Part 2

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.

Self-discharge





Example of self-discharge (for open battery type) depending on battery temperature and discharge time
- A. Acid density in g/cm3
- B. Number of days that the battery was not under load
- C. Acid density at different battery temperatures.
There is always some self-discharge in a battery, when the battery is not in use and during both charging and discharging. If a battery is not used for a longer period, there is considerable self-discharge. The acid density falls and the active material in the plates is converted to lead sulphate. Excessive discharge must be avoided because otherwise there is an increased risk of sulfation. Sulfation may cause permanent damage to the battery. Regular charging of the battery will prevent sulfation. See Sulfation. There is an increased risk of damage from freezing in a heavily discharged battery. See Deep discharging.
The speed of discharge depends on the temperature, time, the condition and construction of the battery. The temperature is particularly influential. The rate of self-discharge is faster at higher temperatures. Batteries should be stored for prolonged periods in a dry, cold place, preferably below freezing.
Ensure that the battery is fully charged if it is to be left unused for a long period. No further charging will be required if the battery is in good condition and is stored in a dry cold place. If the battery is being stored in a warm place, it may require regular charging.
The illustration shows an example of how quickly a battery (of open type) can self-discharge, depending on the temperature of the battery. Note how the density of the acid reduces with time and how the self-discharge speeds up as the temperature increases. For an explanation of the density of the acid, see Acid density.

Acid density





Example of the variation in the stand-by voltage and in the density of the acid with the state of charge in a battery(of open type) at +25°C (77°F) (measured after approx. 2 hours charging or discharging)
- A. Stand-by voltage in V
- B. Acid density in g/cm3
- C. State of charge, SOC, in %
- D. Variation in the stand-by voltage with the state of charge
- E. Variation in the density of the acid with the state of charge.
The density of the acid is a unit showing the concentration of sulphuric acid in the electrolyte. The density of the acid is a measurement of the battery voltage and State of charge, SOC. The density of the acid is measured in g/cm3. Sulphuric acid is required for the chemical processes in the battery.
The higher the value of the acid (i.e. high concentration of sulphuric acid), the higher the voltage and state of charge. A low acid density value means a correspondingly low concentration of sulphuric acid, low voltage and a reduced capability for providing current. The electrolyte in a fully charged battery has a density of 1.28 g/cm3 at +25°C (+77°F). The density of the electrolyte in a fully discharged battery is 1.10 g/cm3 or lower depending on the type of battery.
The illustration shows how the stand-by voltage and the density of the acid drops as the state of charge of a battery reduces.

Hint: For maintenance-free as well as sealed batteries (AGM) the battery acid cannot be accessed and thus its density cannot be measured.

State of charge, SOC
The state of charge (SOC) is expressed as the amount of electrical energy that is stored in the battery at any given time, in relation to how much energy can be stored in a fully charged battery. The state of charge is listed as a percentage of full charge.