Internal Resistance of a Battery Lab

Get the Document: Lab-InternalResistance.pdf

Internal Resistance of a Battery

When you draw current from a battery, the terminal voltage drops because all batteries or cells have an internal resistance which we can imagine as a small resistor in series with the cell. The formula then for the EMF, ε, is 𝜀 = 𝐼(𝑅+𝑟) where I is the current leaving the battery, R is the external resistance, and r the internal.

What this means is that the terminal voltage of the battery (The voltage you would measure if you did so from the black dot to the black dot in the diagram above) will drop as you draw more and more current from the battery. In fact, if we look at the relationship between the current and the terminal voltage, it should be V­_t = ε - Ir, where ε is the battery's EMF, I is the current we are drawing from it, and r is its internal resistance. If we graph V­_t vs I, we will get a graph with a y intercept of ε, and a slope of -r, so that is the plan, to get the internal resistance from the slope of the best fit line.

Directions -

0. With the light bulb unscrewed, read the unloaded terminal voltage of the battery, then screw it in.

1. Choose 5 - 10 different currents by sliding the variable resistor, and record for each the current in amps (mA/1000) and the potential in volts.

2. Graph V­_t vs I, and put a trendline through the points, displaying the equation for the trendline.

3. Unscrew the light bulb, and turn off the meters unless there is someone waiting right there to do the lab.

4. Answer these questions:

a. What was the internal resistance of the battery?

b. How does the unloaded voltage you measured in step 0. compare to the y intercept of your equation?

c. As batteries wear out, their voltage drops more severely with current drawn. What is happening to their internal resistance? (why do you know?)