MPPT maximum voltage - why no safety margin?

Most electronic systems and parts, both consumer and professional, include hefty safety margins in their design and specifications:

Sometimes the true specifications are much more robust than the printed ones (for example, NEC's wire ampacity rules are quite conservative, so you can probably pull 40a from a 12 gauge circuit briefly without the wire melting).

Sometimes there are safety systems in the circuit, e.g. fuses, breakers, or BMSs which will shut down, e.g. if you try to draw 150a from a LiFePo with 100a maximum, the circuit may open but you won't cause any damage.

But, from what I've read, MPPT maximum voltages are a "never to exceed!!!" number with dire consequences (e.g. the device fails, perhaps melts and maybe catches on fire).

Human factors matter, and it seems to me that if most other specifications have a safety margin, people will naturally and reasonably get used to there being a general "safety margin" with most published specs.

Why not in this case?

Additional questions:
* are there other specifications which have no safety margin?
* since MPPTs are for Solar Panels, and Solar Panels have the property where their VOC can go UP in cold weather, is this simply a bad way to specify maximum voltages? E.g. maybe MPPTs should have to give their ratings for maximum VOC for a solar panel at 20,000 feet elevation, -40 temperature, and pointed directy at the sun?

soylentgreen said:

Most electronic systems and parts, both consumer and professional, include hefty safety margins in their design and specifications:

Sometimes the true specifications are much more robust than the printed ones (for example, NEC's wire ampacity rules are quite conservative, so you can probably pull 40a from a 12 gauge circuit briefly without the wire melting).

Sometimes there are safety systems in the circuit, e.g. fuses, breakers, or BMSs which will shut down, e.g. if you try to draw 150a from a LiFePo with 100a maximum, the circuit may open but you won't cause any damage.

But, from what I've read, MPPT maximum voltages are a "never to exceed!!!" number with dire consequences (e.g. the device fails, perhaps melts and maybe catches on fire).

Human factors matter, and it seems to me that if most other specifications have a safety margin, people will naturally and reasonably get used to there being a general "safety margin" with most published specs.

Why not in this case?

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Why does it matter? There probably is a little bit of a margin. Midnite has a "HyperVOC" mode where the chargers go dormant until they drop below safe Voc, but even the hyper mode has a never exceed limit.

soylentgreen said:

Additional questions:
* are there other specifications which have no safety margin?

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Yes. Most of them. A 500A shunt shouldn't even be run at 500A. 400A max continuous is recommended. Many electronic components are actually "run something less than rated."

soylentgreen said:

* since MPPTs are for Solar Panels, and Solar Panels have the property where their VOC can go UP in cold weather, is this simply a bad way to specify maximum voltages?

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No.

soylentgreen said:

E.g. maybe MPPTs should have to give their ratings for maximum VOC for a solar panel at 20,000 feet elevation, -40 temperature, and pointed directy at the sun?

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Why should specifications be for something that 99.99999% of the applications can't apply? That's just insane.

Solar isn't a dude changing his oil in the driveway. It requires a design process and a certain level of knowledge. There's a reason why there's a national electric code and licensed electricians. This isn't trivial stuff. You can't expect an industry to cater to the ignoramus.

Well, the Sol-Ark 15K specs are 125-425V MPPT Voltage Range, 500 max and the manual also says “Damage will occur if PV VOC > 550V”

Danke said:

We, the Sol-Ark 15K specs are 125-425V MPPT Voltage Range, 500 max and the manual also says “Damage will occur if PV VOC > 550V”

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That's Pretty Plainly Written And EZ to Understand. Wish More Mfr's Would Be As Clear.
I called Renogy Tech / Product support with the same VOC questions abt my 100a MPPT. The tech was vy helpful and easy to communicate with .
He listened and answered all my questions... Renogy may be a tad pricey but I've not been unhappy with anything I've bought from them...

soylentgreen said:

Why not in this case?

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The main reason is that the higher the voltage a MOSFET can handle the higher its Rdson is, and so the lower the efficiency is (assuming the same component cost). And the costs increases exponentially when you go for lower Rdson.

I think the main reason is, there are too many options for matching components, and consequently far too many combinations for manufacturers to test. The only thing they can do is to tell you what the design limits are and let whoever is putting the system together ensure whatever random components they throw together will play nice.

Solar panels have a defined STC limit, and a defined temperature correction curve. At one point I got an, IIRC, 25% recommended headroom on Voc input from Outback. That "recommendation" could maybe be a little clearer in the manual, but it's not really needed, IMHO. Anyone who can be trusted to spec out a system and assemble it can look at a datasheet and decide how close to limits they're comfortable running.

