mppt vs pwm

The MPPT controller buck converter does not use "excess voltage" to make more amps as is usually stated in blurbs. It takes the output power/Vbatt to get amps. The "excess" is in the power the panel can put into the buck converter. The sun is still shining on the panel.

I almost understood that part just above, where the buck converter tries to balance the power at its input with the power being used at its output. It has to find a voltage at the input that will get the power down to what is being used at the output.

So is that still "tracking" or not? It is not tracking for Vmp but it is matching demand by fiddling with voltage. When the battery can accept all the power available, the input to match would have to be optimized, which is at Vmp. When demand is less, it still optimizes voltage but to a different amount. So does it do that the same way?

If so, then we should call MPPT "Voltage Optimizing" and it is always on! We say MPPT stops and is not always "on" and it goes to PWM. It seems all that really means is that it stops finding Vmp but it does not stop finding the optimum voltage at the input. ?????

Oops, so that would mean with enough demand, the optimum voltage could be Vmp, so you would say it is back in MPPT -Bulk. But then what about that 13.2v set point for when it kicks back into MPPT?

It would be a coincidence if V/batt were at 13.2 just when demand got up to where the panel was maxed out. Especially with a big battery bank. It would not necessarily be at a "loaded voltage" that pulled Vbatt that far down.

jrnymn7 wrote:
We know MPPT controllers use excess voltage to create amps. But do they also use available amperage to create voltage?

They don't do just one or the other. On the IV curves the maximum power occurs at less than the maximum amps but with more voltage. MPPT is looking for the maximum power which is watts. And that maximum power is for the input to the controller. Then the controller converts that power on the output side (more amps and less voltage) for maximum power that the battery will accept.

No.

jrnymn7 wrote:
We know MPPT controllers use excess voltage to create amps. But do they also use available amperage to create voltage?

One thing I'm wondering about:

We know MPPT controllers use excess voltage to create amps. But do they also use available amperage to create voltage?

... In the mppt vs. pwm test, (the one by cruisemarine? is it?), it was cloudy and rainy. The pwm did poorly, but the mppt did quite well, all things considered. So how did it do it?

I'm thinking, seeing as the 12v panel's Vmp would be low, the pwm had little to no voltage to "pull down", so charging was poor at best, and often not at all. But the MPPT was able to scavenge enough Wattage, regardless of the low Vmp, and make enough power to charge. In other words, available amps were sacrificed in order to increase voltage to a level high enough for charging to take place.

That is because you are only applying a dollar symbol and not looking for the best tool for the job. The only time dollars should be a consideration is if you look at it and either would fill the need. That's when you look at the advantages and cost of each.

I gave up a long long time ago... what I need to know is that if you go with 30V or more panels, and have a 12V system, and go over about 400 watts to 750 watts in 28 -30V panels, then you want to look at a MPPT controller for charging your batteries.

If you are at or under 360W and have 12V panels, PWM controllers that are usually 1/3 the price of MPPT controllers are what you should be considering, and your 12V panels will most likely be more expensive per watt.

Those are the current Break even sizes in watts for charge controllers currently. YMMV... I am in the portable panel camp, none of this really applies to me except as an exercise. My take on it is that MPPT is poorly understood and poorly marketed as to what slight benefits it might provide in the smaller systems, and break even points when it comes to cost per amp now in solar panel costs, which continue to drop, the price of solar panels, in terms of $ per watt.


39 pages, and jrneyman and others, myself included, still don't understand fully, the actual benefits per $ /ah charge rate for MPPT.

BFL13 wrote:

...
That is what I was asking earlier with questioning the panel voltage once no longer at Vmp. Answer was panel voltage goes up higher than Vmp so input and output power is less and so output amps is lower at that Vbatt. But if there is a load greater than battery charging, panel voltage goes down (back up the knee on the IV curve) allowing output amps to go up. (I never quite understood how that panel voltage change happens)
...

Wikipedia "Maximum Power point tracking" wrote:
When the batteries in an off-grid system are fully charged and PV production exceeds local loads, an MPPT can no longer operate the panel at its maximum power point as the excess power has no load to absorb it. The MPPT must then shift the PV panel operating point away from the peak power point until production exactly matches demand.

I also get the impression from Wikipedia that the Buck converter accomplishes this and MPPT by controlling the ratio of input and output impedences (by changing its duty cycle).

Wikipedia "Buck Converter" wrote:
A buck converter can be used to maximize the power transfer through the use of impedance matching. An application of this is in a "maximum power point tracker" commonly used in photovoltaic systems.

The Wikipedia article provides a formula for this. By setting the impedance ratio via the Buck converters duty cycle, the controller can pick the MPP input impedence giving the max panel power.

I presume the output impedence will vary depending upon the batteries SOC and other loads on the system so the duty cycle will have to change to maintain the proper input impedence for MPP operation.

Anyway, more food for thought. Most of this is way over my head ...

jrnymn7 wrote:
RE; Panel Wiring and MPPT:

Correct me if I'm wrong,
(snip)
Assuming ideal conditions, and using two 140w panels with specs of 7.9a Imp, 17.7v Vmp, and 8.2 Isc, and using a Vabs Setpoint of 14.8v:
(snip)
If wired in series:

While in mppt mode, bulk charging at an average of say 14v, average current would be 20a (280w / 14v)... Same as with parallel wiring.

Just before switching to pwm mode, bulk charging current would be at 18.9a (280w/14.8v)... Same as with parallel wiring.

However, as it switches into pwm mode, current would drop to only 8.2a (1x Isc), and amps would taper from there. So if abs charging continued for any significant length of time, there would be significant losses.

Couple of things incorrect here.

Wired in series, the current from the panels would be 7.9A (Imp) but at 35.4 volts (Vmp*2) for a total power of 280 watts. When the Vabs setpoint is reached nothing dramatic happens. Assuming the battery acceptance rate has been intersected, the output voltage will be held at Vabs and the current will begin to taper as battery acceptance declines. If additional loads are added such that the output voltage sags below Vabs, the controller should revert to MPPT to attempt to satisfy the demand.

JiminDenver wrote:
A MPPT controller switches to PWM mode after float for two reasons. There is no longer the loss with the battery being at a higher voltage and the battery could not accept the additional current anyways. The extra current is still there, just not being used to charge with.

Not technically correct. Current only flows when something is creating a demand. It does not "lurk" in waiting. There is however more potential power available from the panel. (Not to be confused with potential meaning voltage difference).

Salvo said,

"Since mppt uses a buck regulator in its topology, it can operate in pwm mode while the panel voltage is higher than Vbat."

So you're saying... Since an MPPT controller uses a buck controller in its topology, IT (that is, the buck converter) can operate while the controller is in pwm mode, as long as Vpanel is higher than Vbatt.

BFL said,

"It is the buck converter, that's the whole point. Isc means nothing when you are using the controller with a buck converter. The amps you get to the battery is always output power/Vbatt. All the MPPT does in Bulk is fine tune the input power to improve the output power somewhat."

O.K., I think it just clicked. I was thinking in terms of an MPPT dropping completely out of mppt AND bucking, and being left with only pwm working with panel Isc. But if the bucker is still functioning, the switch from bulk to abs, at a lower soc, would not cause a sudden drop down to Isc.