Workings Of A MPPT Controller

[jonsun]

But even a few years ago before chinese mppt's hit the market when the panels were half the price but double the voltage it was a ridiculous buy. You'd pay $200 for 100w / 17.xv panel and a $50 controller. Or you'd buy a $120 100w / 34v panel and a $350 controller thinking you'll get 10% more efficiency overall. You spent $225 more for what you thought was 3 amps a day.

Then it turned out your 100w 34v panel outputs 2.9a 95% of the time (instead of 5.5a). In turn you went out and bought another panel or two then swore on the internet that you have a more efficient system AND you need 300w to run a 12v fridge and a laptop. and 120v fridges will fry a 600w system. It is truly amazing how just writing long, technical posts convinces people to spend thousands for a few low-current appliances.

The manuals will not convince the experts, the utterly debunked notions of efficiency will go on and on, and newbies will continue to wildly over-spend.

 

[jimindenver]

Bardo, you are right in that the MPPT program no longer is in use once the controller leaves the Bulk stage. Wow that has been what we have been telling you all along. You of course are still confusing the function of a Program vs the function of the hardware, ie, the buck converter. The controller does not need the MPPT program to allow you to use 100% of the panel in absorb or float, it uses the buck converter for that.

The best I can offer is to show up for the RTR. There I will have use of twin 160 watt portables, one on PWM, the other on MPPT. You can even bring a fancy PWM like the Bogart that while it can not use all of the panels power can somehow charge a battery better. Oncewe wait for the PWM system to catch up bringing its battery's voltage up to absorb levels we will run load test to see how much bigger of a load the systems can handle before their battery loses voltage. Then while you may still not consider the cost worthy, at least you will understand how MPPT works and will not have to look bad as you try to explain what you do not understand.

 

[John61CT]

Jim,

Just making an educated guess here, which do you think will output more total charge

A Victron 75/15 matched with a single 200w Imp panel at say 45Voc (or whatever you think would maximize MPPT)

B Bogart Trimetric controlling SC-2030, same watts rating but nominal 12V, say 20Voc

into a LFP bank (just to eliminate the declining acceptance issue, no mixed loads)

?

Obviously weather latitude are factors, so let's just say average conditions where you go.

 

[jonsun]

Bardo, you are right in that the MPPT program no longer is in use once the controller leaves the Bulk stage. Wow that has been what we have been telling you all along. You of course are still confusing the function of a Program vs the function of the hardware, ie, the buck converter. The controller does not need the MPPT program to allow you to use 100% of the panel in absorb or float, it uses the buck converter for that.

The best I can offer is to show up for the RTR. There I will have use of twin 160 watt portables, one on PWM, the other on MPPT. You can even bring a fancy PWM like the Bogart that while it can not use all of the panels power can somehow charge a battery better. Oncewe wait for the PWM system to catch up bringing its battery's voltage up to absorb levels we will run load test to see how much bigger of a load the systems can handle before their battery loses voltage. Then while you may still not consider the cost worthy, at least you will understand how MPPT works and will not have to look bad as you try to explain what you do not understand.

Just stop. 

why would you go through all this mess to divert low-current loads through the load circuit instead of just charging the damn battery?

Yeah lets spend more money for 1/2 the charge then wire load circuits to a fan so you don't feel foolish wasting everything! efficiency!

 

[jonsun]

Here's a thought, just buy the right panels for the job....they're actually cheaper than high voltage ones now. Or spend hundreds extra in double 0 gauge wire runs, complicated programming, and pointless, random diversion loads that may or may not function with your particular controller.

 

[John61CT]

So much over generalizing there don't know where to start.

OK, here: a 2k AH bank down by 50% will certainly stay in bulk for longer than a 200AH bank. Yes extreme case, not realistic for most "vans" but to make the point.

The early charge stage is the **only** time you need big amps, but that is exactly when you need them, with high AH usage you got to get to Absorb as early in the day as possible otherwise you may never get to 100% by the end of the day.

So that's one. . .

 

[John61CT]

If you want 40V panels just buy 40V panels, why buy 12V and wire in series?

And AFAIK PWM controllers usually have no DC-DC conversion, they just drop the excess voltage and thus waste input power, lose output watts.

 

[jonsun]

Kindly point to your documentation where it shows you mppt functions through diversion. Then explain why do that instead of actually charging the battery.

Ya know since this issue is just brought up for the first time now that youre proven wrong.

 

[jonsun]

So much over generalizing there don't know where to start.

