How Car Battery works and How to Repair

A good supply of electric power is necessary for modern vehicles. The engines require a large current to operate the starter motor and many other systems are electrically powered. All modern cars use a 12V system. The majority of vehicle batteries are of conventional design, using lead plates in a dilute sulphuric acid electrolyte . This feature leads to the common description of ‘lead-acid’batteries. The output from a lead-acid battery is direct current (d.c.).
A rechargeable battery is an electrochemical unit that converts an electric current into a modifi ed chemical compound. This chemical reaction can be reversed to release an electric current. The modifi ed chemical compound in the battery stores energy, which is available as electricity when connected to a circuit. A few batteries have open cells that require routine maintenance to the electrolyte level. This usually consists of adding distilled water at regular intervals. Most modern lead-acid battery designs have improved plate construction and case design, which with precise alternator charge control allows maintenance-free types to be used. A vehicle 12V battery is made up from six cells.
Each lead-acid cell has a nominal voltage of 2.1V, which gives a value of 12.6V for a fully charged battery under noload conditions. The six cells are connected in series, internally in the battery, with lead bars. The cells are formed in the battery case and are completely separate from each other. Each cell has a set of interleaved positive and negative plates kept apart by porous separators. The separators prevent contact of the plates, which would give an internal short-circuit and affect the chemical reaction in the battery cell.
The cell plates are supported above the bottom of the case. This leaves a sediment trap below the plates so that any loose material that falls to the bottom does not cause a short-circuit between the plates. The cell plates are formed in a lattice grid of lead–antimony or lead–calcium alloy . The grid carries the active material and acts as the electrical conductor. The active materials are lead peroxide for the positive plate and spongy lead for the negative plate. When a battery is in a charged state the positive plates of lead peroxide (PbO2) are reddish brown in colour, and the negative plates of spongy lead (Pb) are grey in colour. When the battery is discharging, a chemical reaction with the electrolyte changes both plates to lead sulphate (PbSO4) Applying an electric current to the battery reverses the process .
The charged battery stores chemical energy. This can be released as electrical energy when the battery is connected into a circuit. The electrolyte is dilute sulphuric acid, which reacts with the cell plate material during charging and discharging of the battery. Sulphuric acid (H2SO4) consists of hydrogen, sulphur and oxygen. These chemicals separate during the charge and discharge process and attach to the cell plate active material or return to the electrolyte. During discharge, the sulphate (SO4) combines with the lead to form lead sulphate (PbSO4). The oxygen in the positive plate is released to the electrolyte and combines with the hydrogen that is left, to form water (H2O).
During charging, the reverse process occurs with the sulphate (SO4), leaving the cell plates to reform with the hydrogen in the electrolyte to produce sulphuric acid (H2SO4). Oxygen in the electrolyte is released to reform with the positive cell plate material as lead peroxide (PbO2). Near the fully charged state some hydrogen (H2) and oxygen (O2) may be lost as gas from the battery vent . Some water (H2O) can also be lost by vaporization in hot weather.
With older batteries, this meant that the battery electrolyte needed regular inspection and topping up. Only water is lost from the battery and therefore only water should be used for topping up. Any contaminants will affect the chemical reactions in the battery and, therefore, the performance. Only distilled or specially produced toppingup water should be used.
Tap water is not suitable for topping up a battery. Acid should never be used, as this would strengthen the acid solution and alter the chemical reactions. Most modern batteries, however, include vapour traps and other features to minimize water loss and therefore need little or no attention. The electrolyte chemical composition changes with the state of charge.
It is possible to measure this change using a hydrometer. Sulphuric acid is denser and provides greater buoyancy than water. This property is called specific gravity or relative density. Water, which is used as the base for measurement of all liquids, is given a value of 1 for readings at 15°C (60°F). The dilute sulphuric acid of the electrolyte of a fully charged battery cell has a reading of 1.280. The reading for a half-charged battery cell is 1.200 and for a fully discharged battery is 1.150. A reading below 1.140 may indicate a cell that can no longer be recharged. It is common to write these values with three decimal places but to just say the signifi cant digits (e.g. twelve eighty).
A hydrometer consists of a calibrated fl oat in a glass cylinder . A bulb on the top of the cylinder is depressed so that it acts as a vacuum pump when it is released. A small rubber tube is attached to the bottom of the cylinder and is inserted into the electrolyte in the battery cell. A sample of the electrolyte can, therefore, be drawn into the cylinder. The sample of electrolyte in the hydrometer lifts the fl oat in proportion to the buoyancy of the liquid. The higher it fl oats, the greater the relative density.
Calibrated marks on the fl oat align with the top of the liquid to give the actual reading . This is compared with standard data and all cells are compared with each other to check the general condition of the battery. There should be very little difference between the cells. However, as most batteries are sealed the voltage of the battery can be used to indicate battery condition. A fully charged battery will give a reading of 12.6V and a discharged battery 12.0V. Measurement of battery condition by heavy-duty discharge should only be carried out on a fully charged battery. Heavy-duty discharge is carried out with a specialist item of equipment . The test consists of connecting a low-value resistor in circuit with the battery for a few seconds and measuring the voltage drop.
This test applies a load similar to that of the starter circuit. In general, the voltage should not fall below 10V. shows test equipment for this purpose; as always with this type of thing, refer to the manufacturer’s instructions before use. Battery performance can be determined by its ampere-hour (Ah) rating, but this is also affected by the ambient temperature. A cold battery will have a poor performance in comparison to a warm battery. A more useful measure of battery performance is its ‘reserve capacity’ rating. This refers to the time taken in minutes for a battery to discharge to a cell voltage of 1.75V, when supplying a constant current of 25A. This test refl ects a typical current draw for a vehicle that would be needed if the charging system failed during night driving.
The reserve capacity for a 40Ah battery will be approximately 60minutes. Another measure is the ‘cold cranking amps’ rating, which is a measure of the maximum current that can be supplied for a period of 30seconds before the battery voltage falls below 7.2V. This test is carried out at minus 18°C (0°F) so that it represents the most severe conditions of cold engine starting. The coldcranking amps (CCA) rating of a battery is an important measure in regions that suffer very cold winter temperatures. Some maintenance-free batteries incorporate a built-in hydrometer to indicate the state of charge and condition of the battery.
The hydrometer is colour coded . A green colour indicates that the battery is charged and serviceable. A green–black or black colour indicates that the battery requires recharging. A yellow colour indicates that the battery is faulty. Where a yellow hydrometer is showing, the battery should not be recharged or tested, and the use of jump leads for starting should not be carried out. A new battery should be fitted and the alternator checked for correct operation. Battery charging can be described as slow or fast.
Slow charging is best but in an emergency a fast charge is acceptable. When recharging, a battery should ideally be disconnected from the vehicle electrical systems. There are two types of battery charger: the bench charger, which has a current output of up to about 10A , and the fast charger, which can recharge a battery in about 30minutes, with a current of up to 50A .

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