Replace PWM with an MPPT regulator

Submitted: Sunday, May 12, 2013 at 00:24
ThreadID: 102159 Views:1850 Replies:4 FollowUps:11
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Hi
After reading about the MPPT regulators I would like to replace my PMW with an MPPT.
I have a new 80w folding solar panel.
If I had read these forums I would have brought a solar panel with the MPPT regulator instead of the PMW.
What size MPPT do I need.
10/15/20 or 30amps.
Thanks
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Reply By: Scott T4 - Sunday, May 12, 2013 at 00:29

Sunday, May 12, 2013 at 00:29
And what brand
Thanks
AnswerID: 510836

Follow Up By: Member - Boobook - Sunday, May 12, 2013 at 05:01

Sunday, May 12, 2013 at 05:01
Scott I am a big fan of MPPT over PWM but with only 80 you won't get much of a gain. You would probably get 4.2 Amps peak fight now. With a MPPT you will get about 6A peak. You would get some advantage for about 4 hours so that's 8AH per day.

That's not much of a gain for the price. But if you intend getting more panels or just need to squeeze every bit out of your system then a 10A controller will be good for nearly twice your current capacity, and 15A will be good for about 200W

search ebay with the terms "mppt solar tracer" which is a good brand and economical. You can get them with or without meters. The meter is very useful.
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Follow Up By: Warb - Sunday, May 12, 2013 at 08:34

Sunday, May 12, 2013 at 08:34
An 80W panel will give a maximum of 80W (given the specified light intensity). The panel will also have a maximum current, which no regulator can increase. One supplier of LED camp lighting states in every advert that their MPPT regulator can increase the output of a 120W panel to 180W, but that is an outright lie (which is a shame, because their LED lights are quite good!).

The panel you have is probably specified as 80W at 18V, which is 4.44A. The short circuit current will be listed somewhere in the specs of the panel, as will the open circuit current and voltage and the maximum power voltage and current. The short circuit current (which will be only slightly above the 4.44A calculated above, perhaps around 4.6A) is the maximum current the panel will produce, or allow to pass through it, under any circumstances. Other than in a short circuit you will never see that current!

The problem that an MPPT regulator attempts to address is that if that panel is connected to a partially depleted battery, lets say 11V, it's voltage will be dragged down to 11V. Now 4.44A at 11V is 48W. So under full sun that panel will only be producing 48W when connected to that battery. When the battery is fully charged, lets say 13.5V, the panel will be able to produce 13.5x4.44=59W, but of course the battery is now fully charged so that power isn't needed (unless there are other loads that are using the power).

So the 80W panel will never produce anywhere near 80W when it's voltage is dragged down to batery levels.

An MPPT regulator will allow the panel to operate at it's "best" voltage, whilst reducing still providing the battery with the voltage that it needs to charge correctly. So 4.44A at 18V = 80W, given enough light!

Of course the other point is that no matter what regulator is used the panel can only produce 80W under "test" lighting levels. That is normally 25C and 1000W/m^2 of light, and this rarely occurs! Solar panel output reduces with increasing temperature, and 1000W/m^2 normally only happens for an hour or so at midday in the middle of summer, when the temperatures are far higher than 25C.

The MPPT regulator will, of course, still work under lower light conditions or higher temperatures, but if there is only enough light to produce 40W at 18V, then you will only get 40W! But then without MPPT you will get even less.

So:

1/ MPPT is good, but with only one small panel you have to decide if it's worth the cost.

2/ An 80W panel can only produce 4.4A, so a 6amp regulator is sufficient. Unless you plan to add panels later (which is also more involved than it might first sound!).

3/ A cheap MPPT regulator will not be true MPPT, it will simply lock the panel voltage at 18V. That's better than nothing but will not give the optimum result under all conditions.

4/ A "real" MPPT regulator is definitely not worth the price for an array rated in watts (rather than kW!!)

5/ For an 80W panel and the gains to be had, spending "named brand" money is of questionable worth. These days with panels costing <$1/W (unless you pay the outrageous prices the camping shops still charge!) it might be better to buy more panels. A folding 120W array can be had for about $160, and with your existing regulator it will produce 76W in to an 11.5V battery (given enough light).......A fixed 180W panel (roof top mounting on a caravan) will cost about $200. Perhaps new panels are a better way to go?

