Solar panel conundrum -- Not enough power?

[johnwinter645]

I have a student trying to build a simple solar powered vehicle for a high school design thinking class. He solar panel produces about 3.1 V as measured on a multimeter, but will not power the electric motor she had chosen. She tested the motor with a pair of AA batteries (2.9 V on multimeter) and it ran as expected. We tried troubleshooting using different solar panels, different motors, panels in series (5.9 V), subbed out wires, placing a busser in the circuit in place of the motor. In all cases, the batteries performed and the solar panel did not. What am I missing?

 

[Baluncore]

Welcome to PF.

In all cases, the batteries performed and the solar panel did not. What am I missing?

Sufficient sunlight ?
What is your light source for testing?
What are the dimensions of the solar panel?

 

[Averagesupernova]

You are missing the same thing that is missing when you get shocked by static electricity with several thousand volts. Not enough current to kill you. In the solar powered car situation there is not enough current (amperes) supplied by the panel to drive the motor.

 

[Borek]

What is the power rating of the panel?

What power is required to move the car?

 

[berkeman]

He solar panel produces about 3.1 V as measured on a multimeter

What was that voltage when the solar panel was connected to the motor?

 

[jrmichler]

The performance of a solar panel is shown by a current voltage (characteristic) curve, or family of curves. A typical curve randomly picked from the internet is shown below. It is for a much larger solar panel than your solar cell.

For a given amount of light, there is a maximum voltage when the current is zero. There is a maximum current, at which the voltage is zero. You can measure both of these with a multimeter. Measure the open circuit voltage with the meter set to volts, and the short circuit current with the meter set to current. Then you can draw a "good enough" curve freehand using the above figure as a guide.

Then drive the motor with a battery and measure the voltage across the motor. Connect the meter in series with the motor, set it to current, and measure the current through the motor. Compare the voltage and current to the characteristic curve that you just made. Connecting solar cells in series gives you more voltage, but not more current. Connecting solar cells in parallel gives you more current, but not more voltage. You should be able to calculate how many of your solar cells you need to connect in parallel in order to drive your motor.

 

[Rive]

She tested the motor with a pair of AA batteries (2.9 V on multimeter) and it ran as expected.

A (very) small RC vehicle DC motor (at around 3V) will usually require current in the range of 0.2-0.5A
To supply that you will need a solar panel with 1-2W power, and that surface will be around 10cmX10cm (with good light, likely not something indoor)
A lot depends on the actual product, so take all that as a (very) rough estimate...

 

[Tom.G]

At 40° latitude and Solar Noon, the insolation is about 160W/m² at Winter solstice and 460W/m² at Summer solstice. The best research-grade solar cells get a bit over 21% efficency, figure 1/4 to 1/2 of that for the low-cost readily-available.

Using Winter Solar Noon, no clouds, cell efficency 21x0.33 = 7%, available cell power will be 160 x .07 = 11Watts for a cell 1 meter square!
(Or 1W per SqFt Winter solstice, 2.9W per SqFt Summer solstice)

Looks like a very efficient motor, a very lightweight vehicle (balsa wood?), and maybe ball bearings on the wheels are in order.

Cheers,
Tom

p.s. Please keep us updated on the progress. We like to learn too.

Insolation data from:
https://geography.name/insolation-over-the-globe/