MPPT Con ?

[hellosailor]

Controllers, controller logic, charging algorithms...

"Here Be Dragons!"

I don't think anyone except an engineer in the full-time "twelve volt" business will ever be able to really compare them at that level. And picking a winner?

That's what makes horse racing, right?

[Paul Elliott]

Originally Posted by hellosailor

The "working voltage" of a solar panel is determined by chemistry and physics and you can only regulate the voltage once it gets out of the panels and into a circuit.

The output voltage of a solar panel depends on (among other things) the current drawn from it. Look at the published V/I curves. MPPT controls the current drawn from the panel and monitors the panel voltage. It draws more or less current and evaluates the P=E*I equation, looking for the maximum P. This is MPPT. Battery-charging is the other half of the system, but MPPT itself is pretty simple. Some controllers do this better than others, by merit of their control algorithm and the losses in their circuitry.

The "Maximum Power Point" in MPPT controllers does not refer to a mysterious power point in the solar panels, but as far as I know, to the MPP required to optimally charge the batteries.

Sorry, but MPPT controllers are searching for the panel maximum power point. There is even a specification for this in the panel documents. It varies with illumination and temperature, which is why, rather than being pre-set for a fixed operating point, the MPPT systems have to search for it.

The MPPT controllers don't "dump" any power from the panels. They don't throttle the panels, they don't adjust the panels, they don't turn them on and off. They just soak up all the power (wattage) and then transform it, literally with a transformer circuit or a DC-to-DC equivalent of that (sometimes both) using capacitor charging and pulsing, hence the "pulse width modulation" part of their output. (Once the batteries are fully charged, they will "ignore" the panels, disconnecting from them or doing something else with the solar power. But that's after the charging is done.)

Once the battery charging progresses beyond the bulk stage, if the charging current would otherwise be excessive the MPPT (or any type of) controller will reduce the power they take from the panel.

I'm a bit confused by the concept of trying to control the battery charging voltage. When I used to design battery chargers (not lead-acid), I would monitor the battery voltage, over time, and use this to determine the proper charging current. As I increase the current the voltage rises, but voltage is what I measure and current is what I provide.

[noelex 77]

Originally Posted by hellosailor

John-


But you don't see the "14.4 bulk, Snap!, 13.8 float" etc. changes in the charging voltage that a 3-stage makes. MPPT is a continuous stepping up/down, as it reads actual battery condition (and temperature, if equipped) and makes fine adjustments to match.

Here is the MPPT Outback MX60 charging profile (from their handbook)



You can see how the voltage steadily rises during the bulk phase then it "snaps" to the battery voltage chosen for absorption and float. (see the steady red trace of battery voltage)
This is like most solar regulators. MPPT do not regulate differently from non MPPT units.

MPPT units extract power from the solar panels differently than non MPPT units using a voltage conversion process to try and maximize the output of the solar cells. Once they have extracted this power they regulate the batteries in the same way as any good ordinary solar regulator

Originally Posted by hellosailor

John-


The "Maximum Power Point" in MPPT controllers does not refer to a mysterious power point in the solar panels, but as far as I know, to the MPP required to optimally charge the batteries. Which is confusing because "power" means wattage, not voltage or amperage.

Sorry to disagree, but MPP is a property of the solar panel it is not related to the power to optimally charge the batteries. With respect I think you are misunderstanding how the MPPT system works
.
.

[noelex 77]

OK lots of discussion on how a MPPT regulator works, but not a lot on the improvement, if any, to the the power input on a yacht.
I still have an open mind.
I have however been reding some general solar forums and websites.
The average improvement, from uses, quoted for MPPT on domestic sytems, using a good quality MPPT, seems to average at about 15% (assuming no regulation)
On a yacht I would expect the gains to be significantly lower for the following reasons:

1) Much of the gain in domestic systems is from reduced wire losses (this is not a factor if the panels are in parallel and the is good argument to do this in a boat system even with a MPPT regulator)

2) A series of domestic solar panels generally experiences similar conditions allowing efficient tracking by 1 regulator. Yachts frequently have shadows over some panels. This results in a different MPP for each panel.
.
3) Shade problems are seen less in domestic systems (increasing the solar panel voltage above the battery voaltage)

4) The panels in domestic systems are usually orientated for the latitude

5) The input voltage in a domestic situation can be well above the output voltage without compromises. On a baot when the panels are in series the effect of shadowing over 1 panel is increased

6) Cruising yachts tend to congregate in warmer places (If you are cruising colder places you will get much more advantage from MPPT than my estimation)

7) The larger required parasitic power to drive a MPPT regulator is less significant in the much larger domestic systems.

8) The MPPT voltage conversion and tracking is better at higher voltages.



