Extreme low solar power generation

[ddemunshi]

Hi,

I am trying to hook up a portable solar powered charger for the own use for my phones.

I went out and bought some solar panels, each of which is supposed to give a power of 2.8W in full sun.

However, when I hook up a 4.7 Ohm resistor to the panel and hold it in the sun, I am only gettin a voltage drop of .5V across it, showing a power generation of the order of tens of milliwatts only. The fact that there is nothing wrong with the connection is demonstrated by the change of voltage when I move indoors or cover the panel with something.

So my question is can it be just that it is a bad panel? Or is there some technique to solder the panels? For soldering I initially found that wires dont really want to stick to the electrodes. So then I used quite a bit of flux to get it to stick.

So for more clarity, the circuit consists of two wires coming out of the panel hooked across a 4.7ohm resistor and the voltage is measured across the resistor.

BTW, the panel is supposed to have a 21V open circuit voltage, but when I hold my multimeter across it, it shows only about .5V in full sunlight.

So any advice is appreciated.

[Mike90250]

What is the voltage when there is NO load on the panel. 4.7ohm is a pretty heavy load for a small panel.

If you are DIY soldering cells to make a panel, and the solder does not flow properly/nicely, you are doing something wrong, or the cells are bad. With the right size and temp of iron, real solder and light flux, they should solder perfectly. Factories spit these out by the mile.
If cells are rejected at some point in mfg, they may have skipped the metal layer deposition on the batch you got. Consumer sales cells are all rejects, regardless what the fleabay seller says.

[Sunking]

You are going about it all wrong. Solar Panels are Current Sources, not Voltage and that is your mistake.

Assuming those panels are standard 12 volt battery panels only have an Imp of roughly 0.15 amps. Well 0.15 amps x 4.5 Ohms = 0.675 volts and .675 volts x 0.15 amps = 0.1 watts from your 2.8 watt panel. Nothing is wrong with the panel, it is working perfectly, the problem is Operator Error period. You do not understand Current Sources. Your assuming a panel is a Voltage Source. It is not and is a current source.

Again assuming a 12 volt battery panel @ 2.8 watts you would need a variable resistor of 125 Ohms to 500 Ohms. Then with full noon sun adjust the resistor to read Vmp of the panel from the specs. So let's say you do that and the Vmp is 17 volts, and the resistance ends up being 130 Ohm's. You now know you have a Imp of .131 amps and a power of 2.2 watts at that moment in time. Simple math.

But here is your real problem You need at least a 40 watt panel to charge a cell phone in a day assuming not a cloud in the sky all day long. 5 to 6 watts will take several days.

[ddemunshi]

Hi Mike90250,

Actually the open circuit voltage that I am measuring is about .5V where as the rated voltage is of the order of 21V. This in itself made me suspicious but then when I ran the load to see what is the power delivered I observed the severe discrepancy.

yes the soldering was extremely painful, in fact the most painful soldering I have done in my life and I have soldering stuff even on thin film depositions of weird materials. It finally worked after I used a tone of flux. And I do not doubt the quality of the solder and iron because we use these regularly at our lab to solder even smd components. So there might be some issue with the panels themselves.

[ddemunshi]

Hi Sunking,

I am not sure what you mean by Vmp and Imp. I am a newbie so it would be helpful if you can elaborate on the terminology.

Regarding whether I consider it as a current source or voltage source, I am simplifying the problem by trying to see how much it is delivering across a given load and whether it matches the expected power delivery or not. I can understand fine tuning to reach the specified power, but it something is off the specification by two orders of magnitude, then I guess we have to assume something is quite wrong.

I however would disagree with the time scale of the battery charging. We are actually looking to build a miniature 10W supply to charge up a 4000mAh battery. At 5 V, it would mean it will give us a 2A current. Even if it gives less, then it shouldnt take more than 5-6 hrs to charge up the bank.

[Sunking]

There are 4 specs to a solar panel

Voc Voltage Openn circuit
Vmp Voltage maximum power
Imp Current Maximum Power
Isc Current Short Circuit

The only two you can really measure is Voc and Isc. Vmp is fairly consistant if you can find it, and Imp is a are moving target and changes with each second of the day.

You cannot take a resistor and use it to measure the power like a voltage source, it wil not work because you are dealing with a current source. Your Voc on that panel is 21 volts as you stated like any 12 volt battery panel, and the Vmp is about 17 volts. That means Imp has to be 2.8 watts / 17 volts = 0.16 amps. You only get that much current if it is in direct sunlight on a crystal clear day at noon. Less at all other times of day and conditions. A 4.5 Ohm resistor on a 2.8 watt panel is a dead short. 0.16 amps x 4.5 Ohms is 0.72 volts and .72 volts x .16 amps = .11 watts. FWIW 0.16 amps x 5 volts is 0.8 watts, not 10 watts of the panel. 92% of your power is lost in your regulator. Bet you did not know that.

Lastly a 10 watt battery panel only generates .58 amps, not 2 amps like you are thinking. This time of year you are only getting 2 to 3 Sun Hours and it would take you 4 to 6 days to charge a 5 volt 4 AH battery. You can disagree all you want. My opinion is based on solid fact and math all here in the know will agree with. Your is based on hope and ignorance. You simply do not know what you are doing.

[ddemunshi]

@Sunking

How you measure power or how power is delivered and dissipated is irrespective of whether it is a voltage source or a current source. The difference between the two is one holds the voltage contant irrespective of load and one holds the current constant.
And havent you heard of power conserving voltage regulators by which you bring the output to whatever IV region you want.
And why are you assuming that where I am I have 2-3hrs of sunlight? We have skin burning sun for more than 5-6 hours a day if not more. And finally, since you are bringing in personal credibility, I am an experimental physicist working and building with electronics and optical systems allowing to manipulate matter at the level of single atoms in space for years. So I do have a pretty good idea of what I am doing atleast when it comes to measuring power delivery of an electronic system.

And if you had read what I posted first, you would have seen that primarily my concern is that even in full sunlight the Voc is not increasing to the specified 21 V, not even the 17V that you are suggesting and since this is my first time working with solar panels, what I am worried about is not the electronics but rather if I am making some mistakes in the connections. Hell if I had 17V, we wouldnt have been having this discussion right now because that would work perfectly for my system. My concern was can improper soldering and connections reduce the power generation or power delivery of a solar panel and/or if someone has faced similar situation before.

Also the panels do not have any kind of EVA or any other protective coating. So can touching them by hand (which seems to be leaving fingerprints) cause rapid degradation of the panels to the level that it is unable to produce even 50% of the rated power generation?

[Sunking]

You have no idea what a current source is. You keep thinking you are working with a voltage source.

[Mike90250]

....It finally worked after I used a tone of flux.....

Did the solder actually flow, or is the flux gluing the wire in place ? And if there is no metalization on the cell, nothing is going to fix that.

Surplus flea bay cells are a total waste of time. Last summer I had grade school kids making small PV panels out of factory cells. Spent most of the time teaching them to solder, then out of 6 panels, 2 actually worked. So many ways to go wrong, it's even quite easy to dissolve all the metalization into the solder if you linger too long, then the cell is ruined. Lead free solder does not work (it's not even solder, but marketed as such) Low temp lead solder with low temp flux is the most reliable way to make DIY panels.