to have been begging for it in the past.
Unfortunately, the target system is only 24 V, has Surrrette 1124 Ah L-16s IIRC, and is running two SMA Inverters that total 10 - 11 kW.
But really Doug is already pinging MN Tech, so let's not consume any more of Tech bandwidth -- THANK YOU for offering the added support, Sue.
Wish I had a capable Bench system that could be used for checking out some of the whizziest capabilities of the Classic and other MN products. But not in the cards just now.
Thanks, too, Erik. Hope all is going well with you, and look forward to info on that next system next system when it is operational.
This Global limiting could be helpful for a number of folks with a lot of PV ... think that Doug got some great deal on a couple pallets of PV, so all went from there ... he was running AIMS CCs, which blew up when he upgraded all of the PV cabling to them ... and so on, 'as the world turns'.
Thanks again for the comments! Vic
Vic,
It still exists. You basically need one WBjr per Classic that you want to back off. So lets say you have 4 Classics and each can do 50 amps (200 total) but you want to limit to 50 amps.
Put a WBjr on Classic 1, 2 and 3 and set all 3 to 50 amps as shown in the attached document. Essentially it is not truly global as it only manipulates the one classic that is why the need for multiple WBjr's
Hi Ryan, GTEAT to see you back here, Thanks for the info;
You are saying that all but one Classic would need a Shunt/WBjr, primarily because in your example of 50 A per Classic, the maximum current from any one PV string is about 50 A, so the fourth Classic would realistically be somewhat limited by the available current almost all of the time ?
Will think about this a bit more ...
Thank you very much, Ryan, and it was very nice to meet you at the Intersolar show in SF, last month! 73, TU, Vic -- kilo six italy charlie
It just struck me, that the title of this thread "Global Current Limiting"
sounds like a movie from Al Gore.
GOOOOD one Mike!
Strategy for Limiting Total Maximum Battery Charge Current with seven Classic 250s using 7 Strings of 7ea 180 W 48 Cell PVs.
Have been trying to help a friend with a pretty good sized Grid Backup power system. The Classic 250s have recently replaced some AIMS CCs which got toasted. The PV array is sufficiently large, that some battery charge current limiting seems in order.
System parameters -- Seven Classic 250s, each with 7ea 180 W 48-cell PVs, charging a 24 V Flooded 1070 Ah battery. Total STC PV power is 8820 W. The MidNite White Paper on Battery Charge Current Limiting with Multiple Classics, and the MN String Sizer output for a single Classic 250 using 7ea of the 180 W PVs is are attached.
Using customary Rules-Of-Thumb for estimating the amount of PV power routinely available from PVs, each string of 180 W PVs can produce about 32 Amps of current into the battery bank (180 X 7 X 0.75 = about 945 W (typical). 945/29.5 = 32 A. This takes into account reasonable heating in the PVs, some losses in wiring and in the Charge Controller, etc.
At the NV location the system could see times where this output is considerably higher -- cold Winter days with wind, and especially with reflections from snow, cloud Edge Effects, etc. But 75% of rated STC power is a good general rule.
At 10% of the Surrette S-1475 20 Hour Capacity (C), this maximum battery charge current is about 0.95 X 1124/10 = 107 A for 10% of Capacity (C), and about 140 A for the max rate recommended by most Flooded battery manufacturers of 13% of C. The multiplier of 0.95 represents the Capacity reduction from the Data Sheet C of 5% due to electrolyte of 1.265 SG verses the Data Sheet SG of 1.280, which is customary for Surrete Solar batteries, FYI
Believe that NOT every Classic needs to have its own WBjr and Shunt. Seems to me, that three Classics need not have WBjrs used to limit battery charge current, and the other four will need separate WBjrs and Shunts of their own added to the single one that the system now has, (which will measure total battery charge current).
Saying this another way, you have seven Classic 250s, three of the seven Classics should not need to have their output current limited into the battery. But you would probably want to add WBjrs and Shunts to three of the additional Classics The Classic that has the Shunt directly connected to the battery negative counts as one of the Classics that will not be limiting the battery charge current. The WBjr on this Classic is used to measure the Net charge/discharge current of the battery.
