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Why is mppt better than pwm?

Just an update.

It was a super rainy day today, my 46voc 300watter was sitting at 40v and 0.5a against my pwm controller.
Please give more information: yoo have a panel with 46 Voc, should be 39V Vmp according to the specs? Right?
You used it with a PWM controller and got 40V @ 0.5A Which 40V was it? RMS?
40V looks a bit high, i would expect the Vmp @ 100W/m² irradiance to be around 30 to 36V without a clear maximum...
Which battery have you? at which voltage?
 
DISCLAIMER: I have *not* read this entire (4-page) thread, so this Victron article may have been mentioned:
Which solar charge controller: PWM or MPPT?
One should read that Victron article completely, read it critically and try to understand between the lines as well:

Whereas they explain at chapter 5.3 fig 11 that (assuming a panel with the same nominal voltage than the battery), when cell temperature attain 75°C the Vmp will be equal to the battery voltage, they come to the conclusion that MPPT and PWM are then equivalent.
This is not true!
PWM will still work and MPPT may not even start since the voltage difference will be too low for them to start operation.
And if they ever start, the inherent 1-2 V voltage loss of the Buck conversion will still impact their efficiency by ~7% compared to PWM.

At chapter 6, they deduct that with MPPT, one must chose a panel with a higher nominal voltage or add panels in series: exactly my words!

At chapter 7.3 they treat finally the influence of irradiance, but instead of treating low irradiance like they did for high temperature at chapter 5.3, they took for granted that the nominal panel voltage had already been increased and write:
" an MPPT controller connected to an array with a much higher nominal voltage than the battery, will perform far better than a PWM controller "
which is true, but hides the fact that an MPPT controller connected to a panel with the same nominal voltage than the battery, will perform far worse than a PWM controller.

So under the line, if you don't want to get into the guts:
With the same nominal voltage for battery / panel, take PWM, under the line you will get +-10% the same.
If you chose MPPT, take panels with a higher nominal voltage by at least 15-25V, imperatively.
 
P.S. At § 7.5 Partial shading
They write:
"Partial shading lowers the output voltage. MPPT therefore has a clear advantage over PWM in the
case of partial shading.
"
This is only true if you have a Vmp well over twice the bat voltage and no panel in parallel to maintain the voltage.
So, again, with the same nominal voltage for battery / panel MPPT will provide no advantage in case of partial shading.
 
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P.S. At § 7.5 Partial shading
They write:
"Partial shading lowers the output voltage. MPPT therefore has a clear advantage over PWM in the
case of partial shading.
"
This is only true if you have a Vmp well over twice the bat voltage and no panel in parallel to maintain the voltage.
So, again, with the same nominal voltage for battery / panel MPPT will provide no advantage in case of partial shading.
Going with a high-voltage array makes sense for a lot of reasons, and that can be achieved not just from high Voc/Vmp, but also with strings in Series... Anyway, perhaps the takeaway is to say: If you'd like to reap the benefits of an MPPT controller, be sure "these" conditions are met (insert list here).
 
P.S. At § 7.5 Partial shading
They write:
"Partial shading lowers the output voltage. MPPT therefore has a clear advantage over PWM in the
case of partial shading.
"
This is only true if you have a Vmp well over twice the bat voltage and no panel in parallel to maintain the voltage.
So, again, with the same nominal voltage for battery / panel MPPT will provide no advantage in case of partial shading.
So you seem to be saying that partial shading doesn't lower the output voltage if you are using PWM?

For PWM, the panel voltage needs to be close - but a little above - the battery voltage. So if partial shading lowers the output voltage of the PV, it seems to me that it would cause the the voltage from the panels to go too low for the PWM CC to work effectively.

But, if you are really saying that the partial shading only lowers the voltage for MPPT controllers, maybe you can explain how the PV panels know that you are using PWM vs MPPT. ;)
 
So you seem to be saying that partial shading doesn't lower the output voltage if you are using PWM?

