Design and Function [2 of 4]
Evacuation hose (not sealed batteries)Certain batteries (such as genuine Volvo batteries) have an evacuation hose. This applies to car models where the battery is located in the cargo compartment (not in the engine compartment). The function of the evacuation hose is to lead any gases (oxyhydrogens) that build up in the battery during charging away from the cargo compartment out into the open air.
Caution! The evacuation hose must be connected at all times when the battery is connected. Always remember to connect the hose and ensure that the hose is routed downwards to the dedicated outlet in the bodywork when replacing the battery!
The construction of the cells
The construction of the cells
1. Positive plate in separator
2. Negative plate
3. Positive and negative plate assembly
4. Connection
5. Plate assembly for one cell.
A cells consists of several positive and negative plates. These plates are welded together in groups by plate straps. These plates function as electrodes in the cell. Each positive cell is in a type of isolator, known as a separator. The task of the separator is to separate the positive and negative plates and to catch any particles that have detached from the positive plate, thus preventing short-circuits. Each cell contains a combination of such plates connected in series. The separators consist of an acid resistant plastic.
For AGM-batteries, the pocket separator consists of a thin fibre-glass mat (AGM = Absorbed Glass Mat).
Each plate is made up of a grille with an external layer of active material. This active material contributes to the electrochemical process during charging and discharging. The grille is constructed of a type of a lead alloy which functions as a conductor for the active material and also carries the current. The positive plate has an external layer of lead dioxide, while the external layer on the negative plate consists of porous lead.
The cell is surrounded by electrolyte. This is the final component of a complete cell. Each cell can generate 2.12 V (full charge at 25°C (77°F)).
For AGM-batteries, each cell can generate 2.155 V.
Discharge
The process during discharge
1. Negative plate: Pure lead is converted to lead sulphate
2. Electrolyte: The sulphuric acid is converted to water
3. Positive plate: Lead oxide is converted to lead sulphate
4. Power consuming components.
During discharge, the lead in the negative plate is converted to lead sulphate (PbSO4).
The lead dioxide (PbO2) in the positive plate is also converted to lead sulphate. During the discharge process, sulphuric acid (H2SO4) is consumed while water (H2O) is created. This reduces the density of the electrolyte.
The density drops throughout the discharge process and can be gauged to determine the condition of the battery. The electrolyte in a fully charged battery has a density of 1.28 g/cm3. The density of the electrolyte in a fully discharged battery is 1.10 g/cm3.
Charging
The process during charging
1. Negative plate: Lead sulphate is converted to pure lead
2. Electrolyte: Water is converted to sulphuric acid
3. Positive plate: Lead sulphate is converted to lead oxide
4. The power supply from the generator or the external battery charger.
During charging, energy is supplied to the battery. This causes an electro-chemical process that is the reverse of the process during discharge. The lead sulphate (PbSO4) in the negative plate is converted back to pure porous lead (Pb) and the lead sulphate (PbSO4) in the positive plate is converted to lead dioxide (PbO2).
Water (H2O) is consumed during the charging process. Sulphuric acid (H2SO4) is formed. The density of the electrolyte increase as the amount of sulphuric acid increases.
Caution! For charging AGM-batteries, use only chargers that are both current and voltage-controlled. AGM-batteries are sensitive to overcharging and must be charged with an adapted charger. This since a battery that is charged with too high voltage/current does not absorb all the energy and the excess is converted to heat. When the battery becomes too warm the electrolyte evaporates (acid). When the pressure in the battery becomes too high, the gas is released through the battery box safety valve. When the water volume decreases the acid concentrates to an unacceptable high level, which may destroy the battery!
AGM-batteries may be charged with a max. voltage/current as follows.
* Max. current is calculated with the following formula (Battery capacity Ah/20)*5. For example, for a battery with capacity 70 Ah: (70/20)*5= 17.5 A.
** Charging time depends on how discharged the battery is, however, max. 24 h.
Gas build up
Gas build up
1. Gas build up at the plates
2. Negative plate
3. Electrolyte
4. Positive plate
5. The power supply from the generator or the external battery charger.
Gas builds up at the end of the charging process when charging a lead battery. When the battery has reached 85-90% of the maximum capacity, the water in the electrolyte begins to separate into oxygen (O2) and hydrogen (H2). Oxygen is formed at the positive plate and hydrogen at the negative plate.
Gas build up results in a loss of some of the gas from the battery, because the battery must not be fully sealed. Because the water is lost, the electrolyte level in the battery will drop. New distilled or deionized water must therefore be added to prevent damage to the plates as a result of the electrolyte level being too low. If new water is not added when necessary, the plates may come into contact with the air. This would result in corrosion, reducing the capacity of the battery.
For maintenance-free batteries as well as sealed batteries (AGM), normally no gases are released. This means that the battery water is not consumed in the electrolyte and topping up of battery water is not necessary. Also, the design of the battery box does not permit topping up of battery water.
Warning! If oxygen and hydrogen are mixed in the right proportions, oxyhydrogen is formed. This mixture is extremely explosive. Take great care to avoid personal injuries as well as damage to the battery.
Warning! Make sure that the battery charger is turned off before the terminals are disconnected. This to prevent sparking which may ignite the oxyhydrogen.
Note! Make sure that ventilation is good.