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Students review how solar cells behave in series and parallel, and how they can be cut up and recombined to provide different voltages and currents. They then each design their own panel to power a small device (can be anything, but ideally a rechargeable battery for the solar lamp project).

- Blackboard or other place to draw things in front of class
- Paper and pens/pencils
- Data on cell voltage collected from lesson 1
- Could be useful to have a battery to look at as a demonstration of positive and negative terminals

Batteries require a certain minimum voltage in order to properly charge. This is because the chemical composition of the battery has some internal resistance to taking charge. Electricity and electrons cannot just flow into the battery; they must be “pushed.” The voltage produced by the solar panel and charging circuit will provide the necessary voltage to push electrons into the battery. In order to fully charge the battery, the charging circuit must output a voltage greater than that of the fully charged battery.

A useful analogy to this is pumping water from a well or other underground reservoir. In order to fill a bucket (charge a battery), water must be pushed through the pipes or hose and up into the bucket. Despite how much water is available, the bucket cannot be filled unless the pump provides enough force to raise the water. Similarly, no matter how much current is produced by the cells, they cannot charge a battery unless the force pushing the electricity into the battery—the voltage—is high enough.

A single solar cell can only produce a limited current and voltage, which should be evident by experiment in the previous lesson. Wiring cells together to make a panel will allow students to achieve the higher currents and voltages needed to do useful things like charge a battery. Discovering how cells behave when connected is easy to do experimentally, and should also be predictable given knowledge of circuits in series and parallel and how a solar cell works.

There are two basic ways to connect components in a circuit. In a series connection, two components are wired end to end in a circuit, such that current in the circuit flows through one and then the second, individually.

For instance, in the diagram above, current flows out of the positive end of the battery, through component B and then through component A.

Because current flows through each component in order, the current through components in series is the same, and must be equal to the current through the overall circuit. The voltage across the two can be different, however, and the voltage drop across both components is the sum of the drop across the individual components. In the diagram above, the total voltage drop is V1 + V2

In the parallel connection, components are connected next to each other, such that current through the circuit splits and travels through either component.

Since the cells in this instance are connected at each end, the voltage drop must be the same across both. However, since the current can travel through either component, the sum of the current through each component must equal the total current in the circuit. In the diagram above, the current “splits” at the branch between the two components - some of it goes through one, and some goes through the other.

To raise the voltage of a panel, then, cells must be connected in series. This is because voltages of cells in series add, allowing a series of cells to be connected to add up to the desired voltage. Since currents add when cells are put in parallel, connecting parallel cells or strings of cells allows you to produce higher currents with a panel of the same voltage.

In order to raise the voltage of a panel without raising the current, it is often beneficial to cut cells. This is desirable because the voltage of a particular cell often needs to be increased much more than the current to meet charging needs. When a cell is cut, the pieces of the cell all have the same voltage as the original cell, but smaller currents (students may have noticed this when measuring cells and cell pieces in the first lesson). So rather than create a very large panel that produces too much current, cells can be cut into smaller pieces that are re-wired to raise the voltage, while the current-producing area remains roughly constant.

In this lesson, students will design and build their own solar panels. Students will first determine what to power with their panel, and roughly how much current and voltage is required. They will then discover how connecting cells in series and parallel affects the current and voltage produced by a panel, and will design a setup that can charge their cell phone battery

Before beginning this lesson, students should briefly discuss what they want to power with their panels. The following lessons walk the class through building a rechargeable solar lamp, but feel free to adjust this lesson slightly (e.g. battery charger), to build another project (e.g. USB charger), or to give students more freedom in their designs.

Once students have an idea of what they want to build, they should determine how much voltage and current their application requires. If students are building a lamp or using another project that requires the panel to charge batteries, this will be determined by the type of battery. Mobile phone Li-ion batteries require 4.5V and around 100mA of current to charge.

Once the students have determined their voltage and current requirements, they should see that a single cell cannot achieve the desired numbers. Can students think of another way to achieve this? Given more cells?

At this point, students should either review or be taught the difference between series and parallel connections. Again, a waterfall or other analogy may help visualize the concepts, as might blackboard drawings or other visualizations. Once students have a solid understanding of the difference between these two connections, they should be able to determine how to connect cells to raise the voltage and current of their panels. To achieve a higher voltage, cells should be wired in series so that voltages add (e.g., as with waterfalls stacked on top of one another. If students have not worked with series and parallel connections before, it may be helpful to introduce another hands-on activity here to solidify understanding of these). To increase current, cells must be wired in parallel.

The students should now understand their requirements for a panel, as well as how to increase and decrease the voltage and current of the panel by wiring cells together. Students should now be able to design a panel on their own. If numbers of cells are limited, it is also possible to cut cells into smaller pieces. As students should remember, smaller pieces will have the same voltage and a smaller current than the full cell. Students can use pieces to raise the voltage of their panel without increasing the current far beyond what is necessary, thus using fewer cells. This will be covered in depth in future lessons, but should be introduced to students as a concept now.

Students should design their panels by balancing the requirements they need from their panel with the number of cells they use overall. In other words, they should design a panel such that it can produce the required voltage and current, but without using too many excess cells or unnecessary cuts.

Once students have finished designing, they should work in groups to check each others' designs, explain their calculations, and verify that they should work.

At the close of the lesson, students should think about what else they might need to make a panel. How could you mount the cells? Protect them from dust? Etc.

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