Low amps produced by 100 watt panel? Why?

zoneblue

According to electrodacus, PWM can actually work quite well for LFP using 60 cell panels. His explanation is that:
- 60 cell vmp is closer to the LFP setpoint voltage
- LFP doesnt need eq, or high absorb
- the time when 60 cell panel has lowest voltage is in mid day sun when the bank is likely already full.

The guy is currently building a 3kW PWM targeted specifically at this application.

mike74820

i read on here and on another site about killing battery's with low volts, if like me i got a pwm controller thats not programmable so ive ordered a c35 xantrex controller and it will enable me to up the charging volts 

South Africa

vtmaps said:

In cold temperatures string voltage is higher and the gain can be utilized better by an MPPT controller. 

Once absorb stage of charging is reached, an MPPT controller works like a PWM controller.  

I never knew that once absorb stage is reached, MPPT's are the same as PWM controllers.

In 25-40 deg Celsius, how much more effective are MPPT's compared to high-end PWM's?

BB.

Well, sort of...

Once the solar charge controller is outputting less current than the solar panels can supply, then pwm and MPPT controllers are similar.

Absorb charging is a fix voltage to the battery, and a naturally declining current into the battery bank. And float charging is even a lower voltage setpoint.

However, if you have dc loads (ac inverted, etc.), then you can have a high current (power) draw even in absorb or float mode. Then an MPPT controller will have an advantage over a pwm controller.

Note, the hotter the solar panels, the lower the Vmp, and the less advantage a MPPT will have.

-Bill

mike95490

vtmaps said:
Once absorb stage of charging is reached, an MPPT controller works like a PWM controller.  --vtMaps

Not always.   I've caught my Morningstar MPPT-60 in MPPT Absorb and Float,  Now to get a photo of the readout.,,,,  I believe if it needs the extra power, it will try to find it with MPPT

zoneblue

Once the absorb setpoint has been reached and the sun stays out, the MPPT controller takes the PV up its IV curve to throttle it. Its not actually PW modulating, but teh effect is the same.

South Africa

Good point BB, when the system operates under load all the time.

What is defined as hot from the locals here, i.e. Canadian hot versus Florida versus Cape Town?
I gather +- -5deg Celsius with clear subtly diffused light due to very high clouds (for better reflection of light) is optimal.
22+ deg Celsius upwards is hot.
+30deg Celsius is getting very hot = most inefficient.
+40deg Celsius is getting into the 'moerse' hot region. 

Reason I am asking, on average 28 to 40 deg Celsius, how much more efficient is a MPPT over a PWM? Is it really worth the extra costs?

PNjunction

This is exactly what I'm talking about!

What bugs me is that every time an op with a system loss question comes up, once they find out that they are using a pwm controller, that is used as the bogey-man and the mppt band-aid pill comes out.

I'm not disputing quality mppt efficiencies over pwm.

The problem is that the original op has a problem with his panels, not the controller.  Seeing only half the rated output means either 1) poor environmental conditions or 2) a defect in the panel itself.

The WORST part of it is, is that even Renogy or others here reassure him that it is ok to experience this kind of loss while not taking into account the fact that he is measuring his problem at the panel itself!  And is also the reason I'm suggesting another test another day.

But all too often, out comes the pwm vs mppt thing - and this only serves to promote market "branding" of mppt vs pwm in general, without taking a system-approach into account.

The original op has a systemic problem prior to the controller.  On a good day, seeing only half the rated output is not acceptable whether you are running pwm or mppt - but newcomers might be steered into making a wrong conclusion that lies outside of the controller itself.

mike49

(removed double post)

mike49

Well, I'll take blame if I steered the OP in the wrong direction, and maybe newbies like me should be banned from posting, but I was conveying one of the first things I learned and actually took me quite a while to find out while getting ever confused with all the nuance debates:

Why I am I not getting the face value of the wattage of the panel? 100watt panel should get me 100watts, shouldn't it?
 
Even after lurking on forums etc for over 6 months, I still really didn't start to understand the answer to this question until one day I read a post from BB referring to this FAQ:

http://www.solar-electric.com/mppt-solar-charge-controllers.html/

Then the light finally turned on for me. It is the incorrect newbie expectations that this FAQ directly addresses and then gets into technical reasons why. This FAQ rocks. Every newbie should start with this FAQ.

