Background Information: How Batteries Work

In batteries, electrolytes are substances – often liquids or pastes – that allow electrical current to flow between the positive and negative terminals (the cathode and the anode). They enable the movement of ions (electrically charged particles), making it possible for the battery to charge and discharge. 

A battery is made up of cells, and each cell contains: 

• An anode – the electrode that gives up electrons easily. 
• A cathode – the electrode that accepts electrons easily.
• An electrolyte – the medium that allows ions to move between the electrodes. 

The electrodes cannot be made of the same metal. One must be a metal that gives up electrons easily, and the other must accept electrons easily. Zinc and copper are commonly used. For example, zinc readily gives up electrons in a chemical process called oxidation, which gives the zinc end of the battery a negative charge (anode). At the copper electrode, protons from the acids or salts in the electrolyte accept electrons and react to form hydrogen gas. This process, called reduction, gives the copper end of the battery a positive charge (cathode).

When the anode and cathode are connected by a wire, electrons flow from the anode to the cathode, producing an electric current.

Source: Freddy (https://chemistry.stackexchange.com
/users/5456/freddy), Positive or Negative Anode/Cathode in Electrolytic/Galvanic Cell, URL (version: 2017-01-22): https://chemistry.stackexchange.com/q/16789

A 9V battery, despite its rectangular shape, is actually composed of six smaller 1.5V cells connected in series. 

Source: 9V innards 3 different cells.jpg by Lead holder is licensed under CC BY-SA 3.0 via Wikimedia Commons.

Why can’t one cell just make 9 volts? The voltage of a single cell is set by the chemistry of the electrodes, not by how much electrolyte you add. Each pair of materials (like zinc and copper) has a fixed potential difference based on how easily one metal loses electrons and the other gains them. The electrolyte helps ions move and allows current to flow, but it does not increase the voltage of a single cell. 

That’s why a 9V battery is made of multiple smaller cells connected in series. You can think of each cell as adding one small “push.” More cells add more total push (voltage).  In the following experiment, changing the electrolyte may help the charge move more easily, but the electrodes determine the voltage of each cell. To get enough voltage to light the LED diode, you will likely need to connect multiple cells together.

Introduction

In this lab, you will build a simple battery using metal electrodes and an electrolyte solution. Your group will test one electrolyte to determine how well it produces electrical energy, and then share your results with the class to identify which electrolyte works best. You will also explore how electrolytes allow batteries to move charge (current) and how this works together with voltage to power a device.

Pre-Lab Planning

A. Class Investigation Question: Which electrolyte will require the fewest cells to light an LED diode?

• Prediction (Class-Level Hypothesis): Discuss the list of electrolyte substances provided by your teacher. Based on that list, make a prediction.

• I predict that ______________________________ will require the fewest cells to light an LED diode because _________________________________________________________________________________.

B. Group Investigation Question: How many cells will your assigned electrolyte require to light an LED diode?

• What is your team’s assigned electrolyte? ________________________________________________________

C. Group Investigation Question: How many cells will your assigned electrolyte require to light an LED diode?

• Prediction (Group-Specific Hypothesis)
  
• I predict that it will take ______________________________ cells to light the LED diode because _________________________________________________________________________________________________.

D. Checkpoint: Your teacher will review your hypotheses before you begin the experiment.

Important Safety Reminders!

• Wear safety goggles at all times.
• Do NOT taste or touch the electrolyte solutions.
• Wash your hands after the lab.
• Handle all wires and clips with care — avoid short-circuiting.

Step-By-Step Procedure

Part 1: Set Up Your First Cell (Control: No Electrolyte)

1. Set out one zinc electrode and one copper electrode. Cover the middle of each with a piece of masking tape. The copper is the cathode (+) and the zinc is the anode (-).

2. Take one small cup and tape one zinc electrode on one side (inside the cup) and one copper electrode on the opposite side. 

3. Make sure the electrodes do not touch, and are taped low enough so that the bottom part will be submerged in the electrolyte substance.

Part 2: Measure Voltage

4. Set the multimeter to DC Voltage (20V range).

5. Touch (or tape) the zinc electrode to the black probe of the multimeter (COM), or connect them with a pair of alligator clips.

6. At the same time, touch (or tape) the copper electrode to the red probe of the multimeter (V), or connect them with a pair of alligator clips.

7. Observe and record the voltage in Data Table 1 below.

8. Repeat this measurement 2 more times to improve reliability, and record in Data Table 1.

9. This step acts as a control. It shows what happens when no electrolyte is present, so you can compare it to later results.

