Construction of a traditional Automobile Lead Acid battery
The construction of the traditional Automobile Lead Acid battery comprises the number of cells needed to produce the required voltage for example three cells for a 6 volt battery or six cells for a 12 volt battery .The construction of a cell which is shown can be broken down into four main components, positive plates, negative plates, separators and electrolyte.
The Plates. These are cast lead grids packed with the active materials in paste form. They must be designed to provide secure fixing of the paste while presenting a large surface area to the electrolyte. They must also be strong enough to withstand the stresses set up by vibration and heavy discharge.
The paste. For the positive plates is a mixture of red lead (Pb3 O4 ) and dilute sulphuric acid (H2 SO4 ). Lead monoxide (PbO) and sulphuric acid (H2 S04) are used for the negative plates. The plates are then immersed in dilute sulphuric acid, when by electrolysis the positive plate becomes lead peroxide (Pb02 ) and the negative plate "spongy" lead (Pb). The plates are then made up into groups, the number per section being dependent on the size of the plates and the rating of the battery. The negative sections are made up with one more plate than the positive sections, and the outside plates of each cell in a battery are negative, the positive plates being inserted between them.
The Separators. In order-to keep the dimensions of automobile batteries to a minimum, there is limited clearance between positive and negative plates. Hence, to prevent contact, porous non-conducting separators are used. These use to be may be made of perforated ebonite, specially treated wood or porous rubber.
The electrolyte. The electrolyte is dilute sulphuric acid with a specific gravity normally falling within the range of 1.150 to 1.280 measured at a temperature of 15 C (59 F).
Specific Gravity The strength of the electrolyte is expressed in terms of its "Specific Gravity". The specific gravity of a solution may be defined as the density of the solution compared with the density of pure water, which is said to have a specific gravity of 1. As sulphuric acid has a higher density than water and we have said that the electrolyte
becomes increasingly diluted as the battery becomes discharged, it can be seen that the specific gravity of the electrolyte may be used as a measure of the state of charge of the
battery.
Typical figures for an automobile battery used in normal climates temperature below 27 C would be:
- Fully charged 1.270 to 1.290
- Half discharged 1.190 to 1.210
- Fully discharged 1.110 to 1.130
Specific gravity varies with temperature and values are normally quoted at 15° C. If readings are taken at any other temperature, a correction must be made. For every 5°
above 15° C, 0.0035 must be added to the observed reading. For every 5° below 15° C, 0.0035 must be subtracted from the readings e.g. At 30° C a correction of 0.0105 would have to be added to the reading.
The electrolyte used in vehicle batteries tends to have a higher specific gravity than that used in batteries for other purposes. This is mainly due to the limitation on available
space and the heavy discharge currents imposed by starter loads.
below shows the effect that variation of the electrolyte specific gravity has on the cell voltage and internal resistance. It is apparent from this, that the minimum resistance occurs at a specific gravity of approximately 1.220 and so this would appear to be the optimum operating point. However, at high discharge rates, very rapid localised dilution of the electrolyte occurs at the plate surfaces and to prevent the rate of dilution becoming greater than that of the diffusion of acid, a higher density is necessary.
