MX60 self-protection?

[raysun]

../..
Anyone have a lot of panels and an FM CC they don't mind frying? Let's try setting up an array 200% over the P(max) spec and see what happens. Anyone? Anyone? Buehler? Buehler?
../..

I think my arrays fit your experiment, 2820W feeding a single FM60
But half of the panels are not optimally oriented and the output power setting on the FM60 throttled down to 55A.

I got you beat. I can direct 7kW+ at one hapless FM80 and every panel at 10AM will be exceeding STC by a very large margin.

Still no takers on my pool though...

[drstrip]

I think my arrays fit your experiment, 2820W feeding a single FM60
But half of the panels are not optimally oriented and the output power setting on the FM60 throttled down to 55A.

Your profile says you're feeding this into 24V. If that is up-to-date, you are indeed running at nearly 200% of capacity. How long have you been running like this? Since you added panels in 2018? If so, that's quite indicative. Did anyone from Outback or their reps tell you this was a safe configuration? (Safe = won't burn it up).
How suboptimal is the tilt of those half of the collectors?
Even if you're running at "only" 150% of listed output capability, this is a very valuable data point.

[sbrownian]

Get out your nuclear powered hearing aids, you're gonna need 'em.

and to think I got chewed out in this thread for suggesting that Outback staff didn't pay attention to these forums

Chewed out? If we were chewing you out, you'd be missing limbs. We are sort of a feral pack of dogs here.

Hey now. I've at least had my rabies shots...

[drstrip]

A the computer guys tell you RTFM. I checked the FM60/80 manual and it doesn't list a max power input. Same for my 2004 vintage MX60 manual. The FM100, on the other hand does. Voc is 300V, max short circuit input current = 64A. As quick approximation, let's use Vmpp = 250V. At 64A that gives us 16kW max input. 100A output @ 48V is about 5kW, for a margin of 300%.

Of course, we have no reason to believe one way or the other what the input limits are for the other units.

[raysun]

A the computer guys tell you RTFM. I checked the FM60/80 manual and it doesn't list a max power input. Same for my 2004 vintage MX60 manual. The FM100, on the other hand does. Voc is 300V, max short circuit input current = 64A. As quick approximation, let's use Vmpp = 250V. At 64A that gives us 16kW max input. 100A output @ 48V is about 5kW, for a margin of 300%.

Of course, we have no reason to believe one way or the other what the input limits are for the other units.

I dunno, the math doesn't sound right.

Firstly one cannot have short circuit current being measured and have a measurable voltage, otherwise, it isn't a short circuit.

In the same manner, one cannot measure open circuit voltage and have current flowing, or it isn't an open circuit.

Therefore 64A is not achievable at 260V.

The last calculation is the traditional measurement method that keeps a charge controller out of trouble: 100A rated output current into a 48V nominal battery voltage permits the safe power handling of a 4.8kW (5kW) array.

One can safely assume the design that accomodates the specified output current and voltage will also accomodate the input current and voltage that can deliver the output.

For an FM100 the rated maximum input voltage = 300V. That rating is open circuit voltage of course so has no current, and therefore no power associated with it. For a semiconductor circuit, it can be considered as close to the breakdown voltage of the transistors handing DC to DC conversion as one cares to get.

Looking at the characteristic PV panel IV curves, one can note that as soon as current starts flowing, voltage drops dramatically. At some point on the IV curve V(mpp) and I(mpp) indicate maximum panel power at STC. As conditions vary away from STC (temperature, irradiance, etc.) the maximum power point of the panel shifts away from V(mpp)*I(mpp). A job of an MPPT charge controller is to find that new V*I point for the current conditions.

How the MPPT function does this is by presenting a variable load to the panel, and noting what set of conditions produce W(mpp). (For those following from the peanut gallery, "Load" on the receiving end of an I*V, e.g. "Power" circuit is the device that dissipates 100% of the resultant heat.) Like putting ones hand on a hot stove element, the maneuver is best done quickly so that the heat buildup is not accompanied by unpleasant smell, then dump that heat (power) somewhere else. But I digress.

Once MPP is found it can be used to create AC, fed to a transformer, then fed to a PWM circuit that is attuned to the battery voltage. This backend circuit is the battery charging portion. The PWM circuit is usually a bank of power FETs, excellent high current semiconductor switches ideally suited for PWM operation. FETs will have three design limits: Voltage, Current, and Heat (Power or Watts dissipation.) The FETs are largely what limits the controller's maximum current specification.