Part 3: Add Electrolyte

10. Measure 70mL of your assigned electrolyte substance in the beaker. 

11. Pour the electrolyte substance into the cup. 

12. Make sure that part of the zinc and copper electrodes are submerged in the liquid. This is one cell.

13. Once again, use the multimeter to measure the voltage of the one cell with the electrolyte.

14. Record the results in Data Table 1 below.

Part 4: Test the LED Diode (One Cell)
After measuring voltage and comparing results, you will now test how well your cell powers an LED diode.

15. Disconnect the multimeter. 

16. Using a pair of alligator clips, connect the top of the zinc electrode to the short lead of the LED diode.

17. With another pair of alligator clips, connect the top of the copper electrode to the long lead of the LED diode.

18. Does the LED diode light up? It’s very important that each electrode is connected to the right lead of the LED diode. Read steps 16-17 again and ensure you followed directions exactly.

19. If it still does not light up, then you may not have enough voltage and need to make another cell.

20. Record your results in the first row of Data Table 2.

Part 5: Fixed Cell Comparison

21. Remove the LED diode and alligator clips from the cell.

22. Set up a second cup, with zinc and copper electrodes taped inside.

23. Measure out and add 70 mL of your assigned electrolyte substance to the second cup. 

24. Connect the cells in series. Using a pair of alligator clips, connect the copper electrode of the first cell to the zinc electrode of the second cell.

25. Note: Always connect the copper (+) of one cell to the zinc (-) of another cell.

26. Measure the total voltage of the two-cell system using the multimeter, and record in Data Table 2. Repeat the measurement 2 more times and record. 

27. Then, connect the LED diode to the open ends using two pairs of alligator clips. The free zinc electrode should be connected to the short lead of the LED diode. The free copper electrode should be connected to the long lead of the LED diode. 

28. Check if the LED diode lights up. 

29. Record your results in the second row of Data Table 3.

Part 6: Continue Testing

30. If the LED diode does not light up, remove the LED diode and alligator clips.

31. Build a third cell with electrodes and the electrolyte.

32. Connect the three cells in series.

33. Record the voltage of the three-cell series with the multimeter.

34. Connect the LED diode and see if it lights up. Adding more cells increases the total voltage of your battery.

35. Then, build a fourth cell if needed, record voltage readings, and then test the diode again.

36. Stop when the LED lights or when you reach 4 cells.

Data Tables

Use the data tables below to record important information as you do the experiment. 

Data Table 1: Single Cell Voltage Trials

Data Table 2: Multiple Cell Voltage Trials

Data Table 3: Testing LED Brightness

*Brightness Scale: 0=Off; 1=Very Dim; 2=Medium; 3=Bright

Data Table 4: Class Data
You will complete this table after all groups share their results.

Post-Lab Analysis

A. Your Group Results

1. How many cells did your group need to light the LED diode? 

2. Was your group’s prediction correct? Explain why or why not. 

3. What was the average voltage of one cell with your electrolyte? What was the average voltage when using two cells? What does this show about how cells work in series?

4. How did the LED diode behave as you increased the number of cells (off, dim, bright)? 

5. Did increasing the voltage always cause the LED diode to turn on? If not, what might be another factor affecting whether the LED lights?

B. Class Results

6. Did different electrolytes produce similar or very different single-cell voltages?

7. When all groups used the same number of cells (2 cells), did all the LED diodes behave the same way (off, dim, bright)? 

8. If groups had similar voltage, but different LED behavior, what does this suggest?

9. Which electrolyte required the fewest cells to produce a visible effect (dim or bright light)?

10. Did some electrolytes require more cells to produce any light at all? What might this indicate?

11. Based on the class data, what seems to matter more for lighting the LED diode? Voltage alone, or how easily charge moves through the electrolyte? Explain your reasoning.

C. Understanding the Science

12. In this lab, what mainly determines the voltage of a single cell: the electrode materials or the electrolyte?

13. What job does the electrolyte do inside the battery?

14. Why did water not work well as an electrolyte? 

15. Why does adding more cells help light the LED diode? 

D. Reflection

16. What is one source of error in your experiment that could have affected your results? 

17. What is one way you could modify or improve this experiment if you were to do it again? 

18. If you wanted to build a higher-voltage battery, what changes could you make to the materials or setup? 

Optional Extension Challenge
Modify one variable (such as electrode material or electrolyte concentration/amount) to see if you can reduce the number of cells needed to light the LED diode. Be sure to keep all other variables constant when testing your change.