The "reasoning" for the above is that each Classic could contribute about 32 A of output current. Based on 3 X 32 A = 96 A of maximum nominal Classic output current capability, being less that the 107 A nominal 10% of C limitation. The only real fly in this, is, that during some Winter conditions that cold, windy weather, plus some possible snow reflections might allow the three Classics without WBs could produce more battery charge current than that calculated above. However, most Flooded batteries can accept up to 13% of C, which allows for some headroom for situations with higher-than-normal PV production.
The above started as an e-mail to the owner of this system, and now suffers from far too much editing ... should have started over.
Below is the Classic Sizer output for one string of seven PVs on one of seven Classic 250s.
The brief MidNite White Paper on Battery Current Limiting is at the foot of the first Post in this Thread.
Thoughts? Thanks, Vic
And, as the battery charge ramps up with the rising sun, the battery accepts less amps, and sort of self limits anyway. But a cloudy morning, clearing at noon, can surprise you.
I am still wondering if your friend really has a problem or not given the tendency for these kinds of batteries to self limit.
Obviously a breaker could limit the charge current, but probably that isn't the desired path.
In theory, you could program a PLC, industrial single board controller or hobby controller (such as a beagle board) to detect current levels and use relays to start shutting down controllers as needed. Current detection isn't that hard - even can do with with a hall effect sensor setup.
If extended temperature range is needed, that takes it more towards the single board controller approach.
Quote from: harryn on August 20, 2017, 06:29:03 PM
I am still wondering if your friend really has a problem or not given the tendency for these kinds of batteries to self limit.
Mike, harryn,
The maximum available battery current from the entire array may well not be too much for the battery on a typical day.
Without some form of limiting, there could be problems with intermittent clouds, some shading etc.
If it were my battery bank, would not want the battery charge current exceeding 10% of C, as the battery Gassing voltage is approached, and above it.
The self-limiting of battery charge current that I am familiar with, is the declining battery Acceptance, once the Absorb stage has been reached. IMO at this stage of charge, Flooded batteries are the most sensitive to currents exceeding about 10% of C.
Limiting could be performed by a fairly large Diversion load on the system, perhaps by a relatively large electric heater, perhaps mounted outside, or something similar.
Initially, if had been stated that the PV array was huge relative to the Capacity of the battery, after some checking, the PVs were determined to be 48 Cell 170s, as opposed to some unknown 280W PVs ...
The reduction in the size of the PV array greatly reduced the magnitude of the issues, in string Vmp, Voc, and excessive CC current demands. But, it still seems that some Limiting is in order, for the location of the system.
A number of variables in the system are still unknowns (like the elevation angle, and Azimuth of the PVs, etc), but still would suggest some Limiting in battery charge current.
FWIW, Thanks, Vic
I don't understand why global limiting is an issue with Classics and SMA inverters.
On the SMA menu #222-01 is BatChrgCurMax, which can be set from 10 to 1200 amps. This is the maximum charging current to the battery.
Why wouldn't you just set it here, and not mess with the Classic settings at all?
I ask because I have 2 SMA (12kw total) and 2 Classic 150s, and am strongly considering adding more panels and another Classic. My total output on optimal sunny days could be over 10% C/20, but many days are cloudy in winter, which is when I need to catch every photon. Also I would have significant daytime only loads once absorption phase has started.
Quote from: krementz on August 27, 2017, 04:04:55 PM
I don't understand why global limiting is an issue with Classics and SMA inverters.
On the SMA menu #222-01 is BatChrgCurMax, which can be set from 10 to 1200 amps. This is the maximum charging current to the battery.
Why wouldn't you just set it here, and not mess with the Classic settings at all?
I ask because I have 2 SMA (12kw total) and 2 Classic 150s, and am strongly considering adding more panels and another Classic. My total output on optimal sunny days could be over 10% C/20, but many days are cloudy in winter, which is when I need to catch every photon. Also I would have significant daytime only loads once absorption phase has started.