[raysun]

So how could a charge controller protect itself from the slings and arrows of outrageous array wattage?

Its got to start with the circuits that face the array: the MPPT tracker. There are a bunch of MPPT algorithms, but by far the most common is called "Perturb and Observe". This technique creates a "sweep" by varying its load to force the PV array through a series of voltage steps, and measuring the corresponding current. The best V*I combination is declared the winner.

For an array that, by its nature, can't exceed the V, I, (or W) design limits of the controller, P&O can execute with impunity.

An array that can generate power well in excess of design limits presents a problem. Assuming array voltage stays in line, then Perturb and Observe would seem to be more akin to Drink from a Firehose. How is the excess current to be handled?

In the event of wretched excess, the MPPT could possibly "detune" the V to move lower on the VI curve, but voltage can only be reduced so much before the quantum conversion of photons to electrons would rebel, it would seem.

If the above could be made to work, the mpp would be fairly unstable, and frequent MPPT sweeps would be needed, taking the controller out of production for a greater percentage of the day.

@SandyP, question: what's the lifetime maximum kW recorded from your array?

[raysun]

../..
So what does that tell us? At least for me it seems to say that the MPPT algorithm is smart enough not to always track at the Mpp and just dissipate the excess as heat. This is not surprising, since in float condition even the most balanced system will have input capacity in excess of the output requirement. ../..

^^^ A very good point, otherwise the cooling fan would be running the most during float.

Float is three special cases:
• Battery voltage is high, so nominal power handling capability is higher than during Bulk when the battery voltage is lower.
• Float is a constant voltage phase, the charge controller need not do anything tricky, other than have the PWM voltage regulation circuit hold the Float voltage. The potential for high current delivery is available, however the battery impedance (not controller machinations) isn't permitting it to flow.
• The fan can be running full time during Float - if the battery load is high enough to cause the Float current to be high.

[raysun]

(Sitting through the boring 160kM of a flat Tour de France stage, in order watch the final 2kM where all the action is. At least its a beautiful bike ride through the Loire valley, so deserves half my attention. You get the other half.)

You all need to sit through the abject brain-strain that is Thevinin's Theorum. Especially applicable to DC circuits (like PV-CC-Battery) it posits: For any linear electrical network containing only voltage sources, current sources and resistances can be replaced at terminals A-B by an equivalent combination of a voltage source Vth in a series connection with a resistance Rth.

OK I won't insist, but the takeaway is the charge controller exists only in association to a voltage / current source (the PV array), and a series connection to a resistance (ironically, the battery.)

When the battery voltage is low, its internal resistance is low. This is the state of the "load" when the Bulk charging phase starts. Bulk is a constant current phase. In practice it means drive as much current into the battery as is available - up to the maximum charge limit of the battery.

(Random side question: If feeding maximum rated charge current into the battery is good, why isn't feeding twice the rated charge current into the battery twice as good?)

Bulk is the maximum-stress charge cycle phase for the charge controller - the greatest delta between array voltage and battery voltage, and the highest current demand.

Absorb is a constant voltage phase and starts with a notable battery voltage rise, indicating an increase in internal resistance. Initially, the battery voltage is at its maximum, and current demand (for the phase) is at its maximum too. By the numbers, its the point at which the charge controller can handle maximum power for the nominal battery voltage. Holding voltage constant, internal resistance increases, and current demand decreases. The decrease in current demand is moderated by the battery, not the charge controller.

Float is similar to Absorb in being a constant voltage phase, though the battery voltage is lower, and the internal resistance higher than the start of Absorb. (Try to force more current into the battery by increasing the Float voltage, and watch the excess create a good deal of heat.)

Oh, and that simple Thevinin circuit reduction? Its not so simple. There are multiple voltage and current sources (PV and Charge Controller), and multiple series resistances (Battery and Charge Controller). The dynamic interaction of all three dictates they stay in balance or the circuit will fail.

[sbrownian]

However, the charge controller is anything but linear internally.

It is a switcher at it's core.

Yeah, nitpicking here...

[SandyP]

I got you beat. I can direct 7kW+ at one hapless FM80 and every panel at 10AM will be exceeding STC by a very large margin.../..

So do you actually direct 7kW at one FM80 or is it that you "can" do it?
Since the battery voltage limit for an FM is 60V then why would the 7kW not fry your FM80?

[SandyP]

../..
@SandyP, question: what's the lifetime maximum kW recorded from your array?

