What Makes A Battery Work ?

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A battery is an efficient and compact energy storage centre created by a manufacturing process that combines chemicals, grids, plates and separators to produce electrical power.  Within each cell a battery houses a group of alternate positive and negative plates held apart by separators and bathed in electrolyte.
Since each cell supplies approximately 2.1 volts, a six volt battery would contain three cells connected in a series.  Six cells would prove a “12 volt” battery (actually 12.6 volts).  The size, thickness and number of plates in each cell determines a battery’s capacity.
Taking a look inside a typical battery you will find:

A lead acid battery contains four chemicals:
· Lead – Which is used to make the grids, straps, posts etc.  It is also used in a sponge form as the active material found on negative plates.
· Antimony – Only a small amount of antimony is necessary to harden the grids which would normally be soft and easy to bend.
· Lead Peroxide – Manufactured by oxidising pure lead electrochemically, lead peroxide is the active material found on positive plates.
· Sulphuric Acid – Diluted sulphuric acid forms the electrolyte acid solution which contains about 25% sulphuric acid and 75% water by volume.  This corresponds to a specific gravity of 1.260 at 25C.

The grids within a battery compose a solid framework designed to hold the active material in place.  Cast of lead and antimony, the grids also conduct the electrical current produced by the active materials to the battery terminals.

Positive plates are grids covered in a lead peroxide past and are dark brown in colour.  Negative plates are pasted with grey sponge lead.  The negative sponge lead and the positive lead peroxide create the two different metals necessary to enable a lead acid storage battery to produce electricity.

Separators prevent positive and negative plates from making contact, which results in shorting causing the battery to self discharge.  Separators are thin sheets of non-conducting porous plastic material which are inserted between the plates or completely encasing a positive or negative plate.  A rib faces the positive plates and provides greater electrolyte volume in addition to minimising the area of contact with the positive plates.  Many batteries also feature glass fibre retaining mats placed between the positive plate and the separator to slow the loss of active material from the plate.

The sulphuric acid in the electrolyte is absorbed within the positive and negative plates, producing a chemical reaction which is then released as electrical power.  The electrolyte also carries the electrical current within the battery between the positive and negative plates passing through the separators.

The container houses the pates, separators and electrolyte.  Usually constructed of polypropylene, it must withstand extreme heat and cold as well as shock and vibration.  Bridges at the bottom allow a space for the active material to settle during battery life without the danger of shorting.

Element Construction
The desired number of positive and negative plates are interleaved with separators to form an element or group.  Like plates are welded together at the plate lugs.  Each element connects to the one adjacent to it in a series (positive to negative).  For greater performance there is always one more negative plate than positive.
Any number or size of plate may be used in an element, depending upon how much energy needs to be stored.  Of course, increasing the size and number of plates creates a higher ampere rate during discharge at low temperatures.  (ie: a high CCA rating)

Connecting elements together
Connectors are used to connect al the elements together to form a series within each cell.  They are designed to carry a much higher current than normally required to ensure melting does not occur when starting.

Three types of connectors are used including:

“Conventional old style” connectors which connect elements together on top of the battery.
“Loop-over” connectors which use straps to loop over the cell dividers and are welded and placed under the cover.

“Through the partition” connectors which are made directly through the cell wall and are sealed by crimping under high pressure and lead welding.  This is by far the most common method used.

Covers and vent caps
Covers and vent plugs prevent electrolyte spillage and keep out dirt and other impurities.  A cover may have three or six vent holes depending on the number of cells.  Generally, heat and pressure is used to seal them to the container.  A vent cap closes the vent holes and provides a convenient way to check the electrolyte level.  Vent caps also allow gas to escape when charging the battery.  Some are fitted with a porous disc to prevent externally induced explosions.

Terminals are the battery’s external electrical connection points and are normally located on top of the battery.  They are moulded into the cover and connect internally to the post in the two end groups.  To minimise the danger of installing a battery in reverse, the positive terminal is slightly larger than the negative terminal.

This article is found in the Virtual mechanic CD Rom
You can download it for the price of a latte, but you will learn not to buy a lemon
By Darren Gow-Brown, Melbourne Australia


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