Hi Krementz,
Since I know nothing of SMA Inverters, or Inverter-Chargers, do not know. But, it would be surprising if the SMAs know anything at all of Classics or other Solar battery chargers.
Would assume that the setting in the SMA, is only for its battery charger, and not any other.
FWIW, Vic
SEVEN Classics ??!?!
Well, this global current limit works easiest when there is just a couple of Classics and maybe another charger...
Then, the Classic with the WB Jr. on it can be the one that limits... Assuming that one WB Jr. endowed
Classic is necessary to bring the charge current over the limit.
The software was designed to work with multiple Classics and multiple WB Jr's BUT I haven't
tried it with THAT many Classics before !
Would like to hear how that works !
If it is a large battery bank, then maybe only a couple of the Classics need WB Jr's ???
boB
Hi boB, Thanks for the reply.
My guesses are, that one Classic needs a Shunt and WVjr to monitor all currents going into/out of the battery, and three other Classics need Shunts and WBjrs, allowing those three to have their battery charge current limited, under somewhat unusual conditions.
More Later, Vic
Hello, new to forum, I came across this thread looking for information on current limiting options on the Classics.
I'm working with a scenario similar to what Vic has, except 4 Classics instead of 7.
If more than one classic needs a shunt and a WBjr, what is the best way to accomplish that? You would want all of the Classics referencing the total charge current into the batteries, correct? Can you stack multiple WBjr's on one shunt? (I don't currently have one in hand to check this) Or do you need to wire the shunts in series with one for each WBjr?
Thanks
Yes, you should be able to add more WB Jr.s to one shunt. Place them VERY close to the shunt itself
if at all possible. Stacking them right on top of each other would be great.
Reason why is that if you connect a shunt (only one even) to a shunt with long-ish wires,
then the power draw of the WB. Jr. itself (the shunt is its negative lead) can corrupt the
WB. Jr. readings somewhat.
boB
Quote from: boB on August 28, 2017, 10:20:17 PM
Yes, you should be able to add more WB Jr.s to one shunt. Place them VERY close to the shunt itself
if at all possible. Stacking them right on top of each other would be great.
Reason why is that if you connect a shunt (only one even) to a shunt with long-ish wires,
then the power draw of the WB. Jr. itself (the shunt is its negative lead) can corrupt the
WB. Jr. readings somewhat.
boB
That's good to know. Without having one in hand, I wasn't sure if they could physically stack on the shunt.
Thanks!
Quote from: boB on August 28, 2017, 10:20:17 PM
Yes, you should be able to add more WB Jr.s to one shunt. Place them VERY close to the shunt..
What about just sorting out the code so that up to four Classics share the WBjr readings from a master in the looped serial wiring. Same way the battery sensor temperature data is.
dgd
Quote from: dgd on August 29, 2017, 06:31:45 AM
Quote from: boB on August 28, 2017, 10:20:17 PM
Yes, you should be able to add more WB Jr.s to one shunt. Place them VERY close to the shunt..
What about just sorting out the code so that up to four Classics share the WBjr readings from a master in the looped serial wiring. Same way the battery sensor temperature data is.
dgd
DGD, that is exactly what I wanted to do ! This is exactly what should be done !
Hopefully this will be done.
Unfortunately, we don't have the time to do that right at the moment.
Had to shoot the engineer and ship !
boB
dgd, boB,
Yes that sounds like a great idea ... but for the target system, stacking WBjrs should be fine, especially given the inve$tment made in the system, to date. A few WBjrs, and some SPDs should do almost all that is needed.
boB and the MN Crew are doing wonderful things on the new product development side of things! Thanks!
Of course, we do hope that there will be time in the future to go back and add some additional functions to Classics and KIDs, etc.
73, Thanks, Vic
Right Vic !
There's just way too much to do and so little time !
