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Onboard Electrical Power

Update 12-8-9

The problem

The 48 watt panel I installed is not performing as it should. The new fan that I installed ran the battery down flat. It drew about 2 watts. Nominally this panel should have produced 48watts * 5 hours divided by 24 hours or 10 watts continuous. On an overcast day it should draw about 4 amp hours from the battery. Even a week of overcast days should draw it down by only 28 amp hours. Thus the battery should have remained fully charged even with the fan running at full blast. Clearly there is a problem. The following section details an experiment I performed to understand why.

The following sections describe the experiment, its setup and the results.

The setup. I'm using a 500 watt halogen lamp to simulate the sun.

This setup uses two solar panels in parallel wired to an MPPT controller. I'm using a simple ammeter and voltmeter to measure the voltage and current between the solar panels and the controller. The controller output is wired to a battery that it charges.

Here a solar panel is shaded. It gets less light but it is uniform.

Here A solar panel is completely blocked.

I tried all combinations of shading, blocking and full exposure for the two panels.

I was expecting that the performance would drop to about 2/3rds when one panel was shaded and to about half when one panel was blocked.

As you'll see the results are almost as expected. Apparently the blocked panel allows a slight back current to flow.

The results

Measurements of total output over combinations of shading

Unblocked143 ma 15+ v98ma 15+ v60ma
Shaded98ma 15+ v20ma 15+ v10ma 10.5v
Blocked60ma 15+ v 10ma 10.5v0ma 0v

Measurements of left panel current over combinations of shading

Left\RightLitShadedHeavy shadeBlocked
Lit80 ma80 ma80 ma80 ma
Shaded18.5 ma21 ma22 ma23.5 ma
Heavy shade8 ma12.6 ma12.5 ma12.5
Blocked-4.5 ma0 ma0 ma0 ma

Next steps - first I swap out the MPPT controller and the PWM controller. I also run tests on the fan to make sure it is not drawing more power than expected. I will also put Schottky diodes on each panel to stop the back current.