Test Method PWM vs MPPT ?

[BFL13]

I might get a chance to compare MPPT with my PWM on the set of two 100w panels. I get 12.6 (Isc) now with PWM so that is the "standard"

I was thinking of a quick connect set-up to swap controllers in place, and also to run the two 100s as 12v like now and as a 24v in series.

I gather MPPT does better on lower battery voltage. Does having a loaded voltage on the battery work for that same as if the SOC were lower? If so I could do two voltages, one low one higher on each test. Or?

Is it ok to swap controllers in place? I might have voltage drop that has no effect on the PWM but might bother the MPPT, no idea.

Any suggestions on how to do a quickie test (of course with no clouds that day, and do it mid-day, etc etc.)?

I have no idea how to test for the (bogus?) claim that MPPT will have a greater percentage gain in the shoulder hours. If there is an easy test for that I could do that I suppose (have Trimetric)

Not clear yet whether I will get the chance but hints of it. (borrowing the MPPT) Want to be ready ahead of time so can prepare a test set-up that is fair. (Otherwise, those MPPT guys will moan and whine 🙂 )

[HiTech]

While the IV and PV curves can vary quite a bit from panel to panel, it seems like the shape of each has been constant for each panel I have seen, as Voc shifts with temperature. If that holds true, you should be able to get an upper bound for what MPPT can do with a given multi panel setup by recording the PWM performance throughout the day, and recording the Voc from each panel (only disconnecting one at a time to avoid resetting the controller) each time a PWM reading is taken.

Voc can be used to interpolate IV and PV curves from the standard curve set by temperature/illumination for each specific panel. So you can guess at the Vmpp, Impp and Pmpp that could be produced for each Voc reading taken on each panel.

Draw this curve along with the PWM curve over the course of a day to guess at what the approximate max MPPT advantage could be with a specific mix of solar panels.

MPPT won't produce at this level, but I think it is an upper bound it will not exceed.

If nobody points out a major flaw with this method, I think I will try it.

Jim

[BFL13]

So if I constructed the test where I get 12.6a PWM no matter what, I could see on MPPT:
15.5a at 12.5v batt, 14.9v at 13v batt, and 13.8a at 14v batt (temperature allowing)

[red31]

Panel(s) better, worse or same as nominal?

12.6A vs 14.9A @ 13v

12.6A * 13v = 164 watts 14.9 * 13v = 194 watts /.97 = 200

12.6A * 12.5v = 158 watts 15.5A * 12.5v = 194 watts / .97 = 200

12.6A * 14v = 176 watts 13.8A * 14v = 194 watts / .97 = 200

[pianotuna]

Hi BFL,

I'm just quoting Salvo.

When you do the panels in series test the wiring will be quite adequate for the MPPT.

BFL13 wrote:
I think maybe my wiring lash-up that does not now matter with PWM might be unfair to the MPPT, or at least screw up your estimates. PT keeps saying that. I don't care if it is unfair to the MPPT. If it can't do the job-- too bad! 🙂

[BFL13]

12thgenusa wrote:


...I'll agree with you only if you know your battery acceptance rate is high enough to ensure max panel output.

Not following. If battery acceptance were that high, you wouldn't need to knock down the voltage with a load.

Trying to remember real life example, but something like this, where 12.6a is expected max but batteries only will take 8 amps so need a load:

Turn on some lights to get Minus 10 amps showing. Turn on solar, see Plus 3 amps showing. Solar is doing 13 amps.

[12thgenusa]

BFL13 wrote:

12thgenusa wrote:

BFL13 wrote:
If Battery Acceptance becomes a factor for the higher voltage test in the 14s, I have a drill for that. I turn off solar, turn on some lights to lower the voltage, note the neg amps on the Trimetric, turn on solar, note the new amps showing and the diff is solar. As long as solar amount is the most solar can do, we're good or else I turn on more lights.

All you need to do is turn on enough load at the beginning of the test until the Trimetric shows negative and monitor during the test to ensure that it remains so. This will force the panels to maximum continuous output.

I don't think it needs to remain negative. It can go positive but the spread in amps still has to be at least what you can expect from the panels. I have confirmed I was getting panel Isc this way.

