mppt vs pwm

[pianotuna]

Hi,

A nice little video comparing mppt vs pwm with the batteries in bulk mode.

mppt vs pwm

[pianotuna]

Hi Jim,

You just hit the thumb with the hammer! I could not agree more. Too bad we can't rent controllers before we have to buy them.

JiminDenver wrote:
For each of us different conditions exist. Gear, expected temps, altitude and weather all play a part in what we choose. Formulas and theory don't always that those things into consideration. Only side by side testing in a unique situation will give a person the answer to what is best in their situation. Even that is hard to set up fairly.

[BFL13]

JiminDenver wrote:
I often wake to 12.4 or 12.5v and even tracking the sun the controllers don't ramp up to peak amps without the coffee pot pulling down the voltage temporarily. Once that has happened they will stay at peak until absorb is hit.

So at 12.5v my MPPT system is wont go peak and at a voltage low enough to get it to do so the PWM is really at a disadvantage via losing watts.

.........

I have not seen that but I am not sure if I have ever been in that situation. Maybe this summer I can try out if my Eco-W will do that too.

Be hard to arrange where the day starts at high SOC and you don't get peak amps till mid-day anyway so amps will taper from high SOC before the sun is high enough to make peak amps.

[red31]

mena661 wrote:

MrWizard wrote:

another reason..why i keep saying its about watts not amps (but i'm flogging a dead mule) nobody wants to listen to that one

I'm certainly listening.

zero watts at Isc!! Isc is not at vbatt.

Ppwm? Ppanel? not Pmp for pwm

[JiminDenver]

I often wake to 12.4 or 12.5v and even tracking the sun the controllers don't ramp up to peak amps without the coffee pot pulling down the voltage temporarily. Once that has happened they will stay at peak until absorb is hit.

So at 12.5v my MPPT system is wont go peak and at a voltage low enough to get it to do so the PWM is really at a disadvantage via losing watts.

For each of us different conditions exist. Gear, expected temps, altitude and weather all play a part in what we choose. Formulas and theory don't always that those things into consideration. Only side by side testing in a unique situation will give a person the answer to what is best in their situation. Even that is hard to set up fairly.

[MrWizard]

NOTE
I have often said rainy, overcast, and when the sun comes out late
I have specifically said when the day starts out cloudy, overcast then clears up to be sunny,
It is not that MPPT can make power that isn't there
It is that MPPT can take better advantage of the sun that does appear, to get more amps into the battery, using that power point tracking, to convert volts to amps, above the normal isc

[BFL13]

While PWM sort of chugs along no matter what, MPPT does way better at low battery voltage. We have people who use solar to run loads, and that activity keeps the battery voltage low, so they like MPPT

For battery charging your voltage keeps rising so MPPT might start out with an advantage, and lose it part way up. Depends which to choose on how much of each you do while RVing? If all you do is shallow cycles on solar, your battery voltage stays high, so the MPPT has no chance to shine. if you run loads at night so battery voltage in the morning is low (deeper cycles) then your MPPT has some time in the morning to show off.

Ken showed how MPPT does better in low light when panel voltage is lower using 24v panels, so that there is still some voltage difference for the MPPT (buck converter actually) to work with while PWM has shut down.

But how can Mr Wiz do better with MPPT on cloudy days with MPPT when he has 12v panels? Is there still some voltage the MPPT can do something with that would be noticed?

I wonder if the low light cloudy day MPPT thing might be mixed in with the low battery voltage MPPT advantage. On a cloudy day, the batts would be taking longer to recharge if the amps coming in were lower than on a sunny day, which means they would be at a lower voltage longer in the day, giving MPPT advantage longer to work its magic.

So it might be tricky to separate the two factors in deciding which is doing how much of what you might be seeing. One is based on panel voltage variations while the other is based on battery voltage variation. You can get more voltage difference by moving either one away from the other. You can move both of them either way and get smaller or bigger differences.

If you do a generator recharge first and then go to solar, your solar will be introduced when battery voltage is already up some. So PWM might be just as good by then to finish up.

It is all about the circumstances. I can't see any way to set up a fair fight that would have a general application.

[MrWizard]

The part in the reference that has PWM equal or almost equal to MPPT within that panel temperature range never has PWM doing more amps to the battery than you would get with MPPT. Yet at 25C ambient, and 51C panel with 230w I get more amps to the battery with the PWM 14.5 vs 13.5 amps.

In the example above 14.5 is a 7.4% above 13.5 , in this case in favor of PWM

If you take out 50 amp hrs at 12.5v for 625 watt hrs
And put it back at 50 amp hrs at 13.8 about 782 watt hrs,
There is a direct correlation , this is the POWER needed for the recharge
And everybody is ignoring the power used during the daytime while the charging is going on, which is more power to be generated by the panels
As PT suggested, a proper test would be the same two panels used PWM then again as MPPT in series
Also a fixed load will always generate the same amount of power used regardless of the controller
You need a load such as batteries that are at a known SOC , so low as to be able to accept all power generated, then see which system generates the most power over the period of Sun up to Sun down
Almost impossible to do with batteries
I would suggest a parallel string of 12v dc light bulbs greater than the expected wattage of the panels
These will actually glow a little brighter and use more power as voltage goes up, no batteries, no inverter,
Put an amp meter or watt power meter in series
Read the total power generated , lighting the bulbs..Sun up to Sun down

[JiminDenver]

Salvo wrote:
Did you notice that I said cell temperature? The irradiance for that test is 1000W/m^2. If you have 1 m^2 panel, about 800W goes to heat. Panel temperature rises substantially.

JiminDenver wrote:
75c is 167f

Just where do you plan on camping?