For PWM, the panel voltage needs to be close - but a little above - the battery voltage. So if partial shading lowers the output voltage of the PV, it seems to me that it would cause the the voltage from the panels to go too low for the PWM CC to work effectively.

But, if you are really saying that the partial shading only lowers the voltage for MPPT controllers, maybe you can explain how the PV panels know that you are using PWM vs MPPT. ;)
No.
You have several cases to consider:
a) only one panel at the nominal battery voltage: both PWM and MPPT will fail.
b) with several panels and PWM, you must wire them in // and a partial shading will only reduce the production of one panel, this is the most favourable disposition in case of partial shading.
c) with a panel at a higher voltage or several panels in series, a partial shading will first perturbe the MPPT tracking which will catch up the changed voltage conditions after a certain time.
d) In case of very hard partial shading within a panel's string (e.g a chimney shade by full sun) you have some cells delivering at full power and a few shaded ones providing a high internal resistance. That gives a situation where the MPPT and the collapse voltage are very close, when not identical.
The MPPT algorithm will begin oscillating, returning approx half of the potential power at an average.
Without special software measures to fall back to a PWM emulation at a fixed voltage , the harvest will be highly degraded.
 
MPPT will (almost) always gather more power.

PWM operates by shorting the panels to the battery, forcing them down to battery voltage. If a panel's Vmp is at 17, and you force it to 13.5, it's going to take a 3.5/17 = 20.5% hit to performance. In general, you'll NEVER get more than 80% of rated out of an array connected to a PWM controller in perfect solar conditions.

MPPT allows the panels to operate at peak power.

PWM can't have high PV input voltage, they need Vmp about 1.5X battery nominal voltage, so current coming into PWM controllers is higher and array voltage drop is worse. Your 3S12P array would likely need to be 2S18P and have greater wiring losses between the array and PWM.

MPPT is tolerant of being over-paneled

PWM can't be over-paneled.

Concerning value... If you're talking $/kWh, then you have to look at it this way... How much more does the MPPT cost than the PWM when you consider that you have to buy 10-20% more of an array to match the output of the MPPT?
Not quite true but on the right track.
The current output of a panel is a quasi current source. Meaning that the measured current will be similar regardless of whether the current is being fed into ground or the battery voltage or whatever. This is why simple chargers can be PWM and sill have reasonable efficiency.

However, if you measure the power output of the panel you will find that there is one voltage where the power output is maximized. Power in Watts is equal to the Volts times the Amps. An MPPT charger maximizes the power output of a panel by adjusting the voltage output so that Watts are maximized. This is why you should always use similar panels with a MPPT charger otherwise it will be unable to maximize each panel, whether in a series or parallel connection.

While working I was tasked with designing a MPPT charger for a product and had to dig into the functionality. Sometimes it is almost comical to read the justifications for MPPT that some people pronounce as facts.
 
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I was just pondering what makes an mppt so much better than a pwm? They both do the same thing but a pwm is a better value. I've sat & watched my 2 victron mppt's in full sun doing some kind of internal gymnastics with the output waffling all over the place quite frequently like it's trying to figure out what to do which is wasting lost energy. If you got a 1000 watts going into either type, you get 1000 watts out minus any efficiency loss so I just don't get the point of what makes an mppt better.
With PWM, the rule of thumb is Amps in = Amps out.

With MPPT, the rule of thumb is Watts in = Watts out. With MPPT, you will typically get more Amps out than Amps in. There are a few corner cases where this is not true.

With MPPT, you can run panels in series at much higher voltages than you can with PWM to reduce line losses, have longer wire runs, and use smaller gauge wire if you like. There are situations where that is not desirable though, and the cost-benefit of a PWM SCC is better.

Having said all that, if you are more than a casual user of solar, the MPPT will be the better value in the long run for most folks.
 
