What it shows
A solar panel is not a battery: its voltage and current change with the load. By plotting current against voltage and power against voltage, a student finds the maximum power point, fill factor and effect of shading, which are the basis of MPPT charge controllers.
The science
A solar cell behaves like a current source in parallel with a diode. At short circuit it gives its maximum current (Isc), and at open circuit its maximum voltage (Voc). Between these, power equals V times I and peaks at the knee of the curve. Fill factor is Pmax divided by (Voc x Isc) and shows panel quality. Shading a cell lowers Isc sharply.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 6 V, 1 W or 2 W small solar panel
- A 100 Ω, 5 W rheostat or a 1 kΩ potentiometer with a 10 Ω series resistor
- Two digital multimeters
- A lux meter app or a reference cell
- Cardboard strips for shading
- A spreadsheet
Safety
For this project: Panels of this size are low voltage. Do not stare at bright reflected sun. Rheostats get warm, so an adult must be present, and use fixed connectors rather than loose wires.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place the panel outdoors facing the sun, tilted at about your latitude, and note the time and lux.
Measure the open-circuit voltage Voc with no load, and short-circuit current Isc with the ammeter directly across the panel.
Connect the rheostat as a variable load with the ammeter in series and a voltmeter across the panel.
Starting at the highest resistance, reduce the load in about 12 steps and record V and I each time.
Calculate power P = V x I for each step.
Plot I against V and P against V on the same graph with two y-axes.
Mark the maximum power point, and calculate the fill factor and the load resistance at that point.
Repeat with half of one cell shaded by cardboard and note the change in the curve.
Repeat at a different tilt angle, or in cloud, and compare Pmax.
Write the report with all curves, a comparison table and the meaning for MPPT controllers.
What you should see
The curve shows nearly constant current at low voltage and a sharp drop near Voc, with a peak power at about 75 to 80 percent of Voc. Fill factor is typically 0.6 to 0.75, and shading reduces the current sharply.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the curve have a knee?
The diode junction conducts more as voltage rises, so current falls steeply near the open-circuit voltage.
2What is the fill factor?
Pmax divided by the product of Voc and Isc. A higher number means a better panel.
3Why do charge controllers track the maximum power point?
The best operating point shifts with light and temperature, so tracking keeps the panel at its highest power.
4What happens to Voc as temperature rises?
It falls, so hot panels give less voltage and less power.
Take it further
Build an Arduino I-V tracer that sweeps a MOSFET load automatically and plots the curve on the laptop.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely