Get specs from unknown solar panel/cell

I have a few solar panels/cells lying around that I've gotten from a pack of solar powered garden lights.

The marking on the back of the panels say: Excell EXC53.5*53.5 However I'm not sure if the last "L" in Excell is part of the name or just a graphic part of the logo. The last "L" has two rays coming out of it. The numbers are consistent with the panel/cell size in millimetres.

Anyways I can't find any datasheets for the given info, so before I go fry anything, is there a way to get the absolute maximum ratings for current and voltage?

Any other tips that might be useful when working with unknown hardware would also be useful.

If you have a digital meter you can messure the voltage and the short circuit current in full light. That is as far as i remember the two values normally associated wit a solar cell.

Ok, so I basically have to wait for the sun to come by before I can get some decent numbers from the panel? If that’s the case I would have to wait till spring or something ;).

And correct me if I’m wrong but to measure the short circuit current I would have to connect the multimeter positive probe to + and the ground/negative probe to - on the panel (just like measuring voltage).

If you don't have any sun, of what use is a solar panel?

If you don't have any sun, of what use is a solar panel?

I think the OP is mostly just curious.

To OP: In theory, you could put it under a bright lamp to simulate the sun, but it won't be quite the same as the sun itself.

@PaulS I will in the summer when I will be using my project. Also we do have sun where I live. It's just that it's not as bright as during the summer and also I tend to be at work when it's up.

@TchnclFl Ok so I won't get the max out of the panel unless i have a really bright sun then? But will I get a ballpark figure?

Well, it really depends. It depends on the brightness of the light, the distance, etc. But yes, you will get a general idea of its output :).

In some cases, you may get more electricity out of it than you would from the Sun, but that depends on your location, season, etc. as well.

Ok, so basically I should use some sort of regulator if I intend to use something thats sensitive to high voltage and current?

Ok, so basically I should use some sort of regulator if I intend to use something thats sensitive to high voltage and current?

Yes. I would definitely use a regulator due to fluctuations in lighting, etc.

Since they came from solar garden lights, and are about 50mm square, it's likely that they're good for about 3V, with a "full sun" output on the order of 75-100mA. This is assuming that the lights were the typical sort with a single AA battery: if they had more batteries (I have one with 4 AAs in it), the cells would probably be rated for a higher voltage and lower current.

So you're not going to harm anything or anyone by making mistakes while testing them. Unless you try to force-feed a voltage into the solar cell: in that case, you might turn it into a Smoke Emitting Diode :( Next time you go shopping for electronic parts, pick up a few Schottky diodes with a current rating on the order of .5-1A: if you're going to be doing battery charging, you'll want a diode to protect the solar cell from the battery when it's dark. The Schottky diode is better than an ordinary diode because it wastes less energy when the solar cell is charging.

A google search will turn up examples of what people have done to get more-precise measurements, but you can get a rough idea by just putting it under a bright lamp with nothing but the meter connected (the "open-circuit voltage"), then with a resistor across it that approximates its maximum load. In this case, assuming the guess of "3V 100mA" is correct, the resistor should be about 30 Ohms. Since that guess could be wrong, I'd start with about 50 Ohms, and see what voltage I get. If the voltage is low (closer to 3V than 4 or 5V), I'd try a smaller resistor to see whether the voltage holds up when drawing more current.


Thanks Ran for that extensive answer. I did some measuring a minute ago using a really bright spotlight on the panel and got a voltage of 2.4-2.6 V and a current of 50-110 mA. However I should mention that I've kept the a protective plastic film on the panel so it might not get full exposure. I haven't tried with a resistor yet since I don't have a 50 Ohm resistor neither a 30 Ohm resistor (But if memory serves me right I should get about 50 from two 100 Ohm resistors in parallel, and 33.3 from three 100 Ohm resistors in parallel). Well I'll look into that later.

Thanks for the tip on the diodes aswell I'll be sure to pick some of them up in my next order.

However there is one thing I'm a bit puzzled about. That is when I connected the positive probe to the positive terminal on the panel and the negative probe on the negative terminal i got a negative voltage. So my question is: a. Did i connect my multimeter in the wrong way? b. Could the labeling of the panel be wrong? c. Some other reason as to why a solar panel gives out a negative voltage?

I just took one of my solar lights apart.. the circuit is amazingly easy.. just uses an LDR to turn the LED on and off.

But yeah, diodes are highly suggested, so you don't get discharge when you're not using it.

Generally, if you connect it backwards it will show up as a "negative" voltage, when really it's just connected backwards. But from the solar panel I got, I took the whole circuit, battery and what not, just hotglued the battery and circuit to the back, with the LDR sticking out.

It's outputting about .8 volts in my bedroom with the "real sunlight lightbulbs".. which is nowhere near the sun, obviously. But other thing you can keep in mind if you're playing around with these things, is you can get higher capacity batteries, like 1000maH, 1.2v for $4 each online.

But it sounds like you got the resistor idea down.

It sounds like the panel may be labeled wrong, but that seems kinda doubtful if they're mass produced. I haven't heard of any solar panels outputting a negative voltage (but I haven't searched for one either)

I've been looking for the LDR on this device since I bought it this summer but I really can't find one. Although my experience with LDRs don't really go much further than the CdS I have in my component box. However the circuit is as you say pretty darn simple. But there is a big black blob on the circuit which I'm a bit curious about. I doubt it's a LDR since I can put my thumb on it and shine a light on it without anything happening. Could it perhaps be a diode? The thing is the LED in the circuit lights up when it's dark, or if the solar panel is disconnected. Which leads me to assume that if the battery is getting charged by the panel either the battery does not emit any current or the led is turned off by the circuit.

On another note regarding a regulator. Could one say that the battery acts like a regulator?

The lights I have use the output from the solar panel to determine when they need to switch on. A neat solution and 1 less component :slight_smile:


Could one say that the battery acts like a regulator?

One could, but one would be mistaken :)

As a rough guide, you can continue to feed about .1C ("C" refers to the battery's Capacity in mAH) into a Nicad or NiMH battery, even after it's fully charged, with "no harm" (it might do a tiny amount of damage to its long-term life, but it's so small that the professionals say us ordinary folks can ignore it. It might be different if you were designing a deep space probe, but we're not). So, if you have a 100mA solar cell, and a 1000mAH (or larger) battery, you don't need a regulator: if the solar cell tries to overcharge the battery, the battery will just dissipate the extra energy as heat.

Having a charge controller becomes important when you have a solar panel that supplies a lot more energy than that: e.g., if you design a system that is intended to power the electronics and make sure the battery is fully charged even on a cloudy day, it probably has the ability to damage the battery when it's sunny. Then it's important to include some circuit that can stop the charging when the battery is full.

For your purposes, with these small panels, you can probably get away with leaving out the charge control.