In their favor the higher wind speeds experienced by yachts help cool the panels more than in a domestic situation improving the gains from MPPT.



The impact of these factors is difficult to estimate, but without any hard data my feeling is that on a yacht with multiple panels and a single MPPT the gain would be about ½ to 1/3 of that experienced by domestic users.
This would put the gain from an MPPT regulator in the order of 5-10%.
There are certainly times when the MPPT controller would give much greater gains than this, but also times when the MPPT controller would do no better or even less well than a simpler regulator.
Even this small gain may be helpful for a yacht where there is often no room to add more solar panels and limited options for generating or saving more power.

5-10%, is a much smaller gain than is generally quoted by the manufacturers. This will be reduced further if there are periods of regulation during the day.

[Paul Elliott]

Originally Posted by noelex 77

5-10%, is a much smaller gain than is generally quoted by the manufacturers. This will be reduced further if there are periods of regulation during the day.

You are probably right that the gain from MPPT is less dramatic on a boat, and 5% to 10% improvement may be about right. The variable factors are a huge deal, and it is probably impossible to get solid, broadly-applicable numbers.

But, in my case there does appear to be on average a gain in charging power. For my situation, where I don't wish to add any more panels, even a small gain is worthwhile to me. If I had a tighter budget or had the option of spending the money on additional panels, then the extra cost of an MPPT controller might be a mistake. As long as I get a gain and not a loss from MPPT then even at the current pricing it would make sense for me.

I wish I had better data!

[goboatingnow]

So I hope at this stage we've accepted that MPPT( max power point tracking) is about tracking the max power point of the panel and nothing to do with charging batteries . Mppt will always give some gain unless by coincidence your conventional regulator is sitting on the MPP

As an EE I've found some curious comments in this thread like " snapping to a voltage" LOL

Dave

[mbianka]

Originally Posted by hellosailor

Mike, that ebay controller is absolutely unique. It has
Thunder protection
listed among the feautres. I guess it is the only MPPT controller on the market that doesn't run and hide under the settee when the thunder starts.

Hopefully they paid a little more attention to the electrical part of their design.<G>

I'm gonna hold out for a MPPT controller that is Tsunami protected too. As Cuthbert points out chip sets are coming into the market that should make the cost of the MPPT controllers come down in price. Much like that Kyocera F1010 laser printer I bought in the 1980's. Cost $3,000 dollars but, hey it printed envelopes too! Then there was that brand new printer, scanner, fax unit I bought at a Black Friday sale last week for $30 bucks. Oh yeah and that one prints color photos too! I hope MPPT controllers follow the same cost trajectory. Until then my Morningstar units will do just fine for my needs.

[hellosailor]

Paul-
Apparently there's more going on than I've been aware of. "... MPPT controllers are searching for the panel maximum power point. "
So, the MPPT controller is searching for what in response to what? Is it presenting a different load impedance to the panels, in order to change the power coming out of them? AFAIK the MPPT controllers had no active circuitry on the panel side, they just "fed at the trough" and sucked up whatever the panels put out into a fixed load. Not??


"I'm a bit confused by the concept of trying to control the battery charging voltage."
Well, if a controller has xx Watts available on the imput side, it can give the battery something like 95% xx Watts on the charging side, right? Apparently if it supplies that power at a slightly higher voltage than the battery is at, perhaps 13.3 as opposed to 14.4 volts, and pushes higher amperage instead of the higher voltage, the battery charges faster with less waste heat being generated in it. So the MPPT controllers monitor battery voltage, versus the battery voltage spec and amp capacity that have been programmed into them, and they supply the charging power in the form of "as much amps as I can, at %% over the battery voltage". Subject I'm sure to an amp limit, and the percent may not be a simple percent. That's all where no one wants to talk about proprietary routines, and I can understand they'd want to keep that a competitive advantage, at least until there's some agreement about what actually IS best, and someone publishes it.
I spoke to a lead engineer at one of the largest US battery makers about some of the charging issues (volts & amps versus BOOM) and apparently they're quite convinced that "conventional" 3-stage charging is totally obsolete. And equally convinced that there's enough real-world research to be done, that they won't discuss many specifics at all. Just that PWM is proven more effective than DC charging, and that frequently adjusting voltage-vs-amperage during the charge can also bring about much more effective charging. Within the limits of BOOM, of course.
Apparently the customers and atttorneys all get upset by "BOOM". <G>

[daddle]

The MPPT device has a voltage regulator on both the solar side and the battery side. It adjusts both voltages as needed for best performance of the entire system. Simple in concept.