I'm hoping it helped the OP as well, because what he wrote is precisely what I learned and exactly why I pointed to the FAQ.

Joesv57 wrote:
Quick update.  Looks like after lower my expectations of what could "really" be achieved with 2- 100 watt panels it is performing as it should, getting between 6 -10 amps in direct sunlight.  I was unaware that they were measuring 100 watts as the output at 18-20 volts from the panel vs. me figuring it at 12 volts at the battery.  So in reality, getting 60 watts from a 100 watt panel is more realistic.
Thanks for the help.  Learning a lot!

---

That being said, I want to make sure I understand something in my own system:

I was saying to the OP the most I've seen is 4.5A@13.5v (~61watts ) from my 100watt panel in the peak of summer. So far this winter, the best I've seen is 4.1A@13.5v (55watts) recently, up from 3.5A. I test each panel separately - 20foot 10gauge wire direct from panel to pwm controller. Everything in panel junction boxes looks in order.

Results are consistent for all 3 of my panels with these specs:

2 mono:Vmp 18.2v Imp5.5a Voc 22.7v Isc 5.89a

1poly:Vmp 19.2v Imp 5.23a Voc 22.68v Isc 5.6a

I think the OP was saying he was getting similar results as me.

Are you guys saying we should be getting ~5.5A nomatter that my location is Toronto 44N (OP said he was in midwest) and nomatter the time of year as long as it is a clear sunny day?

Doesn't it depend on location and time of year and solar irradiance/insolation that I am learning so much about this winter?

Here is what is published for my location:

http://pv.nrcan.gc.ca/index.php?n=1752&m=u&lang=e

Based on this, do you guys think my panels are defective or there is a problem with my wiring as well?

thanks a lot for the learning...

mike95490

100 w panels rarely produce 100w.  They can be counted on to produce 80% of their nameplate power in real life conditions.
Just like car mileage stickers, they lie !

zoneblue

There are a number of factors that determine the system output. I explained all of this above. Only the OP will know which of those factors are the limiting ones in his/her use case. As bill said a single leaf can make a huge difference and its not possible for us to know if such things are the case.

PWM is not the bogey it once was, simple PV pricing economics has seen to that. If you have a 100 dollar PWM and the equvilent MPPT controller costs 300 dollars, and it only saves say 15%, maybe a second panel for 100 bucks is cheaper (and more reliable) than the MPPT? Bills rule of thumb is that the break even point is/was around 400Wp, anything under that and PWM is overall more cost effective.

Photowhit

zoneblue said:  Bills rule of thumb is that the break even point is/was around 400Wp, anything under that and PWM is overall more cost effective.

I think it was 'Coot's rule of thumb, I'd say it's higher if you can get panels design in the correct voltage range at near the same price. Indeed it could be very much higher. I've got a quote somewhere, the boB says generally MPPT controllers are about 10% more efficient....

I might well have gone with PWM if I could have gotten the panels in the correct range at the same price. I was very happy with a reliable 1500 watt system on a PWM before my current system, It is still a reliable system for the people I sold it to!

BB.

Rules of thumbs are there to get you "close" without too much extra work (research, math). If you system is in the ~400 Watt to 800 Watt range, either PWM or MPPT can work OK (good price, relatively few issues on installation).

However, price is important. Panels have gotten very cheap... And "brand name" solar charge controllers have "flat-lined" in price. Shipping is expensive, smaller solar panels (140 Watts or less) are ($/Watt) generally more expensive that larger panels (>200 Watt).

You may pay $300 for a single 300 Watt panel (that needs a MPPT controller for optimum charging, Vmp~30-40 volts). While you may pay $600-$700 for 2x 140 Watt panels. But shipping for a single 300 Watt panel could be very expensive (truck, packing, insurance), where a pair of 140 Watt panels may ship UPS or Bus (smaller panels).

In general, I like MPPT controllers. You can run "high voltage" Vmp-array and use much smaller copper wire and run longer distances from Array to Charge controller/Battery shed. Also, if you are in a very hot area, PWM controller with Vmp~17.5 volt arrays may have less than optimum charging current on hot summer afternoons (heat=>hot cells+wiring voltage drop=>depressed Vmp to near battery charging voltage).

In the end, if cost is important to you--Do a paper design with both PWM and MPPT systems (including shipping costs) to see what works out best for you.

-Bill

South Africa

Having started off many years ago with 3 x 310w panels, it has become quite a complicated issue to add more panels onto my PWM controller. Either I exceed the max panel wattage's or the volts. 