OK, here: a 2k AH bank down by 50% will certainly stay in bulk for longer than a 200AH bank. Yes extreme case,  not realistic for most "vans" but to make the point.

The early charge stage is the **only** time you need big amps,  but that is exactly when you need them, with high AH usage you got to get to Absorb as early in the day as possible otherwise you may never get to 100% by the end of the day.

So that's one. . .

You dont get "big amps" when you have the right panels. You get milliamps maybe if yoi can get batteries low enough.

I own an mppt, nothing against them. But i got because it was $120 for user programming and temp sensing, noy because i thoight i could buy cheap high voltage panels or wire in series.

 

[jimindenver]

Jim,

Just making an educated guess here, which do you think will output more total charge

A Victron 75/15 matched with a single 200w Imp panel at say 45Voc (or whatever you think would maximize MPPT)

B Bogart Trimetric controlling SC-2030, same watts rating but nominal 12V, say 20Voc

into a LFP bank (just to eliminate the declining acceptance issue, no mixed loads)

?

Obviously weather latitude are factors, so let's just say average conditions where you go.

Two equally drained batteries on two systems? The MPPT will use 100% of the panels output and bring up the voltage faster than the PWM system accepting 2/3's of its panels output.

 

[jimindenver]

Kindly point to your documentation where it shows you mppt functions through diversion. Then explain why do that instead of actually charging the battery.

Ya know since this issue is just brought up for the first time now that youre proven wrong.

Bardo

I have said over and over that the Maximum power point tracking is not active after the bulk stage. The MPPT program is different than the buck converter. One tweaks the array in bulk only, the other allows for 100% of the panels output to be used in any mode. At the point of absorb and float either system should be able to hold the low amps needed to hold that voltage and continue to charge the battery. At that point the excess power that can be used by either system while still charging the battery.

Simply put my MPPT systems will offer more usable power for charging or running high amp loads in any mode than a PWM system of equal size. This is what I will show at the RTR for those willing to accept it.

 

[John61CT]

> You don't get "big amps" when you have the right panels. You get milliamps maybe if you can get batteries low enough.

Sorry I have no idea what you mean.

A large bank at 50%, totally normal scenario, needs say a hundred AH per day to replace what's used.

In order to get to 85-90% by noon, I need say 30-35A charge output from my SCs in the Bulk and early Absorb stages.

After that point, acceptance declines, ideally down to 1-2A before going to Float, say at 4pm.

So, within the above use case please clarify the point you are trying to make about PWM vs MPPT, and/or the right kind of panels.

If you like, use the example equipment I spec'd in my question to Jim.

 

[akrvbob]

But even a few years ago before chinese mppt's hit the market when the panels were half the price but double the voltage it was a ridiculous buy. You'd pay $200 for 100w / 17.xv panel and a $50 controller. Or you'd buy a $120 100w / 34v panel and a $350 controller thinking you'll get 10% more efficiency overall. You spent $225 more for what you thought was 3 amps a day.

You're not even trying to be fair or reasonable and that makes it hard to hold a balanced discussion with you. You pick absurd situations and say they prove something. But if you compare apples to apples, it looks different. No one who is aware of the superiority of MPPT would spend $120 for a 100w 34 volt panel. They would spend $225 for this 285 34 volt panel that is in the store today, right now at this price:
https://www.solar-electric.com/solarworld-sunmodule-sw285-mono-black.html

Who would spend $200 for a 100 watt  17 volt panel when they could spend $225 for a 285 watt 34 volt panel? The simple fact is that high-voltage residential panels are drastically cheaper than 12 volt panels and go a long way toward offsetting the cost of the superior technology in MPPT controllers. 

Then I'd buy a Blue Sky 3000i mppt controller for $258. Mainly because it is user settable right on the face of it for absorption voltage and time it holds it. Plus, I know for a fact they have superior customer service. That' alone is worth the extra money and the fact it comes wrapped in extremely superior technology is just a huge bonus! 

Your guy spent $250 dollars for a 100 watt system--and he was flushing up to a third of that down the toilet because of the poor technology of PWM. I spent $475 for 285 watt system and got nearly 3 times the power for less than half the cost. Even better, the MPPT will give me 10-35% more of every watt produced than his PWM does. 

Plus your guy will be replacing his batteries 3-5 times as often as I will because his system is underpowered and stupid. It wastes much of the power he bought and can't control the charging of the little power he does make. So, in the long run, he wasted tons of money and created lots of headaches for himself.