[cal40john]

Originally Posted by hellosailor

John-
"It is opening or shunting the panels," Nope, not at all. It is not doing anything to the panels. All it does wrt the panels, is read the voltage coming from the panels.

I think you're wrong.

In the link below is a schemetic of the power circuit of an MPPT controller. The panel output basically goes through a low pass filter then to a sync buck regulator to the battery. There are parts to measure current and voltage at the panel and at the battery. When in MPPT mode (see the text in the link.) The micro adjusts the duty cycle of the buck regulator to maximize power transfer. What happens in this circuit when you no longer want the max power out of the panel? Let's say you don't want any power. Turn off Q400, you now have opened the circuit to the panel as I said before. If you don't want it entirely off, choose the appropriate duty cycle for the power you want, this won't be at the MPPT point.


READ THIS WHITE PAPER
I see Cuthbert posted the link before I got back around to posting
http://www.cypress.com/?docID=17435

How a sync buck converter works.
http://en.wikipedia.org/wiki/Buck_converter

The "working voltage" of a solar panel is determined by chemistry and physics and you can only regulate the voltage once it gets out of the panels and into a circuit. OK, to be picky, you can always make shadows on the panel or reduce the amount of light that hits it, in order to reduce the voltage coming out of the panel. That's still not "voltage regulation" in the sense that charging systems do it. We don't regulate the panel voltage, we regulate the charging system voltage. A difference easily lost in translation.

Look at the first diagram in the MPPT link. The solar panel will always be on that I-V curve (solid line). If you open circuit the panel you'll get 24 volts, if you short circuit it you will get 4.8 amps. Put a resistor across the panel, if pick the resistor such that you measure 2 amps, you will measure 20 volts. The controller is changing the impedence to the panel until the power (dotted line in the diagram) is at a max. Which puts you at about 17.5 volts and 4.6 amps for this panel. You can call it voltage or current regulation, but you're forcing a condition onto the panel. You get to pick one, the other is given by the panel.


Batteries are the same, they have a set of I-V curves for every state of charge. If you pump X amps into the battery you will see Y volts. So for a fixed current going in, the voltage is determined by the battery.

The "Maximum Power Point" in MPPT controllers does not refer to a mysterious power point in the solar panels, but as far as I know, to the MPP required to optimally charge the batteries. Which is confusing because "power" means wattage, not voltage or amperage. And for an optimum battery charge, you need to monitor both voltage and amperage. What an MPPT controller does, is take all the power (wattage) from a solar panel and convert it into the proper combination of volts and amps that a battery needs. This is as opposed to a conventional solar controller, which is either a "dump regulator" or a conventional "down only" regulator that simply dumps (totally wastes) excess voltage.

The MPPT controllers don't "dump" any power from the panels. They don't throttle the panels, they don't adjust the panels, they don't turn them on and off. They just soak up all the power (wattage) and then transform it, literally with a transformer circuit or a DC-to-DC equivalent of that (sometimes both) using capacitor charging and pulsing, hence the "pulse width modulation" part of their output. (Once the batteries are fully charged, they will "ignore" the panels, disconnecting from them or doing something else with the solar power. But that's after the charging is done.)

Transformers, or capacitor charging, can be used to transform voltage into amperage, or vice versa. So instead of dumping excess power, the MPPT controller now has to determine what the power needs of the battery are, and how to adjust the power to best feed them. "Adjust" in this case meaning optimize the voltage and amperage as separate factors. Batteries can pretty much suck in all the amperage or voltage you can supply to them--but if you feed it to them at the wrong levels, they'll go BOOM. For each specific voltage level, there is a limit to the amperage you can feed them, and vice versa. You could go nuts trying to adjust voltage and amperage and compensate the amounts of both every time one or the other changed by 1%, even without trying to remember those numbers also change every time the temperature changes.
And before microprocessors, using just analog circuits, it would have been a nightmare trying to constantly compensate both volts and amps. The charging curves for the batteries are no secret, the problem is that the charger needs to keep checking the battery condition, keep fetching the right information, keep resetting the charging parameters (voltage and amperage based on battery charge state, temperature, etc.) and do it all reliably, to prevent that BOOM.
The two-stage and three-stage chargers are all "simplified" way to do that. They take some broad assumptions about "this can't make it go BOOM" and they stop there. The microprocessor controllers (i.e. MPPT) literally are looking up the right parameters, maybe one per minute, maybe once per second, and they are continuously giving the battery the maximum charging AMPERAGE that is available, at the optimum charging VOLTAGE. A higher voltage just boils the electrolyte and is wasted. Or makes the battery go BOOM.

If you watch the output display on an MPPT controller that has one, you'll see the output voltage keeps creeping up, usually by 0.1 volt increments, as the battery charges. The amperage will also be going up, initially, as the panel output goes up as the sun rises toward noon. Once the batteries are charged, the amperage will creep down again, to a trickle, unless something is drawing power.

