Any problem MPPT charge controllers?

fafrd

Solar Addict
Microinverters can’t be harmed by overdriving with higher-powered panels than they are rated for. The MPPT just sarurates during the highest-power part of the day and potential power is lost (by the Microinverter driving the panel at a higher voltage than Vmppt).

I assume the same is true of MPPT charge controllers but want to make sure.

I’m planning to connect 1.1kW of panels to a 30A MPPT charge controller rated for a maximum power output of 780W when charging a 24V battery.

I typically max out at ~75% of peak panel output, so 1.1kW of theoretical peak output should translate to ~825W of actual peak output.

I’ll lose about 30W to wiring losses (3.4%) meaning about 795W will be available to the charge controller.

The charge controller maxes out at 780W, meaning it will decrease voltage a bit below Vmppt to decrease total charge power to 780W and ~2% of the potential power output from the panels will be wasted (again, only during the brightest peak-output part of the day).

Do I have this correct? Is this how MPPT charge controllers throttle-back PV power output (either because they have maxed out or the battery is charged an no more power is needed).

The only thing I’m a bit confused about is that current only gets driven to zero by driving voltage to Voc, so perhaps MPPTs throttle by increasing voltage (and decreasing current) rather than decreasing voltage (and increasing current)?
 

Luthj

Photon Sorcerer
The majority of MPPT solar controllers cannot be damaged by modestly overpaneling them with regards to wattage. They are output stage limited, and thus can't exceed their rated capacity. Voc however must be limited. As mentioned below though, you need to read the tech manual for the controller in question.
 
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FilterGuy

What, me worry?
First..... It is dangerous to make a blanket statement about 'all' MPPT controllers. The Victron mppt controllers will just not use extra power from the panels (as long as you don't exceed the rated voltage...Most are like the Victron, but the next brand might have other constraints. You have to read the manual of the controller to know what it's limitations are.

As the battery gets toward the top of it's charge, the MPPT controller will start reducing the current it allows through from the panels in order to keep the voltage at the battery from going up. As the current from the PV is reduced by the MPPT controller, the solar panel will start raising the voltage. (This means it is no longer operating at the peak power point.) When the battery is fully charged, there will be no current flowing and the panels will see it as an open-circuit... so the voltage from the panels will be at Voc.

BTW: The Voc rating is at a very specific temperature. If the temperature drops, Voc can go up. In extreme cold Voc can go up by 25%.
 

snoobler

Solar Honey Badger
Panels are a power source. The MPPT is effectively a "smart" variable load. Loads pull amps @ volts. Sources may vary current and voltage according to the load.

MPPT acts as a load pulling current and the panel reacts accordingly. If too much current is pulled, voltage drops. It varies the load until the maximum power is achieved. The only thing the MPPT can control is the current it pulls, so the regulation of panel voltage is done by varying the load.

Different MPPT may react differently to overpaneling. Victron commonly recommends up to 30% over paneling in their online calculators; however, they two hard limits: 1) Panel Voc must not exceed PV input rating and 2) Panel current must not exceed PV input current limit. Midnite solar never recommends more than 20% overpaneling.

My Victron 250/100 has a 5800W limit at 48V and PV input current limit of 70A. Thus, let's say I used 5X 72 cell panels (47Voc/36Vmp) in series, and I could parallel exactly enough of them to equal 69.9A max PV current input... 36V*5 * 69.9A = 12,582W = over 2X what the controller can push to a 48V battery.

Important to check the specs on any MPPT you are considering.
 

fafrd

Solar Addict
Panels are a power source. The MPPT is effectively a "smart" variable load. Loads pull amps @ volts. Sources may vary current and voltage according to the load.

MPPT acts as a load pulling current and the panel reacts accordingly. If too much current is pulled, voltage drops. It varies the load until the maximum power is achieved. The only thing the MPPT can control is the current it pulls, so the regulation of panel voltage is done by varying the load.
Got it - thanks. So throttling is done by reducing current consumed, not my modulating voltage. And so when there is more current available than the MPPT needs (whether because it’s maxed out or because the battery is approaching ful charge), MPPT refuses current consumed and so PV panels increase voltage towards Voc until current output matches MPPTs current consumption.

