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MECHAN ICS Batteries 2 By Preceptor
Page 46
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In the first article in this series, CM, May 20, I described a.simple battery consisting of two plates immersed in an acid solution. The capacity of a battery depends on the surface area of its plates and to provide the capacity required for a motor vehicle one of these plates would have to be made inconveniently large. So instead a number of plates are provided.
Each cell of the battery has a number of positive and negative plates sandwiched together, so that each positive plate surface is faced by a negative one. This form of construction means there is always one more negative plate than positive. The actual number of plates used in batteries in general use, varies from seven to twenty-five depending on the capacity.
Battery capacity is quoted in ampere/hours. Simplified, this means the number of amps a battery will deliver and the number of hours for which it will continue to do so. A 90 amp/ hour battery, for example, will give 2 amps for 45 hours or 5 amps for 18 hours.
In practice a slight complication occurs because the capacity depends to some extent on the rate at which the battery is discharged. At very low discharge rates there is practically no change in the capacity but when _ the rate of discharge is high the capacity is reduced.
For example the 90 amp / hour battery would not give a discharge of 90 amps for 1 hour as might be expected. For this reason battery capacity is always coupled with the discharge rate as, for instance, 90 amp/hours at the 10 hour rate which means that when the battery is fully charged it will deliver 90 amp/hours if discharged in 10 hours or more.
This same capacity must not be expected if it is discharged in a less time. Generally speaking the lower the discharge rate the larger the capacity. The specification published by a leading battery manufacturer quotes a battery capacity as 90 amp/hours at the 10 hour rate and 103 amp/hours at the 20 hour rate.
There must be no direct con tact between the negative and positive plates inside a battery cell. To avoid this a sheet of some non-conducting porous material is interposed between each negative and positive plate .surface. Special types of wood, pvc, rubber or glass fibre are used for this purpose.
The assembled plates and separators are fitted into a hard rubber or plastic monoblock container with three or six compartments for 6 volt or 12 volt batteries respectively. Each cell gives a little over 2 volts when fully charged. The plates usually rest on ribs formed in the bottom of the container which leaves a space between the bottom of the plates and the casing. This prevents sediment which falls from the plates from short circuiting the plates.
The top of the plate assembly is enclosed by a moulded plastic cover sealed to the main case. Screwed stoppers or some other form of removable top is provided so that the cells can be filled originally with the electrolyte and topped up with distilled water as the need arises. Each cell must be vented to the atmosphere to prevent build up of pressure when the battery is "gassing".
The cells are connected in series; that is the negative terminal of one cell is litiked by means of a solid lead connector to the negative terminal of the adjacent cell. in the older type batteries these connecting bridges were exposed but in most modern batteries these connecting pieces are completely covered so that the top of the battery is smooth and flat which facilitates cleaning and removal of moisture. It also improves the battery's appearance. The drawing shows how the cells are connected on a 6V and 12 volt battery.
More about batteries in the next article.
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