My rule of thumb: If you start off and have any inclination that you may one day get a thought of adding more panels, go for a  MPPT controller from the start. Trust me, just do it.

mike49

zoneblue wrote:
"assuming you have equinox sun, zero angle of incidence, youd expect something like: 13V *5.3A= 68W"
"its winter in the north...slight drop in current..and the sun is low in the sky thus, take off say 5% for the thicker air in the lower part of teh sky"
"the rule of thumb is that PWM controllers are 95% or so effcient"

Alright, 68w - 10% = 61.2w expected
I've seen at best this winter = 4.1A@13.5v (55.4w) on pwm controller LCD display from my 100w mono or poly panel

So I'm missing 6 watts and did a preliminary hunt for it today...

The sun wasn't that great, but I did catch some glimpses of brief sunny periods peaking through the clouds...

1) took out my digital multimeter

2) newbie spidey sense starts tingling - spidey sense conversation with myself:
"hey mike, doing this testing should be fun, but didn't you go to great lengths to make sure everything is safe from short circuits?
isn't short circuiting anything, esp a DC current source potentially bad, dude?
remember when you did the newbie ooops and sparked the battery? lucky thing you had a fuse there, eh, and now you love fuses everywhere, don't ya? ah ha, that's why BB kept saying to use a DC clamp meter, not a multimeter. But I don't want to buy another expensive toy and need to save for the MPPT magical cure all. alright then, do the poor man way and cover up the panels, at least if it sparks it won't be as bad, alright, I'll try that mikey..."

3) voltage at the panels seems on spec

4) amps seen at shorted panel I saw at most was 3.5A today during my test period, not that great a testing day as it wasn't a full clear day..

5) amps seen on PWM controller LCD when I plug it back in - from what I could tell was consistently about the same as amps at shorted panel.
This makes sense since my newbie understanding is all a pwm does is directly connect the panel to the battery for bulk/CC stage...

6) When I get a chance next on a more clear sunny day if it ever comes, I'll do more testing like have my 2 identical mono panels out, one shorted with multimeter, the other connected to PWM
so I can compare at exactly the same time instead of eyeballing back and forth like I did today.


This is fun. Thanks all again for the learning.

And if I don't find those missing watts in summer, I will either:
a) blame those dang govt photon blocking chem trails
or
b) blame poor irradiance at my location or at my time of testing   

Then maybe try the magical MPPT cure all

vtmaps

Photowhit said:

I think it was 'Coot's rule of thumb, I'd say it's higher if you can get panels design in the correct voltage range at near the same price. Indeed it could be very much higher. I've got a quote somewhere, the boB says generally MPPT controllers are about 10% more efficient....

Bit of a quibble... Generally, I think PWM controllers are more efficient than MPPT controllers.   I think PWM controllers tend to have lower tare losses and convert less energy to heat.

MPPT controllers are more effective than PWM controllers.  They tend to harvest more energy in a day's time.

mike49 said:

And if I don't find those missing watts in summer, I will either:
a) blame those dang govt photon blocking chem trails
or
b) blame poor irradiance at my location or at my time of testing   

Then maybe try the magical MPPT cure all

An MPPT controller may not work as well as you hope, especially in the summer when your panels are at their hottest.

You have true 12 volt panels with Vmp about 17-18 volts.  At times, you may have your battery voltage up to 16 volts.  An MPPT controller needs headroom, that is, it needs to load the panel (which pulls down the panel's voltage) and discover what amount of load results in the Maximum Power Point.   Thus, for example, if you need 15.5 volts (to the battery) from a hot panel that is only producing 16 volts, an MPPT controller will not work as well as a PWM controller. 

Another way of saying that is "a PWM controller has less voltage drop than an MPPT controller"

This is not just theory.... there are a number of sad stories on this and other forums of folks who switched to MPPT controllers with very poor results. 

There are certainly good reasons to use an MPPT controllers in some systems.  I think you will need to grow your system considerably and raise your string voltage before an MPPT controller makes sense for you, especially in the summer. 

