Solar panel - Logging Power

Hi all,

I have a smallish solar panel (~60mm x 30mm) and I have decided to fly one on my next high altitude balloon launch (payload goes around 30km in altitude to near space) to log various readings to see exactly what power it generates above the clouds :D

However, I'm more of a software person than hardware so require a little bit of help please! My initial thoughts are to log Power from the solar panel by measuring pv across a "load" resistor. I will be using a 150ohm (or around about) resistor as the data sheet for the panel shows that peak power is obtained with a ~150ohm load.

The panel though will produce around 8 to 9v at ~45mA; so I require a pot divider to scale the voltage down to be used with one of the arduino analog pins.

My question is: does the resistor values of the potential divider matter and will they affect the power reading (Using P = (v^2)/R)? e.g. would there be a difference using 100ohm and 50ohm resistors to create pot divider or 100Kohm and 50Kohm resistors?

Any other hints/help would be greatly appreciated :)

Thanks

If the load resistors are the only load on the panel, then I would use that for both. Use a 100 ohm and a 50 ohm resistors in series to create the load. Measure the voltage across the 50 ohm resistor.

I think you need to think about what you're trying to measure.

As I understand it, solar cells essentially produce current in proportion to the light level (within limits) but also have a significant internal resistance so that the voltage you see at the terminals will vary with load.

Are you just trying to get a general idea of how the 'ability to generate power' of the cell varies with height, or are you actually trying to measure output voltage and current accurately and determine the peak power available versus height? In the first case you can just rig up a resistive voltage divider with a total resistance sufficient to keep the output current within the cell's limits under maximum brightness. But in the second case you need to regulate and measure the output current more carefully.