I only have a Victron battery monitor and rarely look on the FM for the max kW (& not at the house at the moment thanks to covid restrictions).
Over the last 3 years the Victron data recorded at 60 seconds intervals seems to show a max battery charging amps of 61Adc (at the second the data was logged) with a load of 3.5Adc so max output from the FM60 of 64.5Adc.
I expect that, due to cloud edge effects, there would have been numerous over amp excursions not logged.

[SandyP]

I think my arrays fit your experiment, 2820W feeding a single FM60
But half of the panels are not optimally oriented and the output power setting on the FM60 throttled down to 55A.

Your profile says you're feeding this into 24V. If that is up-to-date, you are indeed running at nearly 200% of capacity. How long have you been running like this? Since you added panels in 2018? If so, that's quite indicative. Did anyone from Outback or their reps tell you this was a safe configuration? (Safe = won't burn it up).
How suboptimal is the tilt of those half of the collectors?
Even if you're running at "only" 150% of listed output capability, this is a very valuable data point.

Yep it is a 24V system.
It has run like this for almost 3 years (with some tinkering of the charger controller's output limit setting).
I did not consult anyone from Outback.
The additional 1300W of panels added in 2018 are on a roof sloping 15deg east - so I expect you can de-rate them to 80-85% of their nominal output.
We added the panels to get more solar input to the system during overcast/cloudy periods.
As our overnight power usage is generally not high, with the extra panels in summer the system will have reached absorb by 9:00am but after a deeper discharge it reaches the output current limit early and then stays flat(ish) - see attached.

WattsperM2 for the day (from nearby private weather station)

[raysun]

I got you beat. I can direct 7kW+ at one hapless FM80 and every panel at 10AM will be exceeding STC by a very large margin.../..

So do you actually direct 7kW at one FM80 or is it that you "can" do it?
Since the battery voltage limit for an FM is 60V then why would the 7kW not fry your FM80?

I could, however, I wouldn't. It would be insane to do so.

[raysun]

However, the charge controller is anything but linear internally.

It is a switcher at it's core.

Yeah, nitpicking here...

It makes figuring out its operational "safe zone" even more challenging.

To nitpick even further it is actually an AC voltage transformer at its core. A literal transformer, with inductively coupled windings.

On the input side is the MPPT microcontroller and sense circuits, and a DC to AC converter.

On the load side is a PWM voltage controller.

[sbrownian]

Without a schematic to look at..

(Or one to do some reverse engineering on...)

I would be somewhat surprised if it did a full DC-AC-DC conversion.

I was always under the impression that they were "buck" converters, mostly due to the requirement that the PV voltage input always has to be greater than the output voltage.

DC > input filtering > FET chopper > output filtering/integration > battery..

The input and output filters do have inductors and are used as energy storage devices, but generally not transformers, per se.

Not endorsing this outfit, but they have a quick overview of some of the main types of switch mode converters...

https://www.arrow.com/en/research-and-e ... converters

I am guessing that you are thinking the charge controllers use the transformer coupled synchronous type of configuration?

Edit: Also, because of the isolation advantage a transformer based system has?

[drstrip]

After I went to bed last night I realized I erred in using the Impp x Vmpp in suggesting the controller could handle 16kW. That gave you time to catch it first
I went to the spec sheet for RaySun's REC Alpha panels. At STC, the short circuit current is 10.43A for the 450W panels. In my naive interpretation of the specs (and RaySun will provide more knowledgeable input), this says that we can put 6 parallel strings. The Voc limit of the FM100 is 300V, and the same panels have a Voc of 52V at STC. Even at extremely cold temperatures, we're still within the spec with two panels in series. So based solely on the input criteria,we should be able to connect 6P x 2S for 12 panels, or 5.4kW. With a nominal 48V battery, that's not quite 10% over the rated output.

I went back to the MX60 manual and looked more closely. While the spec table doesn't list the input current limit, the text in the installation section says the controller can handle 60A input. A section that baffles me then says that to meet NEC requirements based on the 60A output, the input should be limited to 80% of that value, or 48A. That would allow a 4P x 2S configuration on my MX60s, or 3.6kW. The Voc at STC would be 104V, which at low temperatures is right at the 120Voc limit at -20°F.

Now I have to take some time and read RaySun's tutorial on electrical engineering.

[raysun]

which at low temperatures is right at the 120Voc limit at -20°F.

From the Outback spec sheet on the MX60: This charge controller will support up to 150 VDC open circuit voltage on the input of the solar array...