Different MPPT may react differently to overpaneling. Victron commonly recommends up to 30% over paneling in their online calculators; however, they two hard limits: 1) Panel Voc must not exceed PV input rating and 2) Panel current must not exceed PV input current limit. Midnite solar never recommends more than 20% overpaneling.

For voltage limits and Voc (as well as worst-case Voc at lowest annual temps), I get it; no need to explain further.

For PV input current limit, I’m confused.

first, I’ve never seen a spec for input current limit. The current specs I’ve seen on MPPTs are maximum output currents, not input currents.

second, we just clarified above how MPPT can throttle-back input current (PV output current) all the way to zero.

So other than the MPPTs MPPT ‘range’ extending all the way past the worst-case Voc (where PV output current will drop to 0A), I’m confused about ‘MPPT input current limit.’

My Victron 250/100 has a 5800W limit at 48V and PV input current limit of 70A. Thus, let's say I used 5X 72 cell panels (47Voc/36Vmp) in series, and I could parallel exactly enough of them to equal 69.9A max PV current input... 36V*5 * 69.9A = 12,582W = over 2X what the controller can push to a 48V battery.

Important to check the specs on any MPPT you are considering.
I just looked up your 250/100 and there is is in black and white: maximum PV Short Circuit Current 70A.

For a charger limited to 100A of charge current, that means PV incoming voltage must be at least 150% of battery voltage.

First time I’ve seen that spec but I will circle -back and read my spec sheets more carefully...
 

snoobler

Solar Honey Badger
The Bluetti actually specifies a maximum PV current limit, but it does not indicate any special concern about it. Victron points it out and indicates the safety systems in the controller can only handle the 70A or they may not operate properly.

PV Voltage >= 150% of battery voltage is actually the sweet spot for MPPT efficiency. As you drive PV voltage higher than the battery, slight efficiency penalties are introduced; HOWEVER, those are often offset by reduced % voltage drop.
 

fafrd

Solar Addict
Here is the manual for my 30A MPPThttps://hqsolarpower.com/content/HCC30MPPT-G1-Manual.pdf

It has no spec for maximum PV current. Maximum charge current only.

But it does have a spec for ‘Maximum Input Power’ which may translate to the same thing (and is the spec that motivated me to start this thread to begin with).

So I’m at a loss as to how to interpret that. It that is a maximum on the amount of input power the MPPT can accept and anything over that level it will saturate and throttle back (like my microinverters do), I get it.

But if that is intended to mean that input power should never exceed 780W, I’m lost and don’t know what to do;

Limiting array power to 780W is safe but then I’ll never get more than ~585W in and ~24A out.

Using my local experience to take 75% of my array size as peak power will give me the full charge output of the MPPT during the peak part of the day, and will be under 780W 99.9% of the time, but is not guaranteed. If it’s not self-limiting and can damage the MPPT, it’s only a matter of time.

And if the MPPT can limit actual input power to 780W, then what’s the harm in exceeding that by 10% or 20%?
 

fafrd

Solar Addict
The Bluetti actually specifies a maximum PV current limit, but it does not indicate any special concern about it. Victron points it out and indicates the safety systems in the controller can only handle the 70A or they may not operate properly.

PV Voltage >= 150% of battery voltage is actually the sweet spot for MPPT efficiency. As you drive PV voltage higher than the battery, slight efficiency penalties are introduced; HOWEVER, those are often offset by reduced % voltage drop.
Ok, so if the 70A incoming current is merely a limit and not a problem if (potentially) exceeded, I understand (and hopefully my MPPTs maximum power is similar - this is how microinverters function).

And this would mean there is no danger in having an array that would deliver more current if it could. The panels respond to what the MPPT draws and if the MPPT stops drawing additional current at 70A, the PV array will put out 70A (at whatever voltage that corresponds to).
 

snoobler

Solar Honey Badger
I would not exceed the PV input current limit on a Victron, but I would consider it on other brands.

I would suspect that your controller is just establishing the peak performance of the SCC, but it is a concern.

I would hesitate to deploy a larger array without confirmation from the manufacturer.

I also don't rely on an assumption of <100% power. I've logged 105% rated output, and I routinely get 90%, but I am at 6800 ft, so less atmosphere between the sun and panels.
 

fafrd

Solar Addict
The majority of MPPT solar controllers cannot be damaged by modestly overpaneling them with regards to wattage. They are output stage limited, and thus can't exceed their rated capacity. Voc however must be limited. As mentioned below though, you need to read the tech manual for the controller in question.
For Voc, I understand. The MPPT must be able to handle input voltages all the way up to Voc so it can throttle-back current input from the PV array all the way to 0A.