For those who aren't competent to do this, there's always the "Up in Smoke" section, hopefully as a reminder to check their skill level before connecting wires, but if not, maybe they can at least save someone else.

Yes, it's a lot of numbers, but they are all essential. So be it. Learn them, and be grateful for the skill so generously shared by so many members here to help ensure you get it right enough the first time to avoid letting out the magic smoke.

I have a question I've been meaning to ask for some time: why use Voc?
I mean, once the panels are connected to the controller, they're not going to be open circuit, are they?
I thought, maybe when the battery reaches maximum voltage, PV input goes to Voc. But I measured the voltage at the controller's input, charging 0A as the battery is fully charged, 43.2V, when Voc is 48.5. So it must be for some other reason. Which?
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Don B. Cilly said:

I have a question I've been meaning to ask for some time: why use Voc?
I mean, once the panels are connected to the controller, they're not going to be open circuit, are they?

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They will until they either start drawing current or heat up.

What happens if a BMS cuts off charge due to OVP or low temperature protection?

Well, warm they (probably) were - even though it is February.
But current they were not drawing.
Unless it's the minimal current drawn by the SCC itself. Zero amps.
So whether it's the BMS or the controller itself, I don't see any open circuits.
If I open the circuit, they're not connected to anything, are they?
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An MPPT algorithm, AIUI, will typically scan from open circuit to short (perhaps only as far as the controller is rated for if you've overpaneled there), looking for where the watts out is highest. So yes, you do need the SCC to be rated for highest potential Voc.

Rated for highest Isc is prudent, but there's less chance of a magic smoke emitting event from exceeding it a little.

Don B. Cilly said:

why use Voc?

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Because it’s a number at a given standard temperature and elevation so that all panels can be compared evenly to each as well as for planning purposes.
Because the silicon chips all have the same relatively linear behaviors you can calculate the maximum voltage (VOC maximum) and thereby any panel can be safely configured for the appropriate equipment and its environment.

The manufacturer’s didn’t make up VOC. It was the best way to determine output nomenclature in a fashion that doesn’t leave one asking “what do you mean by maximum? Maximum static at STP? Or temperature compensated voltage?”

It’s not like “music power” -rated speakers that advertise at 300W or 250W or 500W and nobody can be sure what that means as compared to “X”-watts RMS at 8 Ohms for example.
Watts RMS at “X” ohms sortof gives an idea of what to expect for decibels and what characteristics. That is the standard for audio- not max power, not music power, not “300Watts volume” I saw once.

VOC is the standard to let you know how to select equipment for using whatever solar panel. Like kilowatt, horsepower, BTU, or torque.

soylentgreen said:

Most electronic systems and parts, both consumer and professional, include hefty safety margins in their design and specifications:

Sometimes the true specifications are much more robust than the printed ones (for example, NEC's wire ampacity rules are quite conservative, so you can probably pull 40a from a 12 gauge circuit briefly without the wire melting).

Sometimes there are safety systems in the circuit, e.g. fuses, breakers, or BMSs which will shut down, e.g. if you try to draw 150a from a LiFePo with 100a maximum, the circuit may open but you won't cause any damage.

But, from what I've read, MPPT maximum voltages are a "never to exceed!!!" number with dire consequences (e.g. the device fails, perhaps melts and maybe catches on fire).

Human factors matter, and it seems to me that if most other specifications have a safety margin, people will naturally and reasonably get used to there being a general "safety margin" with most published specs.

Why not in this case?

Additional questions:
* are there other specifications which have no safety margin?
* since MPPTs are for Solar Panels, and Solar Panels have the property where their VOC can go UP in cold weather, is this simply a bad way to specify maximum voltages? E.g. maybe MPPTs should have to give their ratings for maximum VOC for a solar panel at 20,000 feet elevation, -40 temperature, and pointed directy at the sun?

Click to expand...

Because they are competing with other companies that do not have to abide the the safety margins either, so each manufacturer will state the absolute limits to showcase their products in the best possible light.

Personally I try to get efficient and quality products that do not require active cooling at the power levels I want to operate them at. Heat (at least in summer) is waste that needs to be removed somehow either through cooling by opening a window, or an air conditioner, the latter would be a waste of even more energy.

Don B. Cilly said:

So whether it's the BMS or the controller itself, I don't see any open circuits.
If I open the circuit, they're not connected to anything, are they?

