Cells

Cells, often incorrectly called batteries (a collection of cells), are devices which can hold electrical energy, which can be used to power applications. They are made with two plates or poles with an electrolyte (Used to transport ions between the positive and the negative plates) between them. Cells fall into two categories; primary and secondary.

The capacity of a cell or battery is measured in Ampere hours (Ah). It is the discharge of current by time. An example would be, 100Ah being used for 5 hours will give:

20A for 5 hours
10A for 10 hours
5A for 20 hours

Primary Cells (non-rechargeable)

Primary cells produce electricity from chemical changes, but when these chemical changes have finished, the cell is useless and can only be thrown away. Some types of primary cells are Leclanch?, Mercuric Oxide, Manganese-Alkaline and Lithium.

Leclanch? Cells

The construction of Leclanch? cells include a pole forming the positive contact with a depolariser surrounding it which is a mixture of powdered carbon and manganese dioxide, which removes hydrogen, and a container made of zinc which makes the negative contact. Between these is the electrolytic which is a paste of ammonium chloride impregnated into a paper lining places close to the zinc.

The disadvantage of this cell is that when left connected, gaps appear in the casing and the electrolytic paste can leak. They can also physically swell which can possibly get them stuck in their container.

Leclanch? cells have an open-circuit (unloaded) e.m.f of 1.5V. When they are stacked, bigger voltages can be created.

They have a shelf life of 1 year and are used in torches, clocks, toys etc.

Mercuric Oxide Cells

Mercuric Oxide cells use an alkaline based electrolyte of potassium hydroxide and are intended for low current applications. They're outer case is made up of two segments which join making the positive contact. This cell will not leak or corrode easily. Types of these cells are button and flat pellet.

If the cell is touched by hand, the cells life with drastically be reduced. The electrolyte is extremely harmful as it can harm eyes and burn skin, so you should never attempt to recharge or open the cell.

They have a shelf life of 2 years and their voltage is 1.35V. Applications of use include watches and small tools.

Manganese-Alkaline Cells

Potassium hydroxide is used as the electrolytic for this cell and is either in jelly or liquid form. Both cylindrical and button types are made with a central steel rod coated with manganese oxide and graphite. The negative contact is formed by the outer case.

Output voltage is constant during discharge and can provide a high current output. These cells can be connected in series to provide a higher voltage.


Lithium Cells

The positive terminal is made from lithium on stainless steel foil. It's separated from the rest of the cell with a porous polypropylene enclosure. the negative terminal is made of stainless steel mesh with a coating of manganese oxide and graphite surrounded by a steel casing.

The cell has a constant e.m.f of about 3.4V to 3.7V and has a shelf life of 10 years or more. They must never be connected in parallel and only two cells can be connected in series.

Overview of Primary Cells

Leclanch? (Zinc Carbon)
Voltage: 1.5V
Shelf life: 1 year
Uses: Torches, clocks and toys

Mercuric Oxide
Voltage: 1.35V
Shelf life: 2 years
Uses: watches and small tools

Manganese-Alkaline
Voltage: 1.5V
Shelf life: 30 months
Uses: devices which need high current

Lithium
Voltage: 3.3V - 3.7V
Shelf life: 10 years
Uses: memory backup and low medium drain applications

Secondary Cells

These cells produce electricity from chemical reactions in the cell. When direct current is fed through the cell, the chemical reactions are reversed and the cel can be reused. Some of these cells are Lead Acid and Nickel-Cadmium.

Lead Acid

The Lead Acid cell is the most common secondary cell. It's made of two lead grids in a plate form. One is covered with lead peroxide and the other contains a spongy lead immersed in an electrolytic of dilute sulphuric acid.

This cell gives an e.m.f of 2.2V when fully charged. This falls to 2V when in use. The cell is discharged at 1.8V and has to be recharged before it can be used again.

When in use, the electrolytic (water) evaporates and must be topped up to the level indicated.

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The cell is made up of positive and negative plates interleaved. They are connected to their respective terminals with lead group bars.

Nickel-Cadmium Cells (ni-Cd)

Nickel-Cadmium are made up of a nickel hydroxide positive plate, a cadmium hydroxide negative plate and an electrolyte of potassium hydroxide in an aqueous solution.

Their displayed voltage is 1.2V but the e.m.f is 1.25V - 1.3V and capacities range from 100mAh to 10Ah. 10 cells can be used in series giving 12V. These are high power cells and are used for applications like emergency lighting and engine starters. ni-Cd cells can practically last forever if kept in accordance to the manufacturers specification.

Overview of Secondary Cells

Lead Acid
Voltage: 2V

Nickel-Cadmium
Voltage:1.2V