6/ Camping shops have no idea about solar panels. Most internet experts have no idea about solar panels. Many companys selling solar panels to campers lie, or don't know about solar panels. If anybody tells you that MPPT will increase the panels output above its specified level, run away vary fast!

7/ People with no understanding buy products from retailers with no understanding (or questional morals!) and then are either disappointed with the results or simply never know their system is not great.

Maximum is maximum. All we can do is to try to recover the losses and get as close to maximum as possible under any given set of conditions.
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Follow Up By: Member - John and Val - Sunday, May 12, 2013 at 09:09

Sunday, May 12, 2013 at 09:09
Warb,

Must disagree with some of what you say. An MPPT regulator CAN increase the charging current above that coming from the panel. Obviously, as you say, it can't create energy. It is a switch mode device capable of changing voltage though, and allows the panel to operate at its optimum, about 18V, while delivering current to the battery at about 13.5-14.5 volts. The controller reduces the voltage by about 25% so if it was 100% efficient we'd look for a corresponding increase in current going to the battery. In practice we can expect maybe 15%.

I agree with your suggestion that with a small system it may be more cost effective to add more panels rather than picking up that 15% by replacing the PWM controller. My personal preference is to go for both - more panels AND an MPPT controller. In my experience, if there is a fridge involved, 80W isn't big enough for a camping setup, but double that is plenty.

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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Follow Up By: olcoolone - Sunday, May 12, 2013 at 09:45

Sunday, May 12, 2013 at 09:45
MPPT reg will give between 10-20% increases over a conventional reg....... the 20% figure is under ideal conditions whereby you get maximum sunlight and the solar panel surface temperature is between 20-25 deg C.

MPPT regs are good but on small panels the advantages of a MPPT reg is outweighed by the cost.

Solar panel ratings can be misleading much like 240v inverters..... sure it will deliver 3000w but with the power consumption of 1000w inverter.

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Follow Up By: Member - Boobook - Sunday, May 12, 2013 at 09:46

Sunday, May 12, 2013 at 09:46
Warb, give it away mate. You obviously don't understand how it works and should not give advice on the subject.
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Follow Up By: Warb - Sunday, May 12, 2013 at 09:50

Sunday, May 12, 2013 at 09:50
John,

In my post above I do not mention the amps an MPPT regulator will supply to the battery, because that number is not relevant to MPPT (being a panel optimising system rather than a charger per se), and it is not a stable figure as the current will vary with battery voltage etc. All the numbers I quote are for power (watts) as the MPPT system optimises the power produced from the panels, rather than the amps sent to the battery.

As you say, the whole point of an MPPT system is to separate the panel voltage from the load voltage. The battery charger section will (should?) always produce a voltage correct for battery charging - so if it is seeing 80W from the "panel", then no matter whether that is 1 amp at 80V or 80A at 1V, it's output to the battery will be a touch over 11V and 7.2A, if the battery voltage is 11V.

MPPT refers solely to the panel "interface" and my comments refer solely to the panel voltages, currents and power.

However, if we look at the current going to the battery then if the MPPT system manages to get the full 80W (4.4A x 18V) from the panel, whilst a non MPPT regulator that allows the panel voltage to be dragged down to the battery's 11V (i.e. 48W from the panel), then the MPPT equipped charger will be able to supply 7.2A to the battery, rather than the 4.4A maximum from the non-MPPT system. Assuming 100% efficiency, of course - at the 85% efficiency that a cheap MPPT system may (?) run at, that drops to just over 6amps.

However it is normal to discuss MPPT in terms of panel output, because what the power is used for is a separate subject. In this case it is a battery charger, at which point the volt and amp numbers from the unit will entirely relate to the size and state of the battery, the type and efficiency of the charger, and any other loads that are connected to the battery. Is is entirely possible for either the MPPT or non-MPPT regulator to actually output almost nothing if the battery is fully charged.