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When the MOSFET is off then it's basically an open circuit, like opening a switch.

In my mind how close the 'spec' is to the 'limit' depends on a bunch of things, but one of the major factors is whether it's a major cross-shopping item that's going to have a huge impact on a customer's decision on one product vs another.

For example, if Ferrari says a car can do 243mph, you can be pretty sure that under the same conditions it will not go faster than that. It may go slower, but not faster! Even though it's nearly irrelevant to real usage it is a spec that may be very influential to 'new money' buyers that are buying the thing purely for bragging rights and don't give a crap about 10 or 15 other more relevant specs.

I teach automotive students about tires and explain that you really can't trust any tire manufacturer's statements about at what age a tire should be replaced, which in my opinion is at about 5-6 years. If one of them takes the high road and says 'a tire's tractive performance falls off severely after 6 years, therefore you should replace them at that point regardless of tread depth etc' then some other less scrupulous tire manufacturer is going to run an ad saying 'our tires last 70% longer than leading competitors!' just because they 'claim' their tires can go 10 years with no data as to how they come to that conclusion.

So manufacturers have to protect themselves both from customers and competitors even if how those groups are using the information you give them is incorrect, immoral, whatever. You still have to hedge. In the case of MPPT operating voltages if someone is buying a high-voltage mppt they are probably buying it primarily for that spec. I would guess if you are talking <150v input limits there is less sensitivity from the customer to slight differences in max input voltage at this point. But they will still correlate to commonly available electronic component ratings, because MPPT manufacturers are still just pulling available parts from the open market, so if there are for example 450v rated caps and 500v rated caps and nothing in between, you are unlikely to see an mppt rated 470. Hope that makes sense..

Vigo said:

really can't trust any tire manufacturer's statements about at what age a tire should be replaced, which in my opinion is at about 5-6 years

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I’ve had a 1971 CB350F at 92mph on early 1980s tube tires on spoke wheels…?

Someone brought me a cb350 with last tags from 79 that had been sitting in a a garage so long a lot of it is seized up solid.. It MAY have original tires.. im jealous you actually got to ride one because ‘mine’ would probably take 200hrs of work to get to the first ride.. ??

Don B. Cilly said:

once the panels are connected to the controller, they're not going to be open circuit, are they?

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It can be danged close when an MPPT needs to remove its foot from the gas pedal. Like when big user loads are removed in late Absorption or Float, or when transitioning down to Float voltage from Absorption voltage. Then factor in perfect storm scenarios that raise Vmp/Voc above rated spec like extreme altitude or extreme cold.

Example: my array's rated Voc is 112.8v. Controller data shows this week Vpanel typically rises to ~109v during the Float transition and other sudden demand drops.

Edited to add this digression: Kawasaki currently makes a Ninja zx25r 250cc inline four.

Perhaps it is just me but when I see the word "maximum" I take it to mean maximum. Most of these units give a operation range for PV voltage. If you stay within it you have sufficient safety margin.

Bit like the concept of speed limit does not mean you think it is a minimum speed you have to drive.

Mattb4 said:

Perhaps it is just me but when I see the word "maximum" I take it to mean maximum. Most of these units give a operation range for PV voltage.

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Culturally there’s a thing, too.

40 years ago the colloquial use of, “I gave it 110%” and student’s graduating high school couldn’t have a 103% grade average. 100% was everything possible.

When you over-drove audio speakers at “150%” you knew not to actually input a wattage to achieve a volume level that put more than the rated speaker watts RMS or you’d blow the cone or fuse the voice coil- 100% was 100%.

Now today overclocking CPU’s or having menu-screen options in your 500hp glitter car to make 700hp or even spending 130% of net income is culturally “normal.” Plus, generations under, say, age 40, will say they are “giving it 110%” but then talk about all their game of thrones and other TV shoes they watch and I wonder, “where do they get the time?!” When I was “giving it 100%” in 2021 I drove 65k miles and was working 7 or 8am and getting home 9pm six (and occasionally 7) days/week.
So we as a culture think 66-2/3% is 110% and know ‘there’s more’ so …

Because the author of the spec sheet saw all the components had a 150 volt rating, so they put 150 volts in the spec sheet. It's simple.

No promises from anyone that it won't blow up at 150.1 volts.

They aren't lying and many aren't paying extra for the cushion, but you can. As already mentioned, Midnight Solar (and others) have features where you can go above the voltage limit. But those are higher end products where engineers have spent extra time designing and specify internal components. VS many import products that are built down to a price.