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diy solar

Why is mppt better than pwm?

Horsefly said:
I have to admit, if I came here looking for a simple answer to the question posed in the title, most of the posts here are of absolutely no use. Sigh....
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In fact, you have got hundreds of answers, most of them containing wrong beliefs.
There is no really simple and unique answer to the question_ what is better PWM or MPPT, except one: it depends !

First is depends on your panel and your battery.
Low or mid power panels (up to 120W) have MPP voltages around 18 V.
Higher power panels have 36V (rarely 30V).
There is a very reason for the panel's MPP to be almost always multiples of 18 V.
This reason is: they are optimised for PWM use with 12 / 24V batteries.

So is the next question to come is: if they are optimised for use with 12/24 volt batteries, why isn't their MPPT voltage closer to the the battery voltage? Basically, there would be absolutely no problem to design a panel with a MPP of 16V...

The ~18V / 36V standard voltage for these panels has been carefully chosen, believe me !

First of all, MPP is not a constant. The label MPP is for a high irradiance of 1000W/m2 and 25°C, which rarely happen together.
Under the low radiance or high temperatures the MPPT can be considerably lower.
And low radiance or high temperatures happen quite often !
Then we have to consider some potential losses in the power leads we need the power at the battery, not at the panels.

Finally a Lead-Acid battery must from time to time be driven into equalisation with voltages up to 15.5V.
So definitely 18V MPP panels are ideal for PWM-operation with 12V batteries and 36V MPP panels are ideal for PWM-Operation with 24V batteries. This is the proven experience of millions of panels over 30 years of practical usage in the field.

But isn't a MPPT controller with a 18V MPP panel with 12V batteries better?
Mostly not.

A MPPT controller needs a good voltage headroom above the battery voltage to be able to function correctly.
A few volts will not do !
The advantage of tracking MPPT over just 2-4V over the battery voltage will never offset the increased system losses of a MPPT Solar controller.

So, if you have a 36V MPP solar panel to feed a 12V battery, go for MPPT.
If you have a 36V MPP solar panel to feed a 24V battery, go for PWM.
If you have a 18V MPP solar panel to feed a 12V battery, go for PWM.
If you have a 24V battery and want MPPT, you will need at least two 36V MPP solar panels and a controller that runs at 80V input or more.

The other aspect to consider is: what are you doing with your solar power?
If you use it off-grid and want a continuous operation, you must design your system for the worst conditions.
These are low irradiance only a few hours a day. Under these conditions PWM works is its best with the 18V panels for 12V batteries. MPPT would only be better, if you had a 36 volts panel. If you have a system, which is dimensioned for those worst-case conditions, you have a pretty good chances to end up with the battery fully loaded before 12 o'clock on sunny days, so you will be throwing energy away the rest of the time.
So it does does not really matter, if your PWM controler is not harvesting the last possible 5%.

Of course if you are grid-connected and feed the power back to the energy supplier, the figure is completely different and there is no other way than using MPPT.

So, you see, the simple question has no simple answer.
 
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rin67630 said:
you will be throwing energy away the rest of the time.
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Ya, the increasing number of over-built solar installations every year is wreaking havoc with how much waste sunlight is contributing to global warming.
timselectric said:
In any condition, in my opinion.
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Wellllllll there is the qualifier you need enough series panels or single panel voltage on the sucky days. But i do get what you’re saying :)
 
12VoltInstalls said:
A) why would cheap batteries mean replacing more often with pwm?!
B) the pwm I used has a remote temperature sensor…

PWM worked well for me for several years.
Blanket statements often don’t benefit anything <
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I did not say cheap batteries need to be replaced more often. I said that, at least with the cheap pwm that I was using, it damaged my battery slightly, and shortened it's life. Therefore, I would not use the same cheap pwm with expensive batteries. The cc that I am talking about was a basic starter kit device. A precursor to the Renogy Wanderer. No temperature probe. No programmability. Simple plug and go. I will stand corrected on my statement. It is more common to find pwm controllers that have fewer bells and whistles, but I will admit that I did not research them much, and simply replaced my own pwm with an mppt that was more programmable and had temp comp when my array became too large for the Wanderer precursor.
 
So what you are saying is, it was the lack of features and programmability that was what lead to a problem with your battery.
Undoubtedly true.
Every controller that I know of uses pulse width modulation, even those that have temperature compensation and power point tracking and programmable voltages.

Adding or deleting these "extra" features, has nothing at all to do with pulse width modulation, they all use pwm.
 
Substrate said:
You are absolutely correct. I guess the main point is that by not knowing that MPPT is efficiency tracking algorithm incorporated into a pwm controller, it makes it much easier for sales departments to prey upon a consumer's ignorance, and accept labels, rather than the science as being "better" as to which to purchase.

"Look - there's no need to buy a Victron! We're mppt too! And we're certainly not pwm! "

The smart one knows to ask "Ok, since I know mppt is an algorithm to modify the pwm technique for maximum efficiency - show me YOUR algorithm!" Salesman gulps - because all he knows is labeling, and not the science.

In the past, the same was done to early pwm controllers that didn't incoporate any mppt algorithm

"Look, we put PWM on the case - we're just like Morningstar! And we're certainly not hysteresis!"

But once opened, all we found in the cheezy unit was NOT pwm, but merely a hysteresis circuit which had CV voltages close to each other - like 14.2 lvd, and 14.4 hvd.

