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Solar power supply (MPPT)?

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froggy

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mppt circuit

Does anyone have drawings for a solar panel power supply? I'm looking for a PSU that implements Maximum-Power-Point-Tracking (MPPT). I want to charge lead-acid cells.

/Froggy
 

froggy

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mppt charger schematic

Thanks alot Hishamsaleh,
but what I specifically looking for is a schematic incorperating MPPT technique to maximize the power output from the panel(s). To my understanding, MPPT means that the voltage output from the panel is kept at the level where I*U peaks (Max watts), while at the same time providing the battery bank with a suitable voltage for charging without any significant conversion losses.

/Froggy
 

flatulent

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mppt schematics

I once looked into this and found that these specialized circuits are never used on small solar stations because their efficiency being less than 100% eats up the gain from using the peak power from the solar array.

Commercial arrays are made (putting the right number of cells in series) to have the opitmum power output to a lead acid battery. The area you can improve things is to track the sun. The simplest part is to change the vertical angle of the array over the 12 months of the year to be square on to the sun. The hard part is changing the horizontal angle over the 12 hours of the day.

It is usually cheaper to buy more arrays than to make a horizontal servo system. Doing the vertical angle change by hand every month is worth the effort.
 

froggy

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solar mppt

This is for a small application 25-200W (RV's, Boats etc).

I dont think what you say is correct, according to the specs on a few commersial products with MPPT, they claim the following:

Power Gain: 10-30% depending on battery status, and solarpanel output.
Efficency: >97%
Power consumption: <3mA @ 12V

The downside is that they are quit expensive(100$+), so in that sense you are right, thats cheaper per W to use more panels instead. But the reason why I'm still is interested in schematics is:
1. I think that it's possible to make these things for a lot less.
2. It's not always practical to use more Solar panels due to space constraints.

/Froggy
 

flatulent

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mppt charger circuit

I find it hard to believe the 97% conversion efficiency for a switching power supply which is what these circuits are. The 10% improvement (which is more realistic for a almost fully charged battery) is removed by the 90% switching efficiency of a real converter.

I once saw advertised optical specifications for binoculars that exceeded the laws of physics. More photons came out the eye end than came in the front end.
 

ELECTRON

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solar mppt schematic

is it 97% conversion efficiency for a switching power supply
 

froggy

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mppt circuit design

Yes, again to my understanding of this matter, a MPPT charger is built around a SMPS which is controlled by a circuit to always get the maximum power out from the source (e.g. a solar panel).

So I think that the stated efficency is in fact the efficency of the switch mode PS.

/Froggy

EDIT:
Sorry I doublechecked the efficienency, and it was 95% not 97%!!!
 

flatulent

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As far as the pointing, a 25 degree error will reduce the solar flux by 10% a 45 degree error will reduce it by 30%. So you can see the advantage of sun tracking. The earth is tilted by about 24 degrees which makes the sun angle that far off in summer and winter if you fix mount the array for the average angle. If you mount for best winter performance, the summer will be 48 degrees off and over 30% down. Anothe thing to keep in mind is if power is to be drawn from the battery during daylight. In that case pointing the array to be square to the sun at that time will help the most as the power will be drawn partly from the array. Charging and then discharging a battery has its own losses.

There are other ways to get more of the power out of the arrays. One is to use two 6 V batteries. Put them in series during high sun flux and in parallel during low flux (sunrise and sunset). This can be done with a timer and magnetically latching relays.
 

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mppt means

Yes flatulent, your point is well taken, but as I said earlier the application for this is RV's and Boats. So even if Sun tracking is doable on a moving platform, I dont think its motivated from a energy or economical point of view. If the solar panels are mounted on a fixed platform I totally agree that the first that should be done to improve the energy effenciy is to use sun tracking. (I have seen a few cheap ones using prabolic dish attenuators)

But in mine application with a moving platform, I started to look at the charger circuit, beacause that the only variable I can change.

In the meantime I actually found one MPPT design, but I'm still looking for more examples on how to do this in the best way.

/Froggy
 

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solar mppt circuit

Your attachment is a very good circuit. It is a buck type regulator which means that it functions when the solar output is high. You should do some experiments with your array and a battery and a current meter to see which orientations of the array will produce charging current. I supsect that with the array pointing directly up there will be only a few hours in the day that there is charging. The rest of the time the sun will be at a wide angle to the array and no charging will occur because this circuit cannot raise the output voltage. During these times a boost type converter will raise the array voltage to a value that will produce charging.

When your vehicle is parked (anchored) you can manually change the orientation of the array to be more directly pointed to the sun. Also, in a motor vehicle you can add a circuit to charge the battery from the engine alternator.
 

Mr.Cool

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mppt circuit

yes for MPPT you must implement some way to control the voltage (or current) output of the solar panel. most people use a SMPS. but really, this does *not* add extra circuitry because often people have a SMPS at the output of their solar panels anyways! to create varying DC bus levels.. then you do not have to have certain # of panels to get certain voltage. nor do you have to worry about power stress due to difficult wiring. no with a SMPS you get any bus voltage you want, plus isolation AND load regulation!

now, all you do is control your switching frequency of the SMPS intelligently and you have a MPPT. :)

the power consumption of a MPPT circuit (if a SMPS is already in place) is not significant, even for small solar applications.

have fun!
Mr.Cool
 

drgeeforce

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mppt shema

I was wondering if the MPPT can be substituted by a generic charge controller connected in series to a DC-DC converter. That way, the panels can deliver all the power they can causing less heat buildup and the DC-DC converter can shunt all of the power to the charge controller. This way the incoming voltage can be high as 48V or more, and the charging voltage delivered to the controller be 12v. You would still get losses from the converter, but the panels would heat up less than a straightforward connection to the battery. I was thinking of buying a 48-12Vin 12Vout DC-DC converter rated at 5A, connected to my Xantrex C35. This will connect 2-80W BP Solar panels 45.4Voc, 2.4amps Isc. The controller cost me $75 and the converter will cost $25. The cheapest MPPT I know of, the SunBoost 3048, costs $448. Any thoughts on this?
 

electronrancher

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simple mppt schematic

Here's an example of MPPT of a small solar cell. Andy Pang sells some hobby style solar cells if you need some.. http://home.ust.hk/~bcandyp/solar-kit.html

To understand MPPT, inspect how the Vout/Iout plots appear similar to bjt or mosfet - long saturation region followed by a dropoff to linear as the cell is loaded with heavier current. If you run a cell right at the edge of linear operation, that is MPPT.

Here is the example: I set up a small, panasonic 243318 under a lamp so it gave about 3v max with no load (Voc), and 1mA max output current short circuit (Isc).

Now sweep a load (50k pot) attached to the cell, and record Iout, Vout at several points. My graph appears similar to the normal saturation plot, but is pretty rough due to few points.. (sorry!) I estimated the linear region to be from about 2.92v to 0v, as my load sweeps from 300uA to 1mA.

I found MPPT at about 2v for this cell in this light. Take a look and let me know if you understand how to find it. The circuit to do so? Haha that is your puzzle! I say multiply Vcell * Iout, and sweep current from 0 to point of dropoff..
 

electronrancher

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simple mppt charger schematic

actually, my plot severely underestimates the MPPT, but finds it at the right point (~2v). The approximation says 1.2mW, but the original measurements say up to 2mW for this tiny cell!

If I get a better load than a 50k, I will do a nice sweep but until then i think you get the picture.
 

Darth Murat

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mppt and alternator

I am using mppt with fuel cells
can you give me equations for calculating the gain i have with a 5kW fuel cell
 

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