--vtMaps

zoneblue

Dont forget the (sometimes much) higher tare draw of MPPT controllers.  For instance the 30A Midnite Kid draws a half watt, the 30A Midnite Brat draws about a fifth of that. A larger mppt controller can draw as much as 6W, doing nothing, 24 hours  a day.  If you only have one or 2 100W panels then most likely your ONLY practical choice is PWM. Morningstar do make a small MPPT, thats certainly the only one id be wanting to experimetn with, dont fall for the ebay faux "mppt" junk.

mike49 said:

2) isn't short circuiting anything, esp a DC current source potentially bad, dude?

Its perfectly safe to measure the short circuit current of a soalr panel as they are inherently current limited. The one thing you should be aware of is not to test directly to the MC4 (to avoid damging the silver plating), connect something to the mc4, and connect your meter to the something.

PNjunction

Mike49 - It sounds like you are doing ok and what you present now, even in winter with long angles even during the best of the insolation period wouldn't set off any alarms like the original op has.  The math works using 18v panel ocv as an average..

68W panel / 18v = 3.77A capable short circuit

You took off 10% for a non-perfect day and expected 61.2 watts:

61.2 / 18v = 3.4A

Holy cow - you were spot on when you actually measured it.  (Tip when measuring short-circuit panel leads - don't forget to move your meter jacks PRIOR to the measurement.  If left on voltage, poof!  Ask me how I know <grin> )

Put this in the solar notebook : the reason you don't kill a panel when you short circuit it, like you would with a battery, is that unlike a battery, a panel is a *current source*, not a voltage source - although obviously both are present.  It's a mind-blower when for most of our lives, we have only used voltage sources.  But I digress....

So here, this doesn't raise any red flags like it does with the original op's problem.  His math doesn't work out, so he either truly has bad panels (missing / blown bypass diode can cause only half-the output), a really poor solar insolation period, or trying to charge batteries that are already in the absorb stage, where current is naturally tapered...

mike49

PNjunction wrote:
 (Tip when measuring short-circuit panel leads - don't forget to move your meter jacks PRIOR to the measurement.  If left on voltage, poof!  Ask me how I know <grin> )

Umm, yup I think I did a similar mistake, my meter beeped like crazy when I had the leads on current but setting on voltage (or was it leads on voltage but setting on current), I quickly pulled the leads, didn't go poof, lucky me I don't need to buy another $50 meter..

Out of curiosity, my meter says I can only test current for max 15 seconds every 30 minutes, I'm guessing it's some kind of shunt that will get too hot and blow if I leave current to it? Kinda hard to do more thorough longer timeframe testing, but glad you think me panels are ok thanks
.
PNjunction wrote:
Put this in the solar notebook : the reason you don't kill a panel when you short circuit it, like you would with a battery, is that unlike a battery, a panel is a *current source*, not a voltage source - although obviously both are present.  It's a mind-blower when for most of our lives, we have only used voltage sources.

Thanks, yes this is why newbie me was scared. Newbie me reading all the time how short circuits are BAD and newbie scardy cat after 1st blown fuse at the battery way back when

---

vtmaps said:
"Thus, for example, if you need 15.5 volts (to the battery) from a hot panel that is only producing 16 volts, an MPPT controller will not work as well as a PWM controller...there are a number of sad stories on this and other forums of folks who switched to MPPT controllers with very poor results..."

bluezone said:
"Dont forget the (sometimes much) higher tare draw of MPPT controllers"

Thanks - more to learn about the reasons behind why an MPPT may not be the magical cure all, this is a good start...
I'll certainly look up those posts from people with poor results...
I guess if I put my panels in series when using an MPPT controller, voltage would be less of an issue?

--

bluezone said:
"The one thing you should be aware of is not to test directly to the MC4 (to avoid damging the silver plating), connect something to the mc4, and connect your meter to the something"

Ah, how'd ya know I just stuck my probes in the panel's MC4 it fits sorta nicely...
I suppose I can make some short MC4 cables to put on the end of my panel's MC4 for testing,
but then again, wouldn't plugging in and out MC4 connectors shave some silver plating off as well?

I'm sure glad I'm taking the poor man's slow learning approach to off-grid solar...
Lots of potentially expensive learning gotchas, lots to learn (very fun, but seems endless:), thanks all for the advice and help...

zoneblue

Yeah i buy the darn fuses in ten packs, duh! Some odd imperial size too.

Its not mechanical damage mike, but 18V is enough to produce an arc. Its that arc that ruptures the plating, spare mc4 pigtail is ideal.

mike49

ah arc, got it thanks

Last night I cosied up with my tablet and spend over an hour reading through old "pwm versus mppt" threads here.
Only got through the first few pages of search results before dozing off......

SO, based on what I read, would it be fair to summarize the general real-world experienced consensus here as:

For a small system of <400-600watts of panels, even put in series, the realistic additional gain that can be expected from use of an mppt controller over a pwm controller is:

around +10% ?

and maybe above +10% during the 6 months of winter up here in the Great White North, the colder the better since voltage increases with cold temps, "up to" but hardly ever maybe brief moments getting +30%?

simmtron

If I were you I would just buy another panel, cheaper and you will get more power than just upgrading to MPPT. You will get max 10% from changing to MPPT and you would get 33 % more from upgrading 1 more panel. Sorry you would actually get 50% more

zoneblue

The nice thing about the extra panel, is they last almost forever, if you have the space. But for bigger systems, mppt is good primarily because of the flexibility it gives you to use 60cell panels in various series configurations, or for really long array cables to reduce cable costs.

mike49

Alllllrighty then...so it looks like I did get brainwashed by the magical MPPT cure all marketing

THANK-YOU Mr. PNJuction for calling it out !!

Up til now, I've always honestly blamed my cheaping out on getting a PWM controller instead of an MPPT and assumed by the marketing that I would get most of the rest of the watts that I've never seen from my 100watt panels if I only coughed up cash and go to MPPT!!

Lowering expectations for newbies on panel performance with BOTH PWM AND MPPT debunking should be on some 1st sticky that every small system newbie should read !!! Maybe it's here somewhere and I missed it all this time?

So you guys have saved this currency collapsed poor man a pile of expensive US dollars and made me much more satisfied with my little system and PWM controller. Now I can forget the MPPT and look forward to other toys like a DC clamp meter, hydrometer, and one day the most expensive of all toys - the "achillies heel", "101 ways to kill it quickly", "I need a pHD before I dare buy them and try to maintain them" - Deep Cycle Lead Acid Batteries!!!
 
Thank you ALL...

westbranch

"101 ways to kill it quickly"    well if you keep coming come back , it is like an insurance policy you won;t have to use.. or buy...
now start reading about AGM and LiFePO4 batteries, the last is looking like the Holy Grail of batteries....
A small, lightly used  set of them is a good learning tool (or lesson), lots around if you look and ask.

mike74820

as a pwm user myself, all i can tell you make sure your pwm controller is adjustable and you should be ok . i can tell you with my 200 watts of solar panels and my c35 xantrex controller set at 14.8 volts for bulk and 14.6 for float my battery's have never been happier. just read handy bob's stuff on pwm controllers and where to set them

mike49

hi mike74820, took me a week, but interesting read through that angry/solar/handy bob's blog
i'm only in grade 1 working my way towards the pHD needed to maintain a battery bank, but if i understand correctly, handy bob was using trojan t105 when he recommended 14.8v bulk and 14.6v float for it, right? i looked up the trojan recommendation and it looks like only 13.2v for float, so i guess it's the 14.6v float that is unique with what he is saying? and that works well, eh?

does anyone else here use 14.6v float and had good results?

and i guess i should add a battery monitor to my growing list of expensive toys to get ;-)

(apologies to OP, don't mean to change this thread to a different topic   )

mike74820

i just know when i do what handy bob say, my battery voltage settles in around 13 volts,  before i doing it the other way it rested at 12.7 volts, and now after i use the battery and wake up in the morning that voltage is usually around 12.65 volts

Photowhit

I think Handy Bob has a lot of good advice. I don't like that he calls forums dangerous and points to NAWS but describes Sunking (a truly dangerous, sometime moderator, of other forums) I don't know of a better forum than NAWS. I might even be the person he refers to that heats water via photovoltaics/electric, though I've had discussion's with many people on the subject. If a flooded lead acid battery is resting at 13 volts, it likely just hasn't been resting very long...

My advice would be to read everything you can find and you will come to a better understanding of how systems work.

I didn't realize Handy Bob had been barred from here, but some people find forums too comfortable to speak their mind. After correctly SunKing for many years on a now nonexistent forum his bloviating just gets my blood up... (He has said off grid is only 90% reliable, and I had to explain Net metering to him, yep I'm that old!) I love that Bill (BB) takes the time to present people with facts and others here like to round things out. We've lost some very good moderators here, miss you guys! But you won't find a more balanced forum on solar. One man's opinion.