Solar Charger Tutorial – Part 1By Phillip Stearns
How do Solar Chargers work?
Solar chargers use solar energy to power my electronics, neat! How do they work?
Short answer: Sunlight hits solar panels -> solar panels generate electricity -> electricity flows into battery -> battery outputs clean power on demand to your device
Long answer: We’ve created a five part tutorial to take you through every stage of the process. Solar is obviously much less predictable than plugging into the grid so we’ll be focusing both on specifications and what to expect in the real world. Bring along a multimeter and some parts from Radio Shack and you can get a pretty good idea of how exactly how solar charger work.
Tutorial 1: How do I measure Open Circuit Voltage and Short Circuit Current? (below)
Tutorial 2: How do I measure total power output?
Tutorial 3: How (and why) do I store power?
Tutorial 4: How do charge circuits protect the battery?
Tutorial 5: How do I use stored power to charge a device?
Tutorial 1: How do I measure Open Circuit Voltage and Short Circuit Current?
There are lots of great resources on how solar panels generate electricity including Wikipedia so we’re going to focus here on measuring the Open Circuit Voltage and Short Circuit Current of a solar panel in “perfect” and less than perfect conditions.
Every solar panel has a rated output that includes its Open Circuit Voltage (Voc), Peak Voltage (Vmp), Short Circuit Current (Isc), Peak Current (Imp). The Peak Voltage and Short Circuit Current tell you the Voltage and Current of the panel before you connect it to anything, e.g. there is no load attached to the panel.
As a reminder, Voltage is represented by the symbol V for Volts and is a measure of the difference in electric potential energy between two points. Like air pressure, it flows from high to low. Current is a measure of the flow of charge through an area over time. We use the symbol I to stand for current and measure it in Amps, or simply A for short.
Let’s measure the output of a solar panel. You’ll need:
- Solar Panel – we use our 2 Watt 6 Volt solar panel that uses Monocrystalline cells, but you can use any panel you have lying around with any type of cells
- Sunlight – alternatively, you could use a couple high-powered incandescent bulbs but then you don’t get to spend the afternoon outside
1. Measure Open Circuit Voltage – The black lead should be connected to COM and the red lead should be connected to V or VDC. Set the dial to 20 which means the Multimeter can measure up to 20 Volts.
Touch and hold the black lead to the “sleeve” of the solar panel connector or the black wire. Now touch and hold the red lead to the red wire or insert it into the “tip” of the solar panel connector.
You’ll notice that the Voltage moves around, but with the panel pointed at the sun, we saw between 6.89 and 6.98 Volts for Open Circuit Voltage. This is close to our specification of 7.0V Open Circuit Voltage on the 2 Watt panel.
2. Measure Short Circuit Current – The black lead should be connected to COM and the red lead should be connected to the mA. Set the dial to an amount greater than what you expect the current to be. In our case, we set it to 10.
We measured got 0.33 Amps or 330 mAmps which is close to our specification of 333mA.
3. Assess the impact of real-world conditions.
In the real world, it is not sunny all the time and our panels are not always pointed directly at the sun. So what happens when we move away from perfection?
Angle the panel so that it is facing the sun and record the voltage. Try slowly angling the panel away from the sun and note the changes in Voltage and current. Try shading parts of the panel and then the whole panel and note the changes in Voltage and current.
Here is what we recorded:
|Position||Voltage (V)||Current (A)|
|Directly Facing Sun||6.82||
|Angled 15 degrees||6.81||0.32|
|Angled 30 degrees||6.78||0.32|
|Angled 45 degrees||6.73||0.29|
|Angled 90 degrees||6.07||.06|
|Angled 180 degrees||5.89||0.03|
|Finger on Corner (half of cell)||6.79||.2|
|Fingers on Cell (full cell)||6.74||.04|
|Close Shadows on Panel||5.78||.03|
As you can see, minor changes in angle don’t have a very significant impact on Voltage or current. However, once you get to about 45 degrees away from the sun, current starts to drop very sharply, meaning total power will also drop.
Similarly, light shadows on the panels decrease current by about 25%, but a heavy shadow over all or part of the panel drop panel output by 90%.
Move on to Part 2 of our Tutorial – How do I measure total output? In this tutorial we start connecting solar panels to loads and measuring how much power it is capable of generating.
Skip ahead to Part 3 of our Tutorial – How (and why) do I store power? In this tutorial we explain how to store solar energy in batteries for use when there is no sunlight available.