So designing to 120V(oc) at STC would leave headroom for the excursion into the -20F zone?

[drstrip]

which at low temperatures is right at the 120Voc limit at -20°F.

From the Outback spec sheet on the MX60: This charge controller will support up to 150 VDC open circuit voltage on the input of the solar array...

So designing to 120V(oc) at STC would leave headroom for the excursion into the -20F zone?

My hardcopy manual that came with the MX60 says 120V. I think they were later upgraded to 150Voc. Early in their life I fried one on a cold morning and they sent new parts (gratis) which were supposed to address the problem. I think I upgraded both controllers, but that was a long time ago and recollection is fuzzy.

[raysun]

Makes sense. They do have words in the manual about debating the design V(oc) for colder climates, but I believe that is to give headroom under the 150V absolute max limit.

[drstrip]

The Voc limit of the FM100 is 300V, and the same panels have a Voc of 52V at STC. Even at extremely cold temperatures, we're still within the spec with two panels in series. So based solely on the input criteria,we should be able to connect 6P x 2S for 12 panels, or 5.4kW. With a nominal 48V battery, that's not quite 10% over the rated output.

What was I thinking? With a 300Voc limit, derated to 250Voc to allow for temperature correction, we can have 6P x 5s, not 6P x 2s. That's 13.5kW, or about 250% of rated output at 50V. In spite of these numbers, the manual says 6kW input for 48V output.

Reading the manual for the FM60 and FM80, these have input limits of 48A and 64A, resp., which is an NEC derating from 60A and 80A resp. For the FM80, with 150Voc, we can configure 6P x 2S for 5,400W, which is 135% of rated output at 50V. The FM60 would accept 4P x 2S for 3600W for 120% of rated output. The manual lists 4000W/3200W at 50V, or 100% of rated output.

[raysun]

With a 300Voc limit, derated to 250Voc to allow for temperature correction, we can have 6P x 5s, not 6P x 2s. That's 13.5kW, or about 250% of rated output at 50V.

Hmmm.... the math still escapes me.

The V(oc) ratings are literally Voltage(into an Open Circuit). There is no current flow at V(oc). The 300V(oc) measure is the static voltage the equipment can withstand without damage.

When the PV arrays start conducting current, the panel voltage drops significantly, typically in half, so a 300V(oc) rated array is putting out around 150V when active.

I do not believe the high (13.5kW) array estimate is achievable on an FM100 (or any 100A controller) without damaging the unit.

V(oc) demonstration:

At 6:10AM, the sun has yet to rise above Mt. Kilauea, and the V(oc) of my 2S PV array is already fairly high.

Three minutes later, one array reaches the conduction threshold and the voltage drop is evident.

[drstrip]

Hmmm.... the math still escapes me.

well, it could be because the math is hosed. But here's what I'm doing.
There are two limits on the input to the controller, Voc and Isc.
As you note, these occur at very different conditions - Voc occurs at 0A, Isc occurs at 0V. The Voc determines how many panels we can put in series. For the FM100, that's 5 panels with Voc of 50V.
The Isc determines how many strings we can have in parallel. With a 64A Isc limit and 10.43A Isc on the panels, I come to 6 strings in parallel.

Thus, I conclude that I can simultaneously satisfy the Isc and Voc limits with a 6P x 5s configuration. As I noted, there may be other constraints at play, but that's what I come up with from the input limits.

[raysun]

The two values, V(oc) and I(sc) can be used for design limits, certainly, but not for calculating array wattage, as both represent a 0 Watt state.

Using the panels I have as the "test subject" we can see the V(oc) is 52V. Prudent design for a 150V(max) controller dictates a 2S array.

The I(sc) of 10.31A might suggest 7P, but in practice, its much too high when the battery voltage is considered. For my lithium battery, I can safely choose a nominal voltage of 50V. The specified maximum output current of the FM80 is 80A. The array power to drive the output to 80A is 80A x 50V = 4000W.

In practice, I initially tested the system with a 4400W array (2S-5P) figuring its only 10% over design limits, what could it hurt? Almost immediately the very intense sun drive the array to a power level that resulted in 94A out of the controller. That was too much for my comfort, so I backed the array down to 2S-4P. Even at 3520W array output reaches 4200W frequently, and 4604W being the maximum achieved in the 8 panel array.

V(oc) can be used to set maximum series strings.

I(sc) can be used to specify the wire gauge and circuit breaker size for each string in the combiner box.