 

[akrvbob]

 Then it turned out your 100w 34v panel outputs 2.9a 95% of the time (instead of 5.5a). 

Here is the heart of the problem as I see it. You misunderstand amps and think an amp is an amp, no matter its voltage.  If that were true, you would be just as willing to touch a live 15 amp, 12 volt wire, as you would be willing to touch a live 15 amp 110 volt wire.  

Believe me, when you got 10 times the charge flowing through you, you would instantly understand that an amp, is not an amp!!!!! It all depends on the voltage of that amp. 

So yes, your 2.9 amps at 34 volts is exactly the same as 5.5 amps at 17 volts. 

BUT... the MPPT wouldn't put in 2.9 amps at 34 volts, because your battery wouldn't accept it. So, instead, (just to pick an example) if the battery will only take 13.6 volts, the controller will step the voltage down by 2.5 times and also step up the amps by 2.5 times and put in 7.25 amps. So it is putting in more than your PWM because it is stupid and can't increase the amps, just reduce the volts--that's it's only job in life. 

Now lets see how many watts our two systems are producing:

MPPT 13.6 volts x 7.25 amps = 98.6 watts
PWM 13.6 volts  x 5.5  amps =  75 watts

That PWM is throwing away 25 watts because it can't step up the amps, only reduce the voltage. 

I don't mean to just argue and prove you wrong. But it's important to me that we do not spread misinformation and the idea that PWM is in any way equal to MPPT is not true.

 

[MarkK]

With the relatively inexpensive mppt Victron why go pwm?

 

[John61CT]

Many are happy with spending $25 rather than $100, and some of them are right, for their situation.

 

[Weight]

For any given battery SOC. There are only so many amps that can be accepted. At the very beginning of the day when the bank is at its lowest SOC, the MPPT may have an advantage over the PWM. But your panels must be producing higher voltage or there is insignificant advantage. My 400 watt solar panel array would not be much different using 12 volts at 32 amps or 24 volts at 16 amps. If my battery bank is charged by 1 or 2 O'clock, I'm good. I chose a PWM from a leading manufacturer rather than the much more expensive MPPT from the same manufacturer. I don't want the cheapest chinadoll. If I had residential high voltage panels, I would have chosen MPPT.

 

[jimindenver]

Weight

As long as your needs are met, you have a great system. Posting what you have, how you use it and why you chose it is much more informative than us debating each system as seen by many different view points. It is the fact that we see and use our systems differently that provides the most options to the people looking to learn about solar.

I see this thread as a chance to demystify MPPT controllers, how they work and how they can be of benefit to a user. It isn't to trash a different type of system, that doesn't add to the discussion at all. Understanding the differences does and allows someone to make a choice based on information not advertising, myths or smoke and mirrors. I have been lucky to be part of this discussion on a few forums and it is more than a debate. There are foot print debates between 12v and grid tied systems. Side by side test for output at various times of the day. Test showing that peak power comparisons on a hot day show the PMW system closer when the battery is up due to the effect of heat on a panels voltage. My test were side by sides comparisons of grid tied polys vs monos, the effects of shading and mixed panels, showing that the buck converter worked even out of bulk and how to use a system like a generator.

That last part does not require MPPT, there is excess power to be used once a battery on a PWM system is tapering its acceptance rate. My friend with the twin 160w portables on PWM sees his GRP 24 charged up quickly every day, after that he runs all kinds of stuff without discharging the battery.

 

[frater secessus]

[deleting; jim's excellent post came in before I finished the post]

 

[frater secessus]

I wonder if the friction isn't caused by one letter deployed by marketers:  the M in MPPT.

It might be more useful to think of them as just PPT controllers since they continually track the appropriate power point for present conditions.  Sometimes that's the Maximum Power Point but not the majority of the time.  As far as I know the MPP proper is deployed in these cases:

1. Bulk charging stage
2. when running loads >= the panels' max present output
3. possibly during the ramp-up to Equalization voltage

The rest of the time the controller is using other, sub-maximum power points appropriate to the amount of power required.  Even the power point providing 99% of the array's presently capable output is not the maximum power point.

In a rare fit of optimism I date dare to hope this nomenclature could reconcile:

• why PPT "gains" are "up to" rather than fixed and can vary wildly in different situations
• why manufacturers' documentation says what it says
• why PWM folk think MPPT is no big deal outside Bulk
• why MPPT folk think MPPT is a big deal most of the time
• the behavior that MPPT controllers display
• why either type might be the best solution for any given individual's build