The overpowering ‘modestly’ I understand if that is merely an attempt to prevent customers from needlessly wasting potential PV output energy, but if there is more to it than that, ie: if it can harm an MPPT charge controller to be powered by a PV array with 200% MPPT power, I’m still confused...
 

Luthj

Photon Sorcerer
Many manuals are not clear on this point, so getting in contact with tech support engineering is best. For example several of the Outback controllers I have worked with have no PV wattage limits. If you hit the output current limit, the controller simply reduces the load on the panels.

There is no single design used for MPPT controllers, so absolute/blanket conclusions don't exist.

Voc actually has more to due with the transistors used. Breakdown at high voltages will destroy them.
 

fafrd

Solar Addict
Many manuals are not clear on this point, so getting in contact with tech support engineering is best. For example several of the Outback controllers I have worked with have no PV wattage limits. If you hit the output current limit, the controller simply reduces the load on the panels.

There is no single design used for MPPT controllers, so absolute/blanket conclusions don't exist.
That is how my microinverters handle it and it’s the assumption I made when I purchased this little 30A MPPT.

If it doesn’t behave that way, it’s honestly far less useful to me.

Reading the manual more carefully, in describing panel connection it says this: ‘Ensure the PV current does not exceed the maximum permissible current’ (without any specification for what that current is).

So putting together what Victron wrote regarding Isc and this statement, I’m going to hazard a guess that connecting panels in daylight can cause damage if Isc is too high. Assuming these MPPTs start ‘open’ for current meaning a virtual ground, if the panels are getting irradiated when connected, they will all see ground and put out Isc. If there is an upper limit on the input circuit protection of 70A or 780W/24V =32.5A and Isc inrush exceeds that, I can see that being a problem.

I believe my Microinverters required connecting at night or covering panels with a sheet first (so Isc inrush could have also been a problem there).

In normal morning operation, PV current will never approach Isc as the the panels wake up and once the MPPT is actively throttling back current input and is no longer ‘open’ it should be a non-issue as well.

So I think I understand the issue and it’s only likely relevant when connecting PV array to an operating MPPT controller during the day (including closing a breaker).

Since Vbattery + 1V is Vmppt min for this controller, an empty 8S battery at 20V would represent a minimum ‘open’ voltage of 21V. So if 21V x Isc is under 780W, I should be safe (meaning Isc under 37A).

The 1.1kW array I’m considering has a total (worst-case @ 40C) Isc of 24.1A, so I think I should be good.

To exceed 780W with 24.1A, I’d need a PV voltage of 32.4V (a bit below Vmppt).

So I think I’m going to go ahead, just being careful when connecting during the day. If I end up fritzing this little unit, it’s not the end of the world (picked it up p on closeout for under $50).
Voc actually has more to due with the transistors used. Breakdown at high voltages will destroy them.

Yeah, input voltage beyond rated maximum can kill transistors. But there must also be some damage that can be caused by inrush current being too high when the PV array is first connected (from heat or who know what).

Isc x Vmppt-minimum should be under rated maximum power (or Isc should be under rated maximum input current) to be safe.

What do you guys think?
 

Hedges

I See Electromagnetic Fields!
This charge controller specifies max 40 A PV current:


This string inverter specifies maximum operating input current 10A per MPPT, maximum short circuit current 18A.


I think both of these have a diode to protect against reverse PV polarity; that would have a limit.
Their algorithms for finding MPPT would ideally back off before exceeding a current which could be harmful, but maybe not?

When you over-panel with 1.1 kW, if you orient some strings different from others, e.g. morning sun and afternoon sun, that would broaden the time you get power and reduce the peak, maybe enough no clipping occurs.
 

fafrd

Solar Addict
This charge controller specifies max 40 A PV current:


This string inverter specifies maximum operating input current 10A per MPPT, maximum short circuit current 18A.


I think both of these have a diode to protect against reverse PV polarity; that would have a limit.
Their algorithms for finding MPPT would ideally back off before exceeding a current which could be harmful, but maybe not?

