Every is fitted with a . It is charged while the is running and provides the vehicle’s electrical needs while the is stopped. It also produces the very high current demanded by the starter motor to re-start the .
This means that the must be capable of being charged and discharged a great number of times without loss of performance, and be able to provide a lot of electrical energy (as much as that used by a three-bar electric fire) for the few seconds that the starter motor is in use. The failure of the to turn the starter motor is usually the first indication of a .
A consists of a number of separate 2-volt cells. The cells are joined so that their individual voltages are added together a 6volt has three cells and a 12volt , six. Each cell has a voltage potential of only 2volts, whether it is as small as a torch or as big as a one-gallon container the size of each cell determines only how long it can provide given amount of current.
A cell consists of a number of lead plates which are alternately connected to its positive and negative terminals. They are insulated from each other by synthetic separators and immersed in a solution of sulphuric acid diluted with distilled or de-ionized water known as an electrolyte.
The number of lead plates in each cell varies, but a total is always an odd number because each cell starts and finishes with a negative plate. This is because the positive plates harder and are usually sandwiched between negative plates in order to realize their full potential. In most , the cells contain seven, nine or eleven plates, the more plates there are, the greater the capacity of the .
How the works
The negative plates are made from spongy lead material and the positive plates from lead dioxide. When an external circuit is connected to the , the acid in the electrolyte enters the plates, gradually turning them to lead sulfate, and this chemical action generates electricity. The can continue to discharge until all the plates have been converted to lead sulfate, at which time the is said to be ‘flat’.
If instead of taking electrical power from the , the is connected to a charger or charging system, and current is passed through it at a voltage slightly in excess of its normal output, the chemical process is reversed. The acid leaves the plates, making the electrolyte stronger, and the plates gradually revert to their original composition of lead dioxide and spongy lead. This process is known as recharging and can be continued until no further chemical change in the plates occurs.
“Although electrical energy can be produced from two plates, these have only a small surface area and very little storage capacity. Increasing the surface area by interspacing positive and negative plates gives a greater capacity”
“Electrical power to start the and operate the lights when the is not running is stored in a . Its capacity is measured in ampere-hours. A 48Ah should deliver a current of 1 amp for 48hours or 2amps for 24hours and so on. The biggest demand on the is when the is started. On a cold morning, the starter may need 300 to 400 amps in order to get the moving”
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