# Why are my solar cell currents not adding up?

I'm outside in full sunlight trying out some new solar cells I soldered together. I have a string of cells lined up like this:

My DMM reads 3.14V open circuit and 0.8A short circuit. I take two more strings and connect them like this:

My DMM should read something like 3.14V and 2.4A. Instead, it reads 3.14V and 1.61A.

I thought I may need diodes for each string, so I did this:

I expected a voltage drop and sure enough my DMM read 3V and 0.52A. Next, I made this:

I was expecting 3V and ~1.5A. Instead, I got 3V and 0.82A.

I've never experienced parallel solar cells having a current drop like that in my other solar panels. Not only is the current lower after I add a diode, it's even lower when I connect them in parallel. Outside of possible bad workmanship on my part, is anyone aware of a possible cause?

EDIT!!!

Here are pictures of the (HOW-IN-THE-WORLD-IS-HAPPENING) phenomenon. Each string of solar cells are color coded. The cells having this current reduction issue are the bigger ones. Each picture shows short circuit current with my DMM (sorry Kevin White haha). I used a lamp, so yes the current is gonna be tiny, but it suffices.

1st string of malfunctioning cells: 0.51mA

2nd string of malfunctioning cells: 0.36mA

1st and 2nd strings of malfunctioning cells in parallel: Should be 0.51 + 0.36 = ~0.87mA. Instead, it's 0.43mA

And for the sake of a control group, 1st string of normally functioning cells: 0.3mA

2nd string of normally functioning cells: 0.28mA

1st and 2nd string of functioning cells in parallel: Should be around 0.3 + 0.28 = ~0.58mA. It's 0.6mA :)

And finally, to validate my point, 1st and 2nd string of normally functioning cells and 1st string of malfunctioning cells in parallel: Should be 0.6mA + 0.51mA = ~1.1mA. Instead, it's 0.53mA.

It's almost as if the bigger cells are siphoning off current. I tested for shorts. Looked for corrosion and bad construction. Nothing stands out.

WHAT IS GOING ON?!

• what's your load? Commented Nov 11, 2017 at 22:58
• @user3528438 no load. Open circuit in all cases. Commented Nov 11, 2017 at 23:08
• @jsotola Yeah, I measured all of the strings separately. All were within +/-0.01V and +/-0.01A of each other. Commented Nov 11, 2017 at 23:09
• Then where comes the current? Commented Nov 11, 2017 at 23:09
• @user3528438 the solar cells short circuited with my DMM. Commented Nov 11, 2017 at 23:10

It is necessary that the illumination of solar cell elements was uniform at the time of measurement.

Elements connected in series must be of the same type. Damage (or shading) of one element leads to loss of efficiency of the whole chain.

The rated open-circuit voltage of the serial-connected elements must be the same (the number of serial-connected elements must be the same).

• Not only uniform but also full sunlight with the same intensity for short circuit current as defined in the datasheet.
– Uwe
Commented Nov 19, 2017 at 22:24
• The same effect was observed outside in full sunlight. The pictures were taken indoors when the sun was down. Commented Nov 19, 2017 at 22:27
• Not only full sunlight, but the panels oriented vertical to the direction of the sun. No shades on the panels. Input resistance of current meter low enough for short circuit measurement as defined in the data sheet of the panels. Look in the data sheet of your DMM for input resistance.
– Uwe
Commented Nov 19, 2017 at 22:49
• Effect does occurs only with one type of solar cell regardless of light direction. Commented Nov 25, 2017 at 21:55
• On the last photo there is a parallel connection of two chains of 14 elements and one chain of 6 elements. Why are you trying to connect in parallel chains with different numbers of elements (and different rated voltage)? Commented Nov 26, 2017 at 2:50

You probably have a wiring error - possibly one array is not connecting properly.

Try disconnecting them each in turn, probably disconnecting one of them will not affect the current. The currents should add up

The short circuit current with three panels will be slightly less than the sum of the individual panels as the voltage across the meter will be higher since it is passing more current but it will not be much in error.