Great, so tell me what conditions you expect to see that in. I have never seen the drop off BFL speaks of and my panels see the 90's at altitude and 100s here in Denver. No one sees more intense sun than we do at altitude so I'm trying to determined just how hot it has to be to get a panel temp of 75c.

[Salvo]

Did you look at Fig. 14? Your answer is all there. As long as the mppt input voltage is a couple of volts greater than Vbat, it doesn't matter what the input voltage is.

pianotuna wrote:
Hi Salvo,

I'd love to see an analysis of Mppt vs Pwm at a vmp of 33 with a panel temperature of 75C. To make it a level playing field use 16.5 v for the pwm.

[Salvo]

It depends what type of shade you're talking about; dark shade, produced by an object real close to the panel or light shade, like from a tree that's farther away.

If the shade is dark, the bypass diodes will conduct. If the shade is light, the diodes will not conduct.

In a series string you're only as strong as the weakest link. That means in light shade the string current will be as little as the lowest producing cell. Series panels have a real disadvantage with light shade when the bypass diodes no not conduct.

I would go parallel, or perhaps series/parallel.

jrnymn7 wrote:

So, if series/parallel is not an option, which way is best, where partial shade is concerned; series or parallel?

Thanks.

[Salvo]

Did you notice that I said cell temperature? The irradiance for that test is 1000W/m^2. If you have 1 m^2 panel, about 800W goes to heat. Panel temperature rises substantially.

JiminDenver wrote:
75c is 167f

Just where do you plan on camping?

[pianotuna]

Hi BFL13,

What I was suggesting was identical wattage of panels with the MPPT wired in series/parallel and the pwm wired in parallel. I used 75C because that is what the study Salvo linked to used. Mine list vmp as 16.5 so that is why the series/parallel configuration would be a vmp of 33.

[BFL13]

Also you can't even set up the fair fight because you would have to pick the battery voltage you will be starting at. The lower the battery voltage the more amps you get from the MPPT.

At what battery voltage would it be a fair fight?

Salvo keeps saying his test only showed MPPT with about an 8% advantage, but his own test shows that the MPPT did 50% better!!!!

Of course that was early in the morning when PWM was 2a and MPPT was 3a. Later in the day when PWM was 8 and MPPT 9, that same 1 amp difference wasn't such a big percentage gain. You can make the results anything you want. Just pick the right conditions! 🙂

[KJINTF]

Agreed a fair fight is difficult to setup

The fight becomes a completely different game with LFP batteries
The guy over at http://www.marinehowto.com/ did a PWM Vs MPPT test
Results a clear 20% gain from MPPT
Suggested reading material

[BFL13]

pianotuna wrote:
Hi Salvo,

I'd love to see an analysis of Mppt vs Pwm at a vmp of 33 with a panel temperature of 75C. To make it a level playing field use 16.5 v for the pwm.

i.e. MPPT configuration series/parallel @ 33 vmp
and PWM configuration parallel @ 16.5 vmp.

Salvo wrote:
Fig 14 pretty much sums up the differences. The RV crowd are for the most part fair weather campers. That means cell temperature is in the range 40 to 80C. In that range, for the most part, mppt gains are between 0 and 10% better than pwm. On average it could be about 5%. Nothing to crow about.

PT , please clarify. Vmp does not apply to PWM and the IV curve for a 12v panel has its amps (Isc) fall off a cliff at 15v (which is battery voltage)

At 25C my panel was at 51C, so if panel is 75C I would not like to be RVing in what the ambient for that would be! ( Mena would probably even think it was a bit warm out)

Setting up a "fair fight" is complicated to say the least. The only thing they have in common that can be compared is amps to the battery.

You can't do PWM on a "24v" panel since the PWM controller has no buck converter. You can do MPPT vs PWM with 12v panels, but you would have to decide if it is "fair" to leave the panels in parallel and just swap controllers, or if it is "fair" to switch the panels to series for when the MPPT controller is used.

You have the wiring from panel to controller. Do you leave it the same? What if it is enough gauge for the lower amp MPPT but not enough for the higher amp PWM? The controller-battery wire would stay the same, both being at 12v.

My own comparison is just based on same panel wattage 230w MPPT vs 230w PWM for what amps I get to the battery in the same conditions.

But I am not using the same panels and I am using the same wire from panel to controller, good for the PWM and more than enough for the MPPT.

The part in the reference that has PWM equal or almost equal to MPPT within that panel temperature range never has PWM doing more amps to the battery than you would get with MPPT. Yet at 25C ambient, and 51C panel with 230w I get more amps to the battery with the PWM 14.5 vs 13.5 amps.

I know this is due to panel temperature because at 44C panel, I get more amps with the MPPT than with the PWM at 15.5 vs 14.5 amps.

So how come the fancy lab tests show only where PWM can be almost equal but never better? IMO it is because they are using "power" instead of amps to the battery as the comparison. The battery only cares about amps. 25C ambient is not terribly hot out either.

At a certain panel temperature in the high 40s I would get 14.5 amps either way from PWM or MPPT. Power at the battery would be the same using amps to the battery and battery voltage. Power from the panel would be different.

Power there cannot be compared to decide who is the winner. It is what the battery sees that matters and that is the amps. What power that comes to doesn't matter, it is just a thing you can calculate. The battery is charged by the amps passing through it to do the chemistry in there that raises the battery voltage. Power doesn't do that--amps do that. So the thing to measure in the fair fight is amps to the battery.

Since MPPT creates amps in an entirely different way from PWM you can't compare them earlier in the process, you have to wait until you see what comes out in each case.

Setting conditions for the fair fight is the hard part. Is it even possible? PWM amps stay the same while MPPT amps are all over the map depending on conditions. So you can get the answer you want by picking the conditions!

So that is ok too. You can pick your set-up based on the conditions you expect to be RVing in.