I think a Protection diode is likely.

The second inverter with a 44% between maximum short circuit current and maximum operating current is almost certainly indicating a protection limit of 18A and a heat-related limit of 10A which probably translates to a power limit above which current will not be drawn in operation.

My little charge controller has that statement about ‘ensuring current does not exceed maximum permissible current’ without any specification for what that maximum is but reading through the whole thing again, I found more tidbits:

“With cuttent-limiting charging mode, when the power of PV panel is oversized and charging current exceeds the rated current, the controller will lower the charging power, which enables the system to function under the rated charging current.”

and “Full automatic electronic protect function.”

And finally, in the section on Protections, I found this:

“PV Overcurrent: The controller will limit charging power in rated charge power. Any oversized PV array will not operate at maximum power point.”

So reading this, I’m pretty certain that my charge controller behaves as I hoped and I can safely oversize the array beyond the 780W maximum.
When you over-panel with 1.1 kW, if you orient some strings different from others, e.g. morning sun and afternoon sun, that would broaden the time you get power and reduce the peak, maybe enough no clipping occurs.

Good ideas but I’m really not concerned about clipping. Wasting a bit of potential energy during the peak 1-2 hours of the day is not a big deal to me for this build. My existing grid-tie system using the West-facing slope of the roof rarely reaches 75% of rated power and these panels will be on the East-facing slope where power will likely be even less than that.

My bigger concern was whether I could connect 3 panels to this little charge controller without damaging it since 2 panels would be safe but below my target power generation. With 3 panels I’ll be safely over my generation target and may even be able to largely pay for the 3rd panel by running 8AWG instead of 6AWG for the home run.

I went through this whole analysis of avoiding clipping for my Microinverter-based system and ultimately decided it was more economical to oversize and not obsess over clipping. In the end, the actual PV performance translates to pretty much no clipping (so thank the lord I didn’t pay up for more powerful microinverters or downsize my panels).

I just have no experience with charge controllers and would prefer not to needlessly damage one if I can. But now that I’m pretty convinced that it clips like my microinverters, a 1.1kW array for this little 780W MPPT seems like the right design decision.
 

rickst29

Solar Addict
Generally speaking, all MPPT Solar Controllers should fall into "PWM" mode on the input side - limiting maximum Solar Power Input, when they can't use all the output power from the PV Array. They separately have maximum Voltage input, and the power limit is also related to avoiding stress on the coil-or-mostly-electronic input inductor. My own "30 Amp" RV Solar Controller can deliver a maximum of only 440 Watts into my LFP battery string. But I could easily connect up to "600 Watts", maybe even "700 Watts" of panels if my battery string wasn't already being fully charged, by mid-afternoon or even earlier, by the "490 Watts" I already have.

At high altitude on a perfect and cool day (think Yosemite Tuolumne Meadows at the start of July), my nominal "490 watts" of panels can already generate quite a bit more than 500 watts near Solar Noon. My batteries and loads can consume the entire "30A" of the SCC, but the SCC is being subjected to more power. It simply falls into PWM mode on the Solar side, leaving unwanted power in the panels and unused for a significant portion of each second. The bigger than panel array, in comparison to SCC current capabilities and battery demand, the higher the proportion of disconnected time.

MPPT Solar Controllers, with a wide variety of designs and hardware, have varying abilities to reject high PV disconnected Voltage and switch their PWM logic circuits. (My own SCC is rated for 150 Volts maximum Array Voltage, but I wouldn't want to go anywhere near that - and my array is currently about 80 Volts Disconnected under strong sun, 64 Volts at maximum MPPT power). The best ones can handle 'nominal' arrays with twice their rated output power - and in a few cases, even more. I'm a bit surprised that your 'Sunny Boy' quotes only 630 Watts PV on a 40 controller, That SHOULD be about the maximum power which it accepts, but should not be maximum power which could be made available.

Is there a reason why the battery bank must be limited to only 40A output current? If it could handle more, then you could go bigger on the MPPT (40A models with decent performance a very common), while leaving the Array at 1.1 kW. They can ALL handle substantially more output current than their input current max values, that's what MMPT is all about.
 
