Problem with 30 watt solar panel

Dashie

I am a student working on the project which we decided to use parallax solar panel kit (#33000) as a charger on a robotic vehicle. We encounter some problem with our solar panel output.
The power produce by our solar panel is not as expected from the results stated in the instruction manual.

value in manual:
Open circuit
voltage 7V
current 4A

result under strong sun lights
voltage 6.57V
current 16mA

We follow the instruction carefully and did not find any errors in our solar panel. We would like to know what is our problem in our solar panel as we did not get enough current from it. Thanks in Advance![noparse]:)[/noparse]

stamptrol

What is the battery voltage you're trying to charge?

From your readings, it looks like the panel voltage is very close to the battery voltage. You need a potential difference of a few volts to drive the current into the the battery.

Also, the panel cannot deliver 4A at open circuit conditions. The write up states maximum power at 6 VDC and 4.9 amps.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Dashie

Its will be use to charge a 12V, 4A Lead Acid BATTERY.

As for my reading, It was mention in the instruction manual that it not uncommon to have 7volt and above but the current is way too low for it be of any practical use. I actually did a test with a small LED and 270ohm resistor connected in series and it produce 5.963V, 13.77mA.

Mike Green

You cannot use a nominally 6V solar panel to charge a 12V battery. You need about 15-18V to charge a 12V lead-acid battery. It would be possible to buy or design and build a switching regulator that would take the 6V from the solar panel and step it up to 15-18V along with regulating the output current so that the battery isn't damaged from overcharging, but that's beyond the scope of this forum.

You could use three solar panels in series along with a charging regulator to charge your battery. There are lots of charging regulators on the market for use with solar panels. You'll have to do some research to find one for the type of battery you're using.

Dashie

We did intend to use to type of regulator to step up the voltage in order to charge up the battery but the problem is the current very low compare to the result we should actually be getting. so i would like to know whether the reading from my panel is normal or maybe i had make some mistake during the assembly of the solar panel.

May i know is there anyone here using the same solar panel?
It would be a great help if u can share your voltage and current reading here so can can make a comparison.

Mike Green

How are you measuring the voltage and current? Typically, you put an appropriate load on the solar panel, expose it to full sun, and measure the voltage across the load resistor. For a 30W panel with maximum power rating at 6V and 5A, that's a 1.2 Ohm 30W resistor. If you connect such a resistor to the solar panel with short, low resistance wiring, you should be able to measure approximately 6V across it in full sun.

Unless you describe how you're making the measurements, they're meaningless.

Dashie

What i actually did to measurement is that i connect a led and a 270 resistor in series.then connect a amp meter also in series which is right after the positive terminal of the solar panel to complete a close circuit to measure the current .then use another volt meter to connect across the positive and negative terminal of the solar panel to get my voltage reading. Its was done under full sun.

Is my way of my measurement wrong?
thanks for you guys' help!much appreciated

Mike Green

Yes, your way of measurement is wrong. The amount of current you're measuring is set by the characteristics of the LED and the 270 Ohm resistor, not by the solar panel. You need to learn Ohm's Law and basic electronics. Try doing a web search for "wiki Ohm's Law". Also look at "wiki ammeter".

stamptrol

To re-state the information in Mike's post: Replace your resistor and LED with a 1.2 ohm, 30 watt resistor and check the current in full sun.

What you previously were measuring was the LED current, not the panel output current.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Dashie

An update on solar panel: I did what was suggested in Mike's Post. Got a 25 watt resistor connected in series with an ammeter and a volt meter parallel across the resistor.
It was place under the sun for approximately 30min and i got a average measurement of 2.6V and 1.33A.
May i know does my reading look normal compare to others who is also using this solar panel?

Beau Schwabe

Dashie,

You need to take into consideration the internal resistance of the solar panel and that it changes depending on what your load is.

The numbers you got are probably typical of using a 1.2 Ohm resistor in series with your ammeter making the load resistance actually closer to 2 Ohms.

With a load of 2 Ohms and an internal resistance of about 2.5 Ohms your numbers of 2.6V and 1.33A work out.

I would do it a slightly different way... measure the resistance without it being a load, and then just measure the voltage across that resistance while it is a load. Use Ohms law to calculate current, the ammeter can introduce additional resistance (mostly in the test leads themselves) ... the higher the current the more this can become a problem.

I have not characterized the 30 Watt solar panel, but I have taken a look at our 1 Watt panel. The concept is the same. If you look at where the mid voltage should be, THIS is where most of your power is, but keep in mind half of that power is dissipated within the panel itself, while the other half is dissipated to your load. There is a sweet spot where you will see the most amount of power able to be delivered to the load but each solar cell type (chemistry, i.e. poly crystal or amorphous crystal) has a different characteristic here and where it matters is what the internal resistance is doing based on the load demand.

I don't have a 30Watt panel, but perhaps when I'm out there for the expo at the end of this month I can characterize our 30 Watt panel as well as some of our other panels.


I have included a screen shot as well as the actual Excel program. (change the x1s extension to xls) None of the cells are locked, so you can see the formulas, but you should only need to enter data in the 'green' fields. All of the other fields, including the graph should automatically update.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.