Charge controller rated output in amps X nominal battery voltage dictates maximum array wattage.

[drstrip]

The two values, V(oc) and I(sc) can be used for design limits, certainly, but not for calculating array wattage, as both represent a 0 Watt state.

By using them to compute the design limits on string length and number of parallel strings, I computed the Pmpp at STC, which is what I was quoting.

Screenshot_20210702-064818_Chrome.jpg

Using the panels I have as the "test subject" we can see the V(oc) is 52V. Prudent design for a 150V(max) controller dictates a 2S array.

The I(sc) of 10.31A might suggest 7P,

This is the same as I was computing, but my numbers are slightly different since I used the 450W panel.

but in practice, its much too high when the battery voltage is considered. For my lithium battery, I can safely choose a nominal voltage of 50V. The specified maximum output current of the FM80 is 80A. The array power to drive the output to 80A is 80A x 50V = 4000W.

small quibble - the FM60/80 manual provides a graph of efficiency for a 24V battery setup. (No corresponding chart for 48V). For 100V input, which is almost on the mark for these panels at Vmpp in a 2S configuration, the efficiency is 95%. So it would take 4200W input to provide 4000W = 80A x 50V output.

In practice, I initially tested the system with a 4400W array (2S-5P) figuring its only 10% over design limits, what could it hurt? Almost immediately the very intense sun drive the array to a power level that resulted in 94A out of the controller. That was too much for my comfort, so I backed the array down to 2S-4P. Even at 3520W array output reaches 4200W frequently, and 4604W being the maximum achieved in the 8 panel array.

This is where my lack of electronics knowledge leaves me baffled. In my experiment with the MX60, the controller limited output current to the value I set in the controller. At the time of the experiment, the panels were providing enough power for the controller to output 42A. (This was determined by looking at the second MX60 with the same configuration of panels. The two controllers produce +/- 5% of each other.) I set the limit to 30A and the output dropped to 31.xx. Why would the FM80 output 94A when the limit is set to 80A? It has no issues holding output to lower levels in float mode.
Adding even more to my bafflement, if the FM80 were connected to a nominal 48V lead-acid battery, the absorb voltage would be more like 58V, which would mean an output of 4840W. With 95% efficiency, it would require 4872W at input. If it's not designed to handle this case, what does the 80A output rating mean?

None of this is intended to contradict your computations or experience. You clearly know a LOT more about this than I do. I'm just baffled at the behavior you observe. In addition, SandyP has something like 200% overpaneling on his system, though one set of panels faces east, not south. He's survived in this configuration for something like 3 years. He has his FM60 set to 55A. Is it possible that setting the output limit to 80A means "output everything you can", while setting it to 79A (or anything else) enables clipping to the limit?

[raysun]

small quibble - the FM60/80 manual provides a graph of efficiency for a 24V battery setup. (No corresponding chart for 48V). For 100V input, which is almost on the mark for these panels at Vmpp in a 2S configuration, the efficiency is 95%. So it would take 4200W input to provide 4000W = 80A x 50V output.

Others may take another tack, but I ignore conversion losses because:
#1 they are hard to compute,
#2 they generally are internal to the circuitry, so mean they are causing things to heat up.

Overdriving the input to make up for losses adds more power to the losses as well.

One should do what one thinks best. I tend to err on the conservative side.

The controllers will limit output to their current limit settings - eventually - but will overshoot them if the input is overdriven. If one watches closely, it can be seen, even under "normal" circumstances. Resonse to an increase in input power is not instantaneous.

the controller limited output current to the value I set in the controller

If the input is overdriven hard enough, the controller will actually lose control, and dump the raw input to the battery. I've seen the results of this with my own eyes, and fortunately for me, not to my battery. Let's just agree that feeding over 100V to a 48V battery can have catastrophic consequences.

I'm not exactly convinced Current Limit = 80A actually engages the current limiter. There are many "magic" patameter settings in OB gear that aren't commonly documented.

My favorite is all the activity behind the REBULK parameter.

Another is Absorb Time. Of course, a non-zero value for Absorb Time causes the controller to run the Absorb phase for that period of time.

So what does Absorb Time = 0H do?
Skip the Absorb phase, you say? Partially correct. However, it also causes the controller to skip the Bulk phase and go directly to Float when it starts a charge cycle.

With that in mind, I'd expect Current Limit of 79A to engage the current limiter function at or around 79A. Current Limit = 80A? I'm not so sure.