Of course when the test is to find what max amps is with the MPPT controller unknown amount, then you have to over-do the neg side with more load so you get it all.

I'll agree with you only if you know your battery acceptance rate is high enough to ensure max panel output.

[BFL13]

12thgenusa wrote:

BFL13 wrote:
If Battery Acceptance becomes a factor for the higher voltage test in the 14s, I have a drill for that. I turn off solar, turn on some lights to lower the voltage, note the neg amps on the Trimetric, turn on solar, note the new amps showing and the diff is solar. As long as solar amount is the most solar can do, we're good or else I turn on more lights.

All you need to do is turn on enough load at the beginning of the test until the Trimetric shows negative and monitor during the test to ensure that it remains so. This will force the panels to maximum continuous output.

I don't think it needs to remain negative. It can go positive but the spread in amps still has to be at least what you can expect from the panels. I have confirmed I was getting panel Isc this way.

Of course when the test is to find what max amps is with the MPPT controller unknown amount, then you have to over-do the neg side with more load so you get it all.

[12thgenusa]

elkhornsun wrote:
My setup provides 3-4 times the charging capability that we have ever used. I optimized everything in the installation and in the equipment selection. I now know it was overkill. Once the batteries are back to 100% and it is 10 AM the rest of the potential output from the panels has zero value.

Not true at all. Everything you power from that point until the system goes to sleep is powered directly by the panels without the losses associated with discharging and charging a battery. That is as close as you will get to free power. Unless you spent a whole lot of unnecessary money in "overkill" what you did was the right thing to do. As time goes on I think you'll find more uses for your "extra" power. I think too many people undersize and then are unhappy with what solar can do for them.

[12thgenusa]

BFL13 wrote:
If Battery Acceptance becomes a factor for the higher voltage test in the 14s, I have a drill for that. I turn off solar, turn on some lights to lower the voltage, note the neg amps on the Trimetric, turn on solar, note the new amps showing and the diff is solar. As long as solar amount is the most solar can do, we're good or else I turn on more lights.

All you need to do is turn on enough load at the beginning of the test until the Trimetric shows negative and monitor during the test to ensure that it remains so. This will force the panels to maximum continuous output.

[BFL13]

Ambient temp for test will be say 23C. Tilted panels, air cooled. Voc might be usually seen 20.3ish instead of rated, so I suppose you could figure panel temp working back using the Sharp voltage coefficient. ISTR you are supposed to add about 20C to ambient when we looked at all that a while ago. It varies though.

I think maybe my wiring lash-up that does not now matter with PWM might be unfair to the MPPT, or at least screw up your estimates. PT keeps saying that. I don't care if it is unfair to the MPPT. If it can't do the job-- too bad! 🙂

[elkhornsun]

There are now 12 volt panels on the market. There are ones producing from 17.7 volts up to 45 volts. A battery can accept up to 13.5 volts so with a standard charge controller the extra voltage from the panel is clipped and lost. With the MPPT all the voltage is down converted so all the available amps from the panel can go to charging the batteries.

Where this is most valuable is with the panels providing 34 volts of output as these enable the use of smaller gauge wiring even when wired in parallel. Optimum for an RV would be these 34v panels wired in parallel with a MPPT controller. Optimum is not the same as most cost effective.

My setup provides 3-4 times the charging capability that we have ever used. I optimized everything in the installation and in the equipment selection. I now know it was overkill. Once the batteries are back to 100% and it is 10 AM the rest of the potential output from the panels has zero value.

[HiTech]

That's what I'm going for. Making an upper bound guess as to what it might do for you if it were a perfectly tracking, lossless MPPT controller. It depends on the real world temps of your panels among other things.

Jim

[BFL13]

IV curves on page two of this
http://files.sharpusa.com/Downloads/Solar/Products/sol_dow_ND130UJF.pdf

Not a clue what all this Vmp business is about of course. 🙂

Can you guys actually calculate what an MPPT might do without me having to run the tests? (Which I might not get to --it is not for sure yet)

[HiTech]

Do you have the model of the sharp panel handy? Looks like your real Vmpp on the Chinayard panel at operating temp might be in the high 16.xv range in good sunshine. Maybe a bit lower if the panels really warm up.

Jim