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I've a two solar panels (CS6C-150P). And each produces:

  • 8,3A (Imp)
  • 18,1V (Vmp)
  • 150W (Pmax)

Charge Controller: EP Solar - LandStar B - LS3024B

They are parallel connected, then produces a total current of 16,6A.

Let's assume that is midday with no clouds on the sky and my batteries have 75% of charge.

In these conditions I saw the panel producing only 11A, why?

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    \$\begingroup\$ Welcome to SE. Please edit your question to include links to product data sheet so we don't have to search for them. Meanwhile, look at the curves on the data sheet and see under what load conditions you will bet 16,6 A. \$\endgroup\$ – Transistor Jan 24 '16 at 16:07
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There are many reasons why the current from the solar panel would vary.

First, let's look at the solar panel in isolation, ignoring any effects the batteries could have. When a manufacturer says that a panel is rated for some amount of power, it's done under specific, not entirely realistic conditions called Standard Test Conditions (STC). You probably aren't getting exactly 1kW/m^2 of sunlight and aren't cooling your panel to exactly 25C for your test, so the rated power won't match exactly.

Next, let's consider how your solar panel behaves when connected to a load.

The range of possible output voltages and currents under specific conditions can be plotted as an "IV curve," usually done by connecting some controllable load to the panel at STC.

Here's an example IV curve for a single cell under some particular amount of light: sample IV curve

From this example with a single cell, you can see that depending on the output voltage when the load is connected, the power delivered to the load will vary. The point where the maximum power is delivered is referred to as the maximum power point. Your Vmp and Imp figures refer to the expected voltage and current when the panel is operating at its maximum power point.

If your charge controller directly connects the panel to the batteries, the panel voltage will follow the battery voltage, so the panel may only briefly operate at its maximum power point (or never, depending on the curve).

Controllers that can adjust the operating point to stay near the maximum power point all the time are known as maximum power point trackers (MPPT).

To summarize:

  • Your panel's output power will vary depending on ambient conditions, which are probably not the same as the manufacturer's test conditions.
  • Even if they are the same, your charge controller might not have MPPT functionality, so the output power/current varies with the battery voltage.
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Your listed solar panel specs are for the maximum power point.

Your listed charge controller is a maximum power point controller. It should deliver the maximum capability of the solar panels for the conditions under which it is operating.

Reasons solar panels may not be operating at the listed specs:

The solar panel surface may not be aligned perpendicular to the direction of the sun. Even if the panel is aligned with the sun, the angle of the sun with the surface of the earth determines the air mass filtering the sun's rays.

Temperature may be more than 25C.

The charge controller may not be properly adjusted.

The load may not be capable of absorbing the available power.

Actual performance of solar panels is warranted to be within 3% of published spec.

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  • \$\begingroup\$ Or the batteries may be well enough charged that the charge controller must transition to trickle charging to avoid over-volting the batteries. \$\endgroup\$ – Brian Drummond Jan 24 '16 at 18:38
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    \$\begingroup\$ At a temperature below 25 degrees centigrade, the panel will work better. A typical reason for not performing as specified is a temperature above 25 degrees. Furthermore the most likely reason is a lower insolation than the exaggregated 1kW/m^2 commonly used in data sheets. \$\endgroup\$ – Michael Karcher Jan 24 '16 at 20:54

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