Now, if we only set our clocks back one hour every day we would have a 25 hour day !
boB
Hi,
It seems like the main problem is that your friend's large system was designed for a 24 volt battery pack rather than 48 volt, but that seems to be a done deal now.
As far as controlling current during "excessive production", this can easily be dealt with using a small PLC controller. With panel prices at $1 / kW vs the old days of $10/kW, there isn't a need to count electrons, just keep things safe.
Any decent controls engineer can program a PLC to detect if things are running too hard and send an output to shut down 50% of the Classics in the system. PLC's are pretty easy to program. One of the electrical engineers that consults for me is a controls guy, feel free to pm me if you want some help. He is fairly familiar with the Classic 150 (not an expert, but fairly familiar).
His normal rate is $80/ hr which isn't bad for an engineering consultant. (1/2 the price of a car repair shop) I am guessing that he could do the project in 40 - 60 hours and then you would have the solution for any future projects as well. Of course we all know how software projects go, so it can easily double. :-[
Harry
Hello Harry, Thanks for this idea.
But, just to me, since this function is already in the later Classic Firmware versions, with only the addition of three WBjrs (for this particular system), at a cost of about U$D 150, or even less, this can be accomplished, smoothly, cleanly and with little additional effort.
And, again, just to me, this Limiting function could also be accomplished by adding a Diversion Load, using the Diversion control function that is also already in the Classic Firmware.
But, that is just my read on things. Have done quite a bit of Microcontroller coding in the past, but that just seems to be more than is required for this task -- KISS -- ... IMO.
To me, the MidNite Classic is really the Swiss Army Knife of Charge Controllers. It has SO many useful functions already built in, that allow SO many additional functions to be performed, generally quite easily!
FWIW, but thanks again for the ideas. Vic
Quote from: boB on August 27, 2017, 06:20:16 PM
SEVEN Classics ??!?!
Well, this global current limit works easiest when there is just a couple of Classics and maybe another charger...
Then, the Classic with the WB Jr. on it can be the one that limits... Assuming that one WB Jr. endowed
Classic is necessary to bring the charge current over the limit.
The software was designed to work with multiple Classics and multiple WB Jr's BUT I haven't
tried it with THAT many Classics before !
Would like to hear how that works !
If it is a large battery bank, then maybe only a couple of the Classics need WB Jr's ???
boB
Hi boB,
Yes there are seven Classic 250s on a 24V 1060 Ah battery. Yes, this is not the best system design. The system evolved over time. It IS a shame that the pre-existing 24 V SMA inverters drove the "decision" to stay at 24 V.
Of course this is a far-too-common situation ... "all I need to do is just replace this one item ... and I will be good to go ... ."
Just re-read your Post, quoted above. There will be four Classics with WBjrs. One Master, and three additional. I DO hope that this does work, well enough.
As an aside, thought I would try to test this battery charge current limiting, on a single Classic system here, and started to slew the Default value in Reg 4405 from its 30000 Default value. But there appears to be no higher slew rate, with increased duration of the Down(or Up) arrow button press. Would guess that this change in the register value might well take 15, or so minutes to reach a test value of about 220.
Assume that this is a Modbus Register value, and it IS nice that there is this register editing function built into the Classic FW ... also assume that if this is Modbus, that there is a Modbus R/W Utility that you Pros use. However, neither my friend, nor I are really computer HotShots ... what is the best way to try to expedite setting of reasonably small values in this target register?
Assume that the VMM for whatever reason might possibly return any custom setting in Reg 4405 to its 30000 Default value (?).
Know that you boB, and all of your crew are quite busy right now.
The friend, Doug, plans to buy the additional WBjrs in the next week, or so ...
73, Thanks again, Vic
Vic,
Why are there seven 250 Classics charging a 24v 1060ah battery bank?
This is close to my system with a 1025ah 24v bank yet all the charging is managed by one Classic 150 and one Classic 250.
It must be the inputs to the Classics, what are these?
Just my opinion but I would be seriously looking at the need for 7 Classics and how to get this reduced o just two or three.
