CELL- WEEK SEVEN AND EIGHT

Week 7 and 8
Topic: CELLS
Specific Objectives: By the end of the lesson, student should be able to
1. Define cell
2. Explain two types of cell
3. State two application of each type of cells
4. Explain the two defects/faults in a simple cell and how to prevent it.
5. Outline five ways you can maintain a lead acid cell.
CELLS
A cell is a device that converts chemical energy into electrical energy. A group of cell is called battery. A cell can be primary or secondary.
Components parts of cell
1. Two electrodes (Anode and cathode):
- An electrode is the terminal through which electric current passes between metallic and non-metallic parts of an electric circuit.
- Cathode: The cathode of a cell is define as the electrode to which cat-ions (positively charged ions) flow within the cell (positive terminal)
- Anode: This is the electrode to which anions (negatively charged ions) flow within the cell (negative terminal)
2. Electrolyte: This is usually a solution of water or other solvents in which ions are dissolved. Example, molten salt such as sodium chloride is also electrolytes.

Types of Cell
There are two main types of cell
a. Primary cell
b. Secondary cell
Primary Cell
A primary cell is a battery that designed to be used once and discarded and not recharged with electricity. Common examples are the alkaline battery used in flash lights and a multitude of portable devices. Most common types of primary cell is the Leclanche cell named after its inventor.
Simple cell (Leclanche cell)
The simple cell consist of four components.
a. Glass container
b. Electrolyte (dilute tetroxosulphate(vi)acid ie H2SO4)
c. Carbon plate (+ ve electrode)
d. Zinc plate (-ve electrode)
The voltage between the plates of a lechanche cell, without the connection of an external resistance is 1.5V. this is termed the open circuit voltage or electromotive force (EMF) of the cell and the symbol is E.
Faults in simple cell
The simple cell has two main faults.
1. Local action: The electrolyte attacks the impurities in the zinc forming small cells and corroding the electrode.
2. Polarization: Bubbles of hydrogen gas (an insulator) from around the carbon electrode when the cell is in use and resist the flow of current.
Prevention of Polarization: can be prevented by either brushing of the plates or by using depolarizer (an oxidizing agent) which is commonly used in manganesedioxide. This oxides hydrogen to form water and helps in removing the hydrogen bubbles
Prevention of local action: Local action may be minimized by using pure zinc, but zinc in its pure state is very expensive. Instead, a cheaper option is alloying the zinc electrode with mercury. This process is referred to as amalgamation and resultant alloy is called zinc amalgam.
Advantages of primary cell
1. Cheap
2. Requires little maintenance
Disadvantages
1. It is not portable in the form
2. They are not rechargeable
3. They are affected by default such as local action and polarization
4. Has a high internal resistance
5. Cannot use to supply heavy current
Evaluation
1. Define cell
2. Explain two types of cells
3. Explain two types of deffact found in a simple cell
Secondary Cell (Week 8)
Secondary cell or accumulators are cells whose chemical actions can be reversed by driving a current through them in a direction opposite to the current they supply. Such cells can therefore be recharged and used for a long time.

Types of secondary cell
Lead acid cell
The lead acid cell is made up of the following
- Container
- The plates
- The separators
- The electrolyte (H2SO4)
NB The separator are made up of glass or more commonly wood (wood sheets which have being made porous by removal of the resion
The correct mixture of texraoxidsulphate(vi)acid and distilled water is obtained by using a hydrometer. The hydrometer is an instrument used to measure the specific quality of the mixture.
The specific quality of a lead acid cell is usually about 1.270 when the cell is fully charged and when the cell is discharged, the specific gravity falls but should not allowed to go below 1.190.
The sequence of discharge
i. The sulphur in the tetraoxosulphate(vi)acid combines with the lead on the plates to form lead sulphate. This is known as sulphation.
ii. The oxygen in the lead peroxide (positive plate) combines with the hydrogen in the electrolyte to form water. The electrolyte is therefore weekened and the specific gravity falls.
Indication of discharge cell
1. Specific gravity low
2. Cell voltage low (emf 1.8v to 2v)
3. Witish deposit on plate (lead sulphate).

Indication of fully charged cell
1. Gassing freely
2. High and constant specific gravity
3. Charging voltage steady
4. Charging current steady
5. Colour of plates: positive-chrocolate brown, negative-bright grey
Methods of charging batteries
1. Trickle charge
2. Constant current
3. Constant voltage
4. Floating system.
1. Trickle charging: A small current passed through the battery for a long period.
2. Constant current: The charging current is held constant by means of a variable resistance in the charging circuit.
3. Constant voltage: The voltage is held constant through the charge by means of a variable resistance.
4. Floating system: In this method (used in car charging system) the DC generator re-charges the battery when it falls below a certain voltage. The charging current is automatically cut out when a certain voltage is reached.
Maintenance of a lead acid battery
1. Re-charge battery when voltage and specific gravity falls.
2. Recharge battery regularly even if not in use.
3. Do not add electrolyte to the battery unless there has been a spillage.
4. Keep battery topped up with distilled water above plate level, otherwise air will attack the plates causing hydration
5. Do not over discharge (boost charge) or over discharge a battery
6. Keep the terminals free from corrosion by smearing with petroleum jelly.

Application of lead acid cell
1. Used in car batteries
2. Traction batteries and stand by supplies
Application of primary cell
- Used in a torch light
- Used in a radio set.
Advantages of lead acid battery/cell
1. Cheaper
2. Higher voltage per cell
3. Lower internal resistance
Disadvantages of lead acid battery
1. Lacks mechanical strength
2. Self discharging
3. Plates shed easily
Advantages of alkaline cell
a. Very roboust
b. Not self discharging
c. Capable of withstanding heavy charging and discharging current
d. Lighter for given capacity
e. Longer life
Disadvantages of alkaline cell
1. Expensive
2. Require more cells per volts
3. Container are live
4. Higher internal resistance
5. Must be kept sealed as air attacks electrolyte
Precaution necessary in charging room
1. Room must be well ventilated
2. Vent plugs must be removed before charging
3. No naked lights to be brought near charging batteries
4. Alkaline and lead acid batteries must be charged separately
5. Always add acid to water (not water to acid).
Evaluation
1. State two applications each both for secondary and primary cell
2. State five ways of maintaining lead acid cells.
Assignment
1. Draw and label a typical simple cell
2. What is sulphation