What is internal resistance of a cell?

Short Answer

The internal resistance of a cell is the resistance present inside the cell that opposes the flow of current. It is caused by the chemicals, electrolytes, and materials inside the cell through which the electric charges must move. This resistance reduces the actual voltage available across the cell’s terminals when current flows.

When no current is drawn, a cell gives its full EMF. But when current flows, some voltage is lost inside the cell due to internal resistance. This is why the terminal voltage becomes less than the EMF during operation.

Detailed Explanation

Internal resistance of a cell

The internal resistance of a cell refers to the natural opposition to the flow of electric current that exists within the cell itself. Every cell or battery has some resistance due to its internal chemical structure. This resistance is not placed outside the cell like a normal resistor; instead, it is built into the cell because the flow of ions and electrons inside the electrolyte, electrodes, and connecting materials faces some obstruction. This internal opposition limits the current a cell can supply and affects its performance in circuits.

Understanding internal resistance is important because it explains why a battery’s terminal voltage decreases when it supplies current, why batteries heat up, and why older batteries perform poorly.

Origin of internal resistance

Internal resistance arises due to several factors inside the cell:

  1. Electrolyte resistance
    The movement of ions in the electrolyte faces obstruction.
  2. Electrode resistance
    Electrodes are not perfect conductors; some resistance exists.
  3. Chemical reaction limitations
    The chemical process producing electrons is not instant or perfect.
  4. Material impurities
    Internal impurities slow down the movement of charge.

All these factors combine to produce the total internal resistance of a cell.

Effect of internal resistance on terminal voltage

A cell has two important voltages:

  • EMF (E) → the maximum voltage a cell can provide when no current flows
  • Terminal voltage (V) → the voltage available at the cell’s terminals when current flows

When current flows, some voltage is consumed inside the cell due to its internal resistance. This voltage drop is:

Voltage drop = I × r

Where:

  • I = current
  • r = internal resistance

So, the terminal voltage becomes:

V = E − Ir

This explains why devices connected to weak or old batteries do not work properly—they receive less voltage because the internal resistance becomes large.

Measurement of internal resistance

Internal resistance is usually measured using a potentiometer or a Wheatstone bridge arrangement. The method involves:

  1. Measuring the EMF of the cell when no current flows.
  2. Measuring the terminal voltage when current flows through a known resistor.
  3. Using the formula:
    r = (E − V) / I

By comparing the drop in terminal voltage, the internal resistance can be calculated.

Factors affecting internal resistance

Several factors affect internal resistance of a cell:

  1. Nature of electrolyte
    Thicker or less conductive electrolytes increase resistance.
  2. Temperature
    Higher temperature decreases internal resistance because ions move more easily.
  3. Age of the cell
    Older cells have higher internal resistance due to weakened chemicals.
  4. Current drawn
    Drawing higher current increases heating and increases resistance.
  5. Size of electrodes
    Larger electrodes reduce resistance by offering more surface area.
  6. Distance between electrodes
    Larger separation increases internal resistance.

Importance of internal resistance

Understanding internal resistance is important because:

  1. It determines how much current a cell can safely supply.
  2. It affects the performance and lifespan of batteries.
  3. It explains why terminal voltage drops under load.
  4. It helps in designing better batteries.
  5. It helps in choosing the right battery for specific devices.

For example, devices like inverters need batteries with low internal resistance, while small toys can work with batteries having higher internal resistance.

Practical effects of internal resistance

  1. Reduction in voltage output
    When a torchlight becomes dim, it is usually because internal resistance has increased.
  2. Heating of cell
    Internal resistance causes energy to be converted into heat.
  3. Limited current supply
    A battery with high internal resistance cannot provide high current.
  4. Performance of old batteries
    As batteries age, internal resistance increases, delivering less power.

Applications where internal resistance matters

  • Portable electronics
  • Electric vehicles
  • Power backup systems
  • Laboratory experiments
  • Battery chargers
  • High-current devices like motors and inverters

Engineers design batteries keeping internal resistance in mind to ensure efficiency and safety.

Conclusion

The internal resistance of a cell is the natural opposition inside the cell that reduces the current and lowers the terminal voltage when the cell is in use. It originates from the electrolyte, electrodes, and chemical reactions inside the cell. A low internal resistance means better performance, while a high internal resistance reduces battery efficiency. Understanding internal resistance is essential for measuring cell performance, designing electrical systems, and analyzing real-life battery behavior.