If you look at the V-I curve diagram:

You can see that even if the voltage across the panel is not quite zero the current remains almost the same - they act as constant current sources.

The diodes are not needed as under these conditions there will not be any current flowing back into the other panels. The main situation where diodes are needed is where you have a solar panel connected to a battery - in this case current can flow back from the battery when the panel is not illuminated. The diode avoids this.

You realize that in general you should not measure the current capability of voltage sources by using a DMM in that way. You would normally put the DMM in series with the load.

For example if you measured the current from a battery that way you would either blow the fuse in the meter or possibly damage it.

Solar cells are a special case in that when you load them heavily their voltage drops and they become constant current sources.

alternative-energy-tutorials

• About 50% of your answer was about the OP's short circuit current measurement technique. But it is unclear to me if you are saying there is a problem with the measurement technique. This is a pretty common measurement for solar cells, no? Since solar cells are not voltage sources. I'm confused about what you are talking about.
– Matt
Commented Nov 12, 2017 at 4:44
• Measuring the solar cells individually doesn't produce a result that indicates a wiring error, so I'm still quite confused. I understand not to make a short circuit with conventional sources of power, but in order to measure wattage of solar cells, this is the technique typically used. Commented Nov 13, 2017 at 13:52

The cells are not matched, this could have been for a few reasons: the factory could have screwed up, they could have been damaged. Regardless it is unwise to use mismatched cells (or string of cells) in the same module because they can dissipate power in the cells and run down your efficiency.

What can you do about it? I would just leave the offending string out. You could find a replacement and match it. Ideally the I-V curve of the module or cell needs to be matched as closely as possible to the other cells.

Mismatch in PV modules occurs when the electrical parameters of one solar cell are significantly altered from those of the remaining devices. The impact and power loss due to mismatch depend on:

• the operating point of the PV module;
• the circuit configuration; and
• the parameter (or parameters) which are different from the remainder of the solar cells.

Differences in any part of the IV curve between one solar cell and another may lead to mismatch losses at some operating point. A non-ideal IV curve and the operating regime of the solar cell is shown below. Although mismatch may occur in any of the cell parameters shown below, large mismatches are most commonly caused by differences in either the short-circuit current or open-circuit voltage. The impact of the mismatch depends on both the circuit configuration and on the type of mismatch, and is demonstrated in more detail in the following pages.

Image and text source: PV education

• As you can see in my post, the effect is only on one type of solar cell, even when not combined with solar cells of different types. Commented Nov 25, 2017 at 21:56
• It doesn't have to be a different type for there to be a mismatch, each cell has a slightly different curve, they are usually matched at the factory. Commented Nov 27, 2017 at 16:20

You are assuming that the Superposition Principle applies to non-linear PV sources just as it does with linear circuits.

It does not apply to non-linear circuits.

Similarily This does not apply to superposition.

## edit

Your test report is disorganized and not concise and have no test specs and less than ideal lighting conditions. First you report 0.8A for 1 string and 1.6A for 3 identical parallel strings without results for each one. Then you report Should be 0.6mA + 0.51mA = ~1.1mA. How, which ones dropped from A to mA?

Your test report is impossible to understand. It should be a concise table of results with clear item numbers. Compare specs , estimate light power and then report results to validate this.

You may be experiencing a leakage resistance under low current conditions.

simulate this circuit – Schematic created using CircuitLab

PV's are almost ideal constant current sources below 1/2 of the Voc, except where the input light power is so low that internal leakage affects the current source. Why are you only getting 1mA?

If you see, your measurement of short circuit current is giving large disparity between two strings of larger cells(0.51 & 0.36) than your two strings of smaller cells(0.3 & 0.28).

I am also suspecting Multimeter problem. It might have internal non-linearity for some portion of measurement range. Try changing the current measurement method. Put 1 Ohm resistor across output of the solar cell string output and measure the voltage across the resistor. This might give you the confirmation that your Multimeter is the problem or not.

Also, try to measure the current directly using your current method of amp meter across panel but with 20A current range. It has different shunt for measurement than your mA range. You might get low resolution reading but will give you confirmation that your problem is not due to your measurement method.