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fafrd

Solar Addict
Generally speaking, all MPPT Solar Controllers should fall into "PWM" mode on the input side - limiting maximum Solar Power Input, when they can't use all the output power from the PV Array. They separately have maximum Voltage input, and the power limit is also related to avoiding stress on the coil-or-mostly-electronic input inductor. My own "30 Amp" RV Solar Controller can deliver a maximum of only 440 Watts into my LFP battery string. But I could easily connect up to "600 Watts", maybe even "700 Watts" of panels if my battery string wasn't already being fully charged, by mid-afternoon or even earlier, by the "490 Watts" I already have.

At high altitude on a perfect and cool day (think Yosemite Tuolumne Meadows at the start of July), my nominal "490 watts" of panels can already generate quite a bit more than 500 watts near Solar Noon. My batteries and loads can consume the entire "30A" of the SCC, but the SCC is being subjected to more power. It simply falls into PWM mode on the Solar side, leaving unwanted power in the panels and unused for a significant portion of each second. The bigger than panel array, in comparison to SCC current capabilities and battery demand, the higher the proportion of disconnected time.

MPPT Solar Controllers, with a wide variety of designs and hardware, have varying abilities to reject high PV disconnected Voltage and switch their PWM logic circuits. (My own SCC is rated for 150 Volts maximum Array Voltage, but I wouldn't want to go anywhere near that - and my array is currently about 80 Volts Disconnected under strong sun, 64 Volts at maximum MPPT power). The best ones can handle 'nominal' arrays with twice their rated output power - and in a few cases, even more. When connected to a big array, they're EXPECTED to fall into a mixed PWM/MPPT mode near Solar Noon, but remain operating at nearly maximum power for a longer period of time surrounding Solar Noon. The excess power from the Array, if show on a graph of "output power" versus "time of day" (on the X-axis), simply gets left behind within the panels after a maximum MPPT Solar Input Power level has been reached, or the MPPT no longer desires to absorb it all.

I’m sorry, but this is nonsense. MPPT charge controllers have no ‘PWM mode’.

When available solar input power exceeds the maximum power rating or battery nears full and the MPPT switches to CV mode, it raises the voltage going to the panels to reduce incoming current and power.

At the extreme, it puts our voltage of Voc and cuts current to zero (and incoming power with it).
 

rickst29

Solar Addict
I’m sorry, but this is nonsense. MPPT charge controllers have no ‘PWM mode’.

When available solar input power exceeds the maximum power rating or battery nears full and the MPPT switches to CV mode, it raises the voltage going to the panels to reduce incoming current and power.

At the extreme, it puts our voltage of Voc and cuts current to zero (and incoming power with it).
Are you sure about that? Here's some chatter from a guy who seems to at least sort of know about these things: https://www.solarpaneltalk.com/foru...r-behaving-as-pwm-of-late?p=372209#post372209

Maybe newer designs work the way which you describe, but my older "big iron" MPPT definitely switched over to PWM mode. (It was build in Oregon, and its builder no longer attempts to complete with more modern product designs.)
 

fafrd

Solar Addict
Are you sure about that? Here's some chatter from a guy who seems to at least sort of know about these things: https://www.solarpaneltalk.com/foru...r-behaving-as-pwm-of-late?p=372209#post372209

Maybe newer designs work the way which you describe, but my older "big iron" MPPT definitely switched over to PWM mode. (It was build in Oregon, and its builder no longer attempts to complete with more modern product designs.)

I suppose what the mode is called is somewhat immaterial.

Once boost charging is complete and the MPPT switches to Constant Voltage mode, it will continue to deliver whatever current is required to maintain charge voltage.

If available power at battery voltage is below the power needed to maintain float voltage, I assume the MPPT is going to maintain a voltage that maintains power level above that.

I suppose the easy way to check is to throw a scope on the charge signal after he charger has switched to CV mode...
 
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time2roll

Photon Sorcerer
My Morningstar controller specifically says some overdrive is OK. Power will be limited to the output specification. I would encourage 20% to 30% overdrive especially if the panels are less than optimal mount. Such as flat on an RV roof.
 

Herb

New Member
This is my personal experience with the Victron charge controller. I tested it with higher than rated voltage and it just restarts, once I lowered the voltage it worked fine, it's been 3 years and it works fine. I would not recommend anyone to do that but it seems like they have over voltage protection built in. I visited their board and there is a pretty good discussion about that specific topic.
 
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