Battery: Description and Operation
PrincipleThe principle of the battery is based on a galvanic reversible process. The battery converts electric current into chemical energy which is stored and reconverted into electrical energy.
The function of a battery in a car is to store electricity which can be used later when the alternator does not charge e.g. for parking lights and starting the engine. The battery also acts as a buffer when variations in load occur.
Construction:
The battery is housed in a vessel made of plastic. It contains six separate water tight sections which are used for the cells. Small chambers in the bottom of the battery collect any fallen waste deposits. These waste deposits can cause short circuits between the plates. The electrolyte consists of a mixture of 36% sulphuric acid (H2SO4) and 64% water (H2O) or deionized water. The electrolyte is often called battery acid. It has a specific gravity of 1.28 when fully charged.
Inner construction:
The cells consist of positive and negative plates which are welded in groups to pole bridges. The plates are kept apart by separators which are used to avoid short circuits. Each cell contains such a combination of plates connected in series.
The positive and negative plates consist of lead grids which hold an active material (positive plate = lead dioxide, negative plate = spongy lead). The lead grid carries the current. Each cell has a rest voltage of 2.12 V (fully charged, at 20 °C).
Discharge:
During discharge the negative plate's lead (Pb) is converted into lead sulphate (PbSO4). The positive plate's lead dioxide (PbO2) is also converted into lead sulphate. Sulphuric acid is consumed during the discharging process and water (H2O) is formed. The strength of the electrolyte is thus reduced.
Charging:
On charging the cell the negative plate's lead sulphate (PbSO4) is converted into pure lead (Pb) and the positive plate's lead sulphate (PbSO4) into lead dioxide (PbO2). Water (H20) is consumed during the discharge process and sulphuric acid (H2SO4) is formed. The strength of the electrolyte is thus increased.
Gassing:
At the end of the charging process when the voltage of the battery has risen to 14.4 V (at 25 °C) the battery begins to bubble strongly. This is due to the fact that water dissociates and forms oxygen (02) at the positive plate and hydrogen (H2) at the negative plate.
Oxygen And Hydrogen Mix Are Extremely Explosive:
WARNING: If oxygen and hydrogen mix in the correct proportions oxyhydrogen is produced. Such mixtures are extremely explosive and great care should be taken since the risk of personal injury and damage is great. Bear this in mind and make sure that you switch off the battery charger before disconnecting the terminals.
Self discharge:
Care must be taken to avoid running down a battery too much because insoluble crystals of lead sulphate will be formed. The crystal formation reduces the capacity of the battery accordingly.
Batteries which are not used discharge by themselves; the rate of discharge depends on temperature, time and the condition of the battery.
If a battery is to be left unused for a long period of time make sure that it is fully charged and stored in a clean dry place, preferably below zero. If the battery is in good condition no further charging will then be necessary. However a battery stored in a warm place must be recharged every second month.
Capacity
The capacity of a battery is given in ampere hours (Ah) and usually applies to a battery discharged at 25 °C for 20 hours to a final voltage of (10.5 V).
The capacity of a battery is not constant and depends upon specific gravity, temperature, discharging current, the age of the battery and the charging time.
E.g.
A battery rated as having a capacity of 70 Ah would provide a discharge current of max. 3.5 A (3.5 x 20h =70 Ah) during 20 hours at 25 °C. A low discharge current i.e. a long discharge time, provides more Ah than if the discharge current is high for a short period of time.
Freezing:
Differences in capacity at +20 °C and -18 °C
Temperature +20 °C -18 °C
Capacity 100 % 55 %
70 % 35 %
40 % 25 %
Freezing point of battery in relation to specific gravity
State of charge Specific gravity Freezing point
Discharged 1.12 -10 °C
Half-charged 1.21 -27 °C
Fully-charged 1.28 -65 °C
The porous plates in a battery expand somewhat during discharge. If the discharge takes place during a long period of time, a greater area of the porous plates will be affected and the resulting expansion may cause the plates to misform or split.
Apart from this, large insoluble crystals of lead sulphate are formed at the negative plates. This reduces the effective surface of the plates and the capacity of the battery is reduced. If sulphation has just started then it may be possible to renew the plates by means of a long slow charge.
Low maintenance batteries:
Antimony is added to normal batteries to achieve a desired grid hardness. It does, however, cause to some extent gassing and thereby more water is used up.
In low maintenance batteries the amount of antimony used is reduced and the same hardness is achieved by "ageing" the grids. Water consumption is thereby reduced to approximately one third which means that the battery's electrolyte level need only be checked at normal service intervals i.e. 10.000-15.000 km.
Dry charged batteries:
The consistency of the plates in a dry charged battery is such that the battery can be used approx. 20 minutes after adding the battery acid and without the need to charge.
Dry charged batteries can be stored for long periods of time provided that they are kept dry and cool. If they are kept warm and damp the coating on the plates becomes neutralized with time and the battery must be charged, after addition of battery acid, using approx. 2/3 normal charge current.
Batteries for tropical climates
Batteries designed for countries with tropical climates are constructed in the same way as normal standard batteries. The density of the battery acid is however different. It is made weaker to slow down the rate of the chemical reaction and to reduce self discharge.
Comparison of densities (kg/dm3)
State of charge Standard battery Tropical battery
Fully charged 1.28 1.23
Half-charged 1.21 1.15
Discharged 1.12 1.08
The service life of a battery is usually 3-5 years. Certain factors do however have an adverse effect:
- too high or too low charging
- vibrations
- high temperatures
- heavy loads
- a low state of discharge