Regarding fridges, once again nothing is straightforward! My Engel draws a maximum of 2A, but what percentage of time it draws that 2A depends almost entirely on the ambient termperature. In the Vic high country in winter it almost never runs, and so uses almost no power. If shut in a car/trailer/enclosure that acts like an oven it will run almost continuously in 45C (outside) and sunny weather. So it's consumption, and therefore battery charging requirement, could be anything from zero to 48Ah per day. At the high end of that scale a 200W panel would be hard pressed to keep the fridge running, I would need at least 200Ah of battery to keep it going (without risking compromising battery life), and a cloudy day would create a deficit from which it would be hard to recover.
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Follow Up By: Warb - Sunday, May 12, 2013 at 09:58

Sunday, May 12, 2013 at 09:58
"Warb, give it away mate. You obviously don't understand how it works and should not give advice on the subject."

Hmm.. I design and install domestic off grid solar power systems and have somewhere over 20kW of my own solar power including grid-tie and off-grid (battery charging!) systems, so I'm pretty sure I know how they work. However if my advice is not required it's no big deal to me.

I shall no longer contribute to solar power discussions on this site.
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Follow Up By: Member - Toyocrusa (NSW) - Sunday, May 12, 2013 at 10:46

Sunday, May 12, 2013 at 10:46
Check who gets the most thanks first Warb before you make your decision. Electrical posts run on and on and nearly always degenerate. Bob.
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Follow Up By: Member - John and Val - Sunday, May 12, 2013 at 12:25

Sunday, May 12, 2013 at 12:25
Warb,

Perhaps I misinterpreted your initial comments, which to my mind implied that the battery charging current was limited to the panel output current.

I stand by my response that the battery current CAN exceed the panel current, indeed that is the whole point of using an MPPT controller.

I also stand by my comment "In my experience, if there is a fridge involved, 80W isn't big enough for a camping setup, but double that is plenty." Of course actual demand varies according to thermal loading and many other factors - but why argue over something that starts "IN MY EXPERIENCE......" That's a plain statement of fact that you are in no position to question/argue.

I've had enough of this debate, and like too many other valued contributors will not contribute further. Too often in electrical threads, well intentioned input leads to a fight between the players, to the huge detriment of the game.

John
J and V
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Reply By: Member - Broodie H3 - Sunday, May 12, 2013 at 12:48

Sunday, May 12, 2013 at 12:48
Hi Scott t4,
I don't know about you but now I am totally confused, I think I may go and talk to the experts,lol thanks for the question, as I am in the same position with the caravan and the solar set up, and was an interested bystander, that is allergic to electronics. L OL
Broodie H3
Have car will travel

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AnswerID: 510874

Follow Up By: Scott T4 - Sunday, May 12, 2013 at 19:30

Sunday, May 12, 2013 at 19:30
I think I might of opened up a can of worms here.
Thanks for everyones info.
I am confused but will read through the posts again.
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Follow Up By: Member - Broodie H3 - Sunday, May 12, 2013 at 19:40

Sunday, May 12, 2013 at 19:40
I think I shall do the same thank you every one for the contributions, my grand son is now telling me what you blokes are talking about, he's not allergic to electronics and plays with solar. Thank god for grand kids.
Broodie H3
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Reply By: Scott T4 - Monday, May 13, 2013 at 01:49

Monday, May 13, 2013 at 01:49
Hmmm
I might have to bite the bullet & buy another panel.
I see on ebay that I can buy a folding solar panel 160w for $269.00.PWM regulator
That should cover my needs.
If I can use the 80w panel with it than it's all good.
I should of asked the question on this forum before I bought the 80w panel.
Thanks again for the info.
AnswerID: 510935

Reply By: Jason – Perth - Sunday, May 26, 2013 at 18:06

Sunday, May 26, 2013 at 18:06
Wouldn't it be nice if we were all able to discuss these things face to face and over a beer? Perhaps people wouldn't have to resort to the "stuff it, it sick of this" approach and stop posting to the detriment of others who want to learn something.
Very sad state of affairs and one I am sure the forum owners are concerned about.

They say the pen is mightier than the sword. Hmmm. There have certainly been a few battles fought amongst these pages. No victories I might add.
It is sad how many times people have misinterpreted others writing and slammed them for it.
This forum has now lost two very knowledgable contributors which is to its detriment.


AnswerID: 511874

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