mppt vs pwm

[pianotuna]

Hi,

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

mppt vs pwm

[BFL13]

jrnymn7 wrote:
lol, yes, but I'm saying, unless the bank is very near full, mppt should REMAIN in bulk/mppt mode throughout nearly the entire charge. I don't see why a large load is needed to get it back into bulk, during a 50-9? % soc charge. With solar being at such a low amperage/C-rate, Vbatt should remain low enough to not initiate abs/constant voltage mode. Yes, once the bank is very near full, and now in cv, or already in float, then I can see a switch from cv back to cc when a significant load is applied.

I don't imagine many folks have a solar array large enough to put out more than a C/10 charge rate. (i.e; 700w solar for a 400Ah bank).

Yes the low charging rate means you get to Vabs at a high SOC in the 90s before amps taper. But what would they taper to? The acceptance rate is very low per battery by then.

I use 5 amps per 110AH as 90% SOC at 14.5v Say it is 2a per at 97% at 14.8 whatever it really is So that would be 8 amps with four batts.

So by the time you drop out of MPPT the acceptance rate could already be tapering the amps and the taper down would be at a deep slope with so little SOC to go.

Same whether MPPT or PWM for that. With the low battery acceptance rate, if this happens with the array capable of doing more, you get your "free" load running so you get a jump in amps when turning on a load. Not from some MPPT vs PWM thing but from in increase in the voltage difference with the load on.

[jrnymn7]

Salvo wrote:
I don't know what you're talking about.

In my scenario, some panels or a part of a panel has full sun while other panels or parts of panels are in shade. There are two types of shade. Some shade will cause the bypass diodes to conduct while with "soft" shade they will not conduct. If the diodes do not conduct then series is in a world of hurt.

jrnymn7 wrote:
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)

Yes, thus my confusion... From what I can recall, I've been told that an obstruction, i.e; deep partial shade (from a rooftop antenna, or a rooftop a/c unit, or a nearby tree branch or building, etc., will cause excessive power loss if the array is in 1) series and 2) parallel.

I'm trying to find out which configuration will suffer more in such a circumstance, series or parallel?

[BFL13]

Jim, I think the buck converter in the "MPPT" controller is always working the same way even with the MPPT not working anymore once the set Vabs is reached.

So the controller does not suddenly turn into a straight PWM that has no buck converter and can only do panel Isc. (or does it?) The real issue is what happens with panel voltage now that it is no longer being held at Vmp?

You have that mystery voltage and panel amps as the input watts to the buck converter with almost the same watts as output. Divide that by Vbatt and that's your amps now the controller is not in MPPT. As always.

I don't think that would be panel Isc unless the panel voltage were now the same as battery voltage. If it were then the buck converter would have the same voltage at each end and do nothing. So how could you get any amps? With PWM you get panel Isc at that voltage on its IV curve. If the buck converter is doing nothing does that act the same as if not even there so it just passes Isc through?

Adding a load in Float so amps go over the 24v panel's Isc shows the buck converter IS working but just not at Vmp. So what panel V is it at? Once Vbatt drops to 13.2 then the controller's MPPT kicks back in so then panel v would be back at Vmp.

Somebody with MPPT solar in Absorb or Float with just battery charging happening, please rush out right now and measure panel (or controller intake) voltage and also note what battery voltage is.

Then add a load so amps are above panel Isc. What's panel voltage now compared with Vbatt? Thanks. 🙂

[jrnymn7]

lol, yes, but I'm saying, unless the bank is very near full, mppt should REMAIN in bulk/mppt mode throughout nearly the entire charge. I don't see why a large load is needed to get it back into bulk, during a 50-9? % soc charge. With solar being at such a low amperage/C-rate, Vbatt should remain low enough to not initiate abs/constant voltage mode. Yes, once the bank is very near full, and now in cv, or already in float, then I can see a switch from cv back to cc when a significant load is applied.

I don't imagine many folks have a solar array large enough to put out more than a C/10 charge rate. (i.e; 700w solar for a 400Ah bank).

[JiminDenver]

Salvo wrote:
Don't expect to be handed everything 0n a silver platter. Google away. Or, put a thermocouple on the panel and run some tests. That's what I did.

JiminDenver wrote:

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.

A thermometer lived on the back of a panel for over a year starting with below freezing test and running through out the summer. I never saw those panel temps and it gets plenty hot here. That's why I asked what conditions you were in to see it.

[Salvo]

I don't know what you're talking about.

In my scenario, some panels or a part of a panel has full sun while other panels or parts of panels are in shade. There are two types of shade. Some shade will cause the bypass diodes to conduct while with "soft" shade they will not conduct. If the diodes do not conduct then series is in a world of hurt.

jrnymn7 wrote:
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)

[pianotuna]

Hi jrnymn,

I use the 1200 watt water heater when I have "solar to burn". That puts enough of a voltage drop to get me into bulk/mppt mode. Solar to burn = less than one amp of charging in good solar conditions.

jrnymn7 wrote:
PT,

Yes, but I'm asking whether a heavy load on the system is even required to kick the mppt "back into bucking mode". Seems to me, unless one has a rather large array in relation to their bank, bulk (and thus bucking) would occur very deep into the charge, regardless. I often read, mppt only applies in bulk, but if amp tapering doesn't happen until that last few % of soc, then it's kinda irrelevant, isn't it?

[Salvo]

Don't expect to be handed everything 0n a silver platter. Google away. Or, put a thermocouple on the panel and run some tests. That's what I did.

JiminDenver wrote:

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.

[JiminDenver]

The controller only maximizes charging in bulk mode. Outside of bulk mode is when the battery voltage is high enough that you are not loosing to PWM or so it seems. The bulk converter is still converting excess voltage to amps so you should realize a small increase in PWM charging too IF the battery isn't already tapering. The Isc of my panels is close to 10a but I can see 16a when the controller first goes into absorb or I pull a load in float.

[pianotuna]

Hi,

I get 7 amps from 256 watts MPPT in leafy shade at solar noon in June.

[jrnymn7]

PT,

Yes, but I'm asking whether a heavy load on the system is even required to kick the mppt "back into bucking mode". Seems to me, unless one has a rather large array in relation to their bank, bulk (and thus bucking) would occur very deep into the charge, regardless. I often read, mppt only applies in bulk, but if amp tapering doesn't happen until that last few % of soc, then it's kinda irrelevant, isn't it?

[pianotuna]

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?

[pianotuna]

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.

[jrnymn7]

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)

[jrnymn7]

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?