Question 1
AIM: To test candidates’ knowledge of specific gravity of a motor vehicle battery.
You are provided with a serviceable 12 V, 45 Ah lead-acid battery.
- Remove the battery caps.
- Measure and record in Table 1 the specific gravity of the electrolyte in each cell.
- Complete Table 1 by commenting on the specific gravity of each cell in relation to the state of charge.
- Compare the specific gravity of the cells and comment on the overall state of charge of the battery.
- State two precautions taken in carrying out the activities.
- Describe briefly the process involved in the preparation of an electrolyte.
- Complete Table 2 by stating the effect of electrolyte level on the cell of a motor vehicle battery.
- State five:
Table 1
Cell |
Specific gravity |
Comment |
1 |
|
|
2 |
|
|
3 |
|
|
4 |
|
|
5 |
|
|
6 |
|
|
Table 2
|
Electrolyte level |
Effect of the electrolyte level on the cell |
(i) |
Over-filled |
|
(ii) |
Below the cell level |
|
(i) safety precautions taken in charging a lead-acid battery outside a motor vehicle.
(ii) steps that should be taken to install a battery into a motor vehicle.
Observation
The expected responses were:
Table 1
Cell |
Specific gravity |
Comment |
1 |
1.260 – 1.300 |
Fully charged |
2 |
1.260 – 1.300 |
Fully charged |
3 |
1.260 – 1.300 |
Fully charged |
4 |
1.260 – 1.300 |
Fully charged |
5 |
1.260 – 1.300 |
Fully charged |
6 |
1.260 – 1.300 |
Fully charged |
(d) (i)Comparison of specific gravity of cells
- The specific gravity of the cells are approximately the same
- The specific gravity of the cells vary slightly. The specific figure shows that the battery is fully charged.
(any correct comparison reflected in Table 1)
(ii) Comment on state of charge of battery
- The battery is fully charged(any correct interpretation of values in Table)
(e)Precautions taken in carrying out the experiment
- Ensure the use of personal protective equipment /devices such as hand gloves
- Avoid error due to parallax in hydrometer reading
- Always empty hydrometer before taking the next reading
- Avoid naked flames and sparks
- Avoid contact with electrolyte
- Avoid spillage of electrolyte
- Handle tools with care
(f)Description of process involved in the preparation of an electrolyte
- Get a glass beaker/plastic jar
- Measure quantity of water into the jar
- Measure quantity of sulphuric acid, [H2SO4, Tetra-oxosulphate six acid] at a ratio of
1:3, 1:4, 20%:80%,35% : 65% (acid to water).
- Pour the measured acid into the jar of water gently
- Stir the mixed solution gently with a wooden, plastic or glass stirrer.
(g)Table 2
|
Electrolyte level |
Effect of the electrolyte level on the cell |
(i) | Over-filled |
|
(ii) |
Below the cell level |
|
(h) (i) Safety precautions taken in charging a lead-acid battery outside a motor
vehicle
- Ensure proper ventilation
- Avoid naked flame or fire
- Avoid overcharging
- Ensure tight connection
- Ensure correct polarity
- Add acid to water NOT water to acid
(ii)Steps that should be taken to install a battery into a motor vehicle
- Place battery in its position
- Clamp the battery
- Connect the positive terminal
- Connect the negative terminal
- Apply grease to the terminals
The Chief Examiner reported that majority of the candidates who attempted question1 were able to read the hydrometer accurately. It was however stated that, only a few candidates gave reasonable comments as deductions on the read values.