mppt vs pwm

Hi jrnymn,

I routinely get 17 amps from 256 watts. MPPT controller.

When your bank is at 85% state of charge you could still want ~65 amps of charging. Mine wants about double that. But you can equalize and I can not unless I use bank switching.

Since I paid $8 per watt installed I suspect your system cost 25% of mine. At that price, the extra 8 to 10% of solar harvest made lots of sense. ( ~20 extra watts @ $8 per watt = $160)

jrnymn7 wrote:
For example, my 280 watts of 12v panels have a combined Isc of 16a. Let's say that drops slightly to 15a.

On my bank, that's... 430Ah / 15a = ~ C/28

When charging at a C/4.8 rate, I see a switch from constant current to tapering current at ~83% soc. So at C/28, wouldn't that keep either controller in bulk/cc very deep into the charge?

If so, shouldn't mppt have the advantage (panel temp aside) throughout nearly the entire charge, even without any added loads, seeing as my bank can accept over 30a at 90% soc and a Vabs setpoint of 14.8v?


Moreover, wouldn't solar charging Lithium have the same effect, as they can accept huge amounts of current well into the charge?

Hi,

The reputable sources seem to say that MPPT works in bulk mode only.

If this is so, then during the day, if loads create a situation where the controller "sees" a bulk load, MPPT wins.

The question is by how much. Salvo is now saying it may only be 5%. Other sources say up to 21%. Pretty much everyone thinks 40% is pie in the sky (that's Canuck for "salesman speak").

MPPT wins at panel temperatures over 75 C (167 F). BFL13 sees temperatures of 51 C (~129F). I see winter temperatures of -37 C (~-34F).

So what works for me doesn't "fit" well for BFL13.

Folks need to make this a "dollars and cents" decision. If they are going to "big wattage" MPPT may (or may not) be the right choice.

I only know of one person who seems to be dissatisfied with their system, but perhaps someone sold them some of that "pie", instead of a good solid solar system.

Sorry Salvo,

By "shade", I meant partial obstruction, not low light/clouds.

I think someone told me an obstruction could shut down my entire array, if in parallel? (But I could be mistaken)

How does C-rate play into all of this?

For example, my 280 watts of 12v panels have a combined Isc of 16a. Let's say that drops slightly to 15a.

On my bank, that's... 430Ah / 15a = ~ C/28

When charging at a C/4.8 rate, I see a switch from constant current to tapering current at ~83% soc. So at C/28, wouldn't that keep either controller in bulk/cc very deep into the charge?

If so, shouldn't mppt have the advantage (panel temp aside) throughout nearly the entire charge, even without any added loads, seeing as my bank can accept over 30a at 90% soc and a Vabs setpoint of 14.8v?


Moreover, wouldn't solar charging Lithium have the same effect, as they can accept huge amounts of current well into the charge?

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.

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.

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

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.

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

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.