[cuthbert]

Originally Posted by daddle

The MPPT device has a voltage regulator on both the solar side and the battery side. It adjusts both voltages as needed for best performance of the entire system. Simple in concept.

Nicely put in very few words, If I may tweak that just a little bit to reflect finding the "knee" on the V/I curve on the panel side and to give the bulk/absorption/float on the battery side......

The MPPT device has a voltage and/or current regulator on both the solar side and the battery side. It adjusts both as needed for best performance of the entire system.

[cal40john]

Originally Posted by cuthbert

Nicely put in very few words, If I may tweak that just a little bit to reflect finding the "knee" on the V/I curve on the panel side and to give the bulk/absorption/float on the battery side......

The MPPT device has a voltage and/or current regulator on both the solar side and the battery side. It adjusts both as needed for best performance of the entire system.

I don't understand regulators on both sides. There's just the one buck converter in the white paper that is linked in my last post (You referenced it also.). The buck converter function is V-out/V-in = D(uty cycle). For a given panel temperature and light amount the V-in (panel voltage) is fixed for MPPT operation. Ignoring losses, we know how much power is being transfered to the battery, it's I times V at the panel. For a given state of charge of the battery which has a V-I curve, there is only one point on it that will match that power, so the voltage and current at the battery are predetermined as well. As the battery charges, the V-I curve changes (up), so for the same power input the current has to decrease, the battery voltage rises (V-out) and the duty cycle of the converter has to rise as well to maintain the constant V-in.

Once you are out of a bulk phase you're not doing MPPT anymore.

Battery SOC chart:
http://www.scubaengineer.com/documen...ing_graphs.pdf

[donradcliffe]

Whatever magic MPPT you buy, be sure that it was built for marine use. I had the same experience as Paul with my Blue Sky controller--one drop of saltwater and it was smoking history. I went back to the simpler, cheaper, and potted marine controller.

[noelex 77]

Originally Posted by goboatingnow

Mppt will always give some gain unless by coincidence your conventional regulator is sitting on the MPP


Dave

Yes if the MPPT voltage conversion could be done without loss.
Unfortunately there are reasonable losses in this conversion. The manufacturers are reluctant to tell us what the efficiency of the voltage conversion is other than the best case results.
If you look at at the Outback MX60 handbook for example the graph stops at 90%. They don’t want you to know about efficiencies less than this.
The 90 % efficiency is at about 125W. The graph has a very steep slope at this point so lower wattages are going to be significantly worse.
These low wattage outputs are not often seen in domestic solar systems (because they use a large number of panels), but they are common in boat systems.
There is also some losses determining the MPPT point. Some MPPT are reluctant to start until the solar panel voltage has risen and the controllers themselves consume slightly more power than none MPPT units. Once again these losses have little effect if you have a 2KW domestic system, but they are more significant on the smaller systems installed on yachts.

So in practice I believe that a MPPT regulator on a small boat system can be reasonably often putting in less power than a non MPPT unit. These times however will be offset when the MPPT are putting out more power.
The difficulty is knowing how these effects balance out.

[goboatingnow]

There's still basic confusion over ohms law going on here.

Firstly leaving aside specific MPPT controllers like the outback and blue skies units. MPPT is a technique to extract the maximum power from a panel for a given sunlight level. Solar panels do not have a fully linear V/I relationship there's s particular power on the output curve where it's best that's the MPP .

MPPT controllers search for that point continuously using several methods most by controlling input current and looking for the best V/I point. They arnt regulating the panel.

At this point we now have a power source and now we need to charge batteries . That can be done via any number of methods. Often the pwm switcher Techology is used as it's more efficient then series pass regulators. But in theory any form of regulator could be used. Most systems now deploy 3 stage charging usually IUI or IUU . Direct pwm ( or pulse) charging is not recommended by any battery manufacturer that I know of.

Of course if your battery doesn't need the power then the controller will drift away from the optimum V/I point. ( that's a function of ohms law) but that isn't an
argument against Mppt. Most solar panels have power outputs well below the battery and hence in any charging sequence the battery ( or perhaps the dc circuits) can accept the current hence Mppt makes sense. If not why do you need panels in the first place.

So if your just using a small panel just to trickle charge you batteries then the likely hood is that Mppt is a waste of time. However if you have loads that work the panel to the max ( either battery charging or just DC loads) then by definition MPPT provides more power from a given panel

Note regulators or panels don't " dump" power the panel is a current source the load is a current sink. The effective input resistance of the load rises as the required current falls. The current isn't " dumped".


Dave