Two Classic 250 should easily provide the c/10 106a this bank needs for max Absorb although even if you pushed each to 60A output that would not be detrimental to the bank.
Also with EA at 2% to 3% or thereabouts thats only 20A to 30A.
Then just think of all that $ from selling the spare 4 C250s, keep 1 as maintenance spare..
Also easy wiring just two CL, one with Wbjr and follow-me between them will keep the charge cycle running nicely.
dgd
Quote from: dgd on September 03, 2017, 11:26:12 PM
Vic,
Why are there seven 250 Classics charging a 24v 1060ah battery bank?
This is close to my system with a 1025ah 24v bank yet all the charging is managed by one Classic 150 and one Classic 250.
It must be the inputs to the Classics, what are these?
Just my opinion but I would be seriously looking at the need for 7 Classics and how to get this reduced o just two or three.
Two Classic 250 should easily provide the c/10 106a this bank needs for max Absorb although even if you pushed each to 60A output that would not be detrimental to the bank.
Also with EA at 2% to 3% or thereabouts thats only 20A to 30A.
Then just think of all that $ from selling the spare 4 C250s, keep 1 as maintenance spare..
Also easy wiring just two CL, one with Wbjr and follow-me between them will keep the charge cycle running nicely.
dgd
Hi dgd,
Thanks for taking the time to think about this system, and reply.
Attached is the Classic Sizer output for two strings of seven 180 W Pvs ... seems excessive to me.
This system has "evolved", as noted before.
The roof of the building with the PVs on the roof, had trusses reinforced, was re-roofed (IIRC), and the PROS came out to spend days racking, wiring and running conduit for the hard-fixed array configuration of 7 PVs in series times 7 strings. (this is a FIXED parameter).
SO, OK, Classic 250s (in place of the burned up AIMS CCs).
With this fixed String Vmp, and power, I would NOT run two parallel strings of 7 PVs into a single Classic 250, unless it was temporary (and the CC output current limited) ... that's just me ...
I got kinna involved in this late in the process, well after too many things were FIXED. In retrospect, these type systems always seem a bit difficult to understand, but "at the time, all made perfect sense".
Just my read on things, 73, Thanks, Vic
Ok, thanks for info, I now see what you are trying to deal with and the fixed config 7 by 7 array of 180watt PVs is the crazy part. About 9kW of PV charge potential seems way overkill for the battery bank capacity, it would ve interesting to see what one WBjr shows as total KWhr in and out per day
Dgd
Hi dgd,
Yes, and from appearances, this system will continue to evolve.
This system does have a number of Opportunity loads, including an electric domestic water heater, but only the Air Conditioning loads are controlled (manually).
It appears that the inverter pair runs close to maximum output on occasion ... there had been some thought of changing-out the inverters ... but, then there should really be a change to 48 V (the batteries are about six months old) ... and so on ... It is too bad, but this is not an uncommon issue.
The system owner has been very, very self-sufficient for a number of years, so I had not tried to pry too much into the details of the system. He is a Hammie Radio friend, who's site is about 250 Km from here, so have never seen the site.
Believe that the availability of a couple of pallets of PVs at a very good price is really what started this adventure in Grid backup PV power.
But, the system seems to perform well, and happiness abounds. Doug is very happy with the MidNite Classics (and other MN products used there), and the Support from MidNite.
FWIW, 73, Thanks, Vic
Quote from: Halfcrazy on August 03, 2017, 05:41:40 AMVic,
It still exists. You basically need one WBjr per Classic that you want to back off. So lets say you have 4 Classics and each can do 50 amps (200 total) but you want to limit to 50 amps.
Put a WBjr on Classic 1, 2 and 3 and set all 3 to 50 amps as shown in the attached document. Essentially it is not truly global as it only manipulates the one classic that is why the need for multiple WBjr's
The above, is the way that I understand how this current limiting does work. I have not tested it, or played with this function, for too many years.
Halfcrazy (now using the Moniker, "FNG"), and boB know much more about this function, than I.
malkierian, Glad that you were able to find this old Thread! Vic