The moral is that marketing only wants a consumer to make blind assumptions based upon labels. They *certainly* don't want an educated consumer asking about how good or bad their mppt algorithm is!

The hope is that you will buy based solely on incomplete knowledge based on labels alone (pwm and mppt), and not dig too deep. Heh, show me the algo frightens them off! :)

As you can imagine, these algorithms are usually proprietary and you won't get too much detail - other than perhaps charts. At the low end of the spectrum, those who say they are using mppt may be using such a poor algorithm that doesn't actually track well at all, leaves you ripped off.

So the devil is in the details. For the shady products, the hope is that you won't go there and just blindly accept that mppt is almost a brand-name in itself. :)

Ie, real-world issues to look out for. With MPPT, you *do* get what you pay for. As the brain of your system, is it wise not to skimp out.
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I disagree that, "...mppt is an algorithm to modify the pwm technique...". You can't step up the incoming Amps with an algorithm. You need hardware changes to do that. That is why legit MPPT charge controllers are more expensive, and typically larger and/or heavier than PWM controllers.
 
Warpspeed said:
Yes exactly.
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Nope. You can't step up the incoming Amps with an algorithm. You need hardware changes to do that. That is why legit MPPT charge controllers are more expensive, and typically larger and/or heavier than PWM controllers.
 
100 Proof said:
That is why legit MPPT charge controllers are more expensive, and typically larger and/or heavier than PWM controllers.
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Probably a good time to point out the inclusion of a mppt mechanism on top of pwm controllers as distinct from the scam of relabeling pwm cc as a mppt cc. Both would be ''legit'' mppts however one would use pwm the other would use * ( what is the generic name for the regulation circuit in these older type mppt CC ?)
 
100 Proof said:
Nope. You can't step up the incoming Amps with an algorithm. You need hardware changes to do that. That is why legit MPPT charge controllers are more expensive, and typically larger and/or heavier than PWM controllers.
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Actually all the power handling hardware part is identical, it does exactly the same thing. The big expensive stuff will be identical and cost no more.

What is different is HOW that power hardware is controlled, and that is all done with either an analog feedback loop, or with software control.
 
Its rather like the off road guys arguing which is better, four wheel drive, or all wheel drive (assuming four wheels).

They get very serious and angry about it too, without really understanding that both are really the exact same thing.
But try to say that on an off road forum, and you will be mercilessly attacked.
 
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I can just imagine what my favorite engineering professor and friend over many years would have to say about that !

Things seem to have changed rather a lot since I was a student.
 
Warpspeed said:
Actually all the power handling hardware part is identical, it does exactly the same thing. The big expensive stuff will be identical and cost no more.

What is different is HOW that power hardware is controlled, and that is all done with either an analog feedback loop, or with software control.
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From Victron's literature, "The MPPT charge controller is a DC to DC transformer that can transform power from a higher voltage to power at a lower voltage." and "A PWM controller is not a DC to DC transformer. The PWM controller is a switch which connects the solar panel to the battery." You can't differentiate that with an algorithm.

Legit MPPT controllers are NOT the same as PWM. That is why MPPT controllers are typically larger and/or heavier than PWM units.

 
Warpspeed said:
I can just imagine what my favorite engineering professor and friend over many years would have to say about that !

Things seem to have changed rather a lot since I was a student.
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Snark much?

100 Proof said:
From Victron's literature, "The MPPT charge controller is a DC to DC transformer that can transform power from a higher voltage to power at a lower voltage." and "A PWM controller is not a DC to DC transformer. The PWM controller is a switch which connects the solar panel to the battery." You can't differentiate that with an algorithm.

Legit MPPT controllers are NOT the same as PWM. That is why MPPT controllers are typically larger and/or heavier than PWM units.

Click to expand...

You're arguing with a selective pedant. While the way they function is substantially different with actual hardware differences, they'll argue that since MPPT use PWM as part of the DC-DC conversion mechanism, there's no difference between them.
 
But... it's not exactly DC-DC, it it? If you Modulate the Width of a Pulse, it has to be (at least square wave) AC.
-
 
100 Proof said:
From Victron's literature, "The MPPT charge controller is a DC to DC transformer that can transform power from a higher voltage to power at a lower voltage." and "A PWM controller is not a DC to DC transformer. The PWM controller is a switch which connects the solar panel to the battery." You can't differentiate that with an algorithm.

Legit MPPT controllers are NOT the same as PWM. That is why MPPT controllers are typically larger and/or heavier than PWM units.

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Sorry, but that is technically quite wrong, you have been taken in by sales mumbo jumbo.

Oh by the way, I am a retired power electronics design engineer. I will go into discussing the electronics as deeply as you wish, its what I do.
Before retirement I used to design this type of equipment for mass production and commercial sale.

Both use the classic buck converter circuit. This chops up the incoming dc voltage into discrete pulses of varying width.
The resulting rectangular waveform is then averaged in a series choke to produce a steady dc output of lower voltage and higher current.
The technique is commonly called pulse width modulation.

There is no such thing as a dc transformer, transformers use magnetic coupling of two or more windings, and can only work with ac.

However, clever sales and marketing people like to invent technical terms that makes their products stand out.
The idea that adding different software to control it, makes the product much larger and heavier is just laughable.

Legit MPPT controllers use a PWM power stage with a bit if smart software that can locate and track the power peak.
PWM controllers use some less sophisticated control method, but the power conversion stage is identical.
 
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