Auto Electrical Works Paper 3 2019

Question 2

 

AIM:   To determine the relationship between amount of charge in a motor vehicle battery
and time taken to charge.
            question

  1. Connect the circuit as shown in Fig. 1.

  2. Ask the supervisor to check the circuit connection.

  3. Set the battery charger to 12V range.

  4. Plug the battery charger to an a.c. source.

  5. Read and record the voltage, Vo, across the battery

  6. Switch on the battery charger and start the stopwatch simultaneously.

  7. Read and record in Table 3 the voltage V at time t = 10 mins, 20 mins, 30 minsand 40 mins respectively.

  8. Table 3          


    Time (mins)

    Voltage, V (V)

    V – Vo (V)

    10

     

     

    20

     

     

    30

     

     

    40

     

     

  9. Complete Table 3 by calculating V – Vo.

  10. Plot a graph of V – Vo (V) on the vertical axis against time (mins) on the horizontal axis.

  11. Comment on your graph.

  12. State one application of the experiment.

  13. State two safety precautions taken in carrying out the experiment.

 

Observation


 

 

    The expected responses were:
(e)        Voltage across the battery Vo = 10V
                                                                         

Table 3


Time (mins)

Voltage, V(V)

V – Vo (V)

10

11.00

1.00

20

11.20

1.20

30

11.30

1.30

40

11.38

1.38

Voltage V (V) –values ± 0.9 with respect to Voltage, V (V) in Table 2
V – Vo (V) –values with respect to Voltage, V (V) in Table 2


(i)         A non-linear graph with appropriate:


  • choice of scales.

  • pick of points.

  • joining of points.

  • labelling of axes                                                                                                                                 

(j)         Comment on graph


  • It is non-linear
  • The charging rate decreases with increase in time

(k)        Application of experiment


  • To determine the quality of a battery

  • To determine the charging rate of a battery

                                                                             
 (l)        Precautions in carrying out the experiment


  • Ensure tight connection of the battery terminals

  • Ensure the correct polarity of battery

  • Ensure good ventilation of the room/area

  • Avoid flames/sparks in the area

Questions 2 required candidates to charge a lead-acid battery and measure the terminal voltage of the battery with increase in time of charge. The Chief Examiner reported that only a few candidates attempted question 2 with good responses and deductions.