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While working I was tasked with designing a MPPT charger for a product and had to dig into the functionality. Sometimes it is almost comical to read the justifications for MPPT that some people pronounce as facts.
Very true!
I especially love those RV owners, replacing the original PWM controller with a MPPT one without rewiring their panels from parallel to series, just because they have been told that MPPT is always superior, who will teach you, that now they have that much better harvesting under cloudy weather...
?
Or those letting their same 12V panels cooking at 100°C and explaining how better the brand new MPPT controller is!
o_O
Or those having a couple of 24V panels in // on the roof with a MPPT controller, replacing the 12 V battery by a 24 V one, just because the manual explains, that the controller can work with both voltages and a 24V battery is better...

And if you try to explain them their mistake, they take you for a stupid...
:love:
 
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TL;DR (also trollbait, probably): if you spend even a little bit of time engineering your system right, MPPT is superior. If you don't, it'll still probably perform better for the most part, but it will perform worse under some rare conditions.

Somewhat related: I found a 55W panel the other day that was made for 12V operation with MPPT - Voc of 24V, Vmp around 20V.
 
...under some rare conditions...
These "rare conditions" are not that rare:

You have got a lot of regions with predominant cloudy conditions and long winters.
Even in region with a lot of full sun, you get hard shades, trees, walls, chimneys etc.. on boats you have sails, permanently run on the deck in front of your panels shading them and -every day- inevitably dusk and dawn.
A well designed MPPT overall system will overcome all these odds, but really do not under estimate the number of people, who just do not think a bit further and have their 12V RV systems not running like they should.

Under the line they will also get some solar power, being happy with their "superior" MPPT stuff and looking with condescension to people running well matched PWM stuff.

Basically, with 12v nominal components and PWM you hardly can make any mistake and it will practically never run under -10% compared to the optimum reachable.
With badly matched MPPT components -and, believe me, there are a lot- you can easily lose 50% of the optimum, when it really matters.
 
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Yep for the low wattage and high temperature applications that i have here i stick with 12V PWM systems.

All my larger systems use MPPT.
 
One of things that's not been mentioned on this thread is that the short circuit current, Isc, is 7-10% higher than the Imp. When used with a PWM controller, a depleted battery will draw the short circuit current. Another item, almost all MPPT controllers, when operated at the higher end of their specs (70-80 V PV input & 30 Amps output for 100/30 MPPT) are *not* at their peak efficiency; they're probably at 88-92% efficient. These two factors make PWM a compelling choice for small (under 600W) solar installations - most RV rooftop installations.

RV installs with MPPT controllers & series connected panels have another issue - a break in the panel connection due to vibrations can shut down the entire system. With several MC4 connections, it's not easy to identify & fix the problem, esp. when boondocking.
 
"When used with a PWM controller, a depleted battery will draw the short circuit current."

This is a misconception. It doesn't have to be a low voltage. Short circuit current pretty much occurs almost up to power point. With a PWM it would always see the short circuit current. MPPT occurs when there is a trade off between voltage and current
 
This is a misconception. It doesn't have to be a low voltage. Short circuit current pretty much occurs almost up to power point. With a PWM it would always see the short circuit current. MPPT occurs when there is a trade off between voltage and current
This would happen only in panels with a vey high fill factor, where the I-V curve is almost like a rectangle. In most panels, & esp. in those using poly-Si cells, the I-V curve has a rounded 'knee'. A panel with lowered Voc/Vmp (due to temperature) & batteries near the end of bulk phase won't draw the Isc.
 
So, just like I said. Hot panels at the end of a bulk cycle isn't a majority of the operating time. I'm trying to get across a point that short circuit isn't just about a dead short.
 
Hot panels at the end of a bulk cycle isn't a majority of the operating time.
It's the very regular situation at noon, when the sun burns and your battery got fully charge and still draws some current. After that the current decreases and the cells can cool down. Then you have also the low irradiance situations and the dusk-dawn.
Panels are more frequently than you think in sub-optimal situations, they are practically NEVER at their tag values (1000W/m² at 25°c) and often at a much lower Vmp.
 
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