Part 2
Design and function (Continued)
Charging
The process during charging
1. Negative plate: Lead sulphate is converted to pure lead
2. Electrolyte: Water is converted to sulfuric 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. Sulfuric acid (H2SO4) is formed. The density of the electrolyte increase as the amount of sulfuric 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. Such mixtures are extremely explosive. Be extremely careful to avoid personal injuries and material damages.
Warning! Make sure that the battery charger is turned off before the terminals are disconnected. This is to prevent sparks, which may ignite the oxyhydrogen.
Note! Ensure good ventilation.
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 sulfuric 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. Sulfuric acid is required for the chemical processes in the battery.
The higher the value of the acid (i.e. high concentration of sulfuric acid), the higher the voltage and state of charge. A low acid density value means a correspondingly low concentration of sulfuric 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.
Capacity
The capacity of a fully charged battery is its ability to give a constant current during a certain time and is stated in the unit ampere-hours (Ah). The time for discharging varies depending on the battery's purpose.
In principle, the capacity is controlled by the plates' area as well as thickness. In a battery there is a big area on the plates, on which chemical reactions can take place, and also where a great amount of electrons (current) can be generated between the plates. This gives them a higher cold-start current value (CCA = Cold Cranking Amperes).
A start battery has big area but thin plates.
For a deep cycle battery, e.g., battery for leisure use, it is important that the chemical process can continue for a long time with lower current use, that is why the plates have to be thick. This gives them many ampere-hours or "spare minutes". Since the plates are thick, they need longer time to charge so that the chemical reaction also takes place deep inside the plates.
For start batteries, 20 hours' capacity (K20) is mentioned. This refers to how much current the battery can deliver during 20 hours in an ambient temperature of +25°C (+77°F) without the terminal voltage going below 10.5 V.
Example: A battery (of open type) with a stated capacity of 70 Ah should be able to provide a constant current of a max. 3.5 A (3.5 A x 20 h = 70 Ah) for 20 hours.
The capacity of a battery is not only dependent on the size and construction of the battery, but also varies considerably for batteries of the same type with the size of the discharge current. The lower the discharge current the higher the capacity and vice versa. The capacity of the battery is also affected by the temperature and the age of the battery. The nominal value is given at +25°C (+77°F). The capacity of the battery reduces considerably at low temperatures.
The table shows the difference in capacity in a battery at +25°C (+77°F) and -18°C (-0.4°F).
Battery service life
The service life of the battery depends primarily on its construction, maintenance and operating conditions. The following factors may shorten the service life of the battery:
- High temperatures
- Cycling
- Low electrolyte level
- Deep discharging
- Incorrect charging
- Sulfation
- Corrosion
- Vibrations
To maintain the best possible service life and capacity, the battery must be maintained and charged in accordance with Volvo's recommendations.
High temperatures
A high ambient temperature speeds up the chemical processes in the battery during charging and discharging. For every 10°C (18°F) increase in temperature, the reaction speed of the processes doubles. The risk of corrosion, self-discharge and sulfation increase at a high temperature and the service life of the battery is reduced. The service life of a battery improves in colder surroundings.
Cycling
Cycling means all the discharging and charging in a battery. A battery is always cycling. Discharging, or cycling, can have varying degrees of depth. Deeper discharge is more damaging than lesser discharge. Each discharge results in stress to the plates which deteriorate accordingly. Each instance of cycling cause the material in the plates to become more fragile and ultimately some of this material will separate from the plates. Cycling results in a reduction of capacity.
A lead battery cannot tolerate infinite cycling. Deep discharges should be avoided in order to maintain as long a service life as possible.
Low electrolyte level
The electrolyte level in an open battery (not maintenance-free battery and AGM-batteries) must be checked regularly. The water in the electrolyte is consumed by gas production and absorption. Batteries have different water consumption. This depends on design and ambient temperature. The water is used more quickly in hot climates.
If the electrolyte level is too low, the battery may suffer corrosion and the capacity of the battery may be reduced. Corrosion may occur in the connections between the cells. The consequence may be an open-circuit in a connection which will prevent the battery from supplying a current.
The capacity of the battery is reduced if the electrolyte level is so low that some of the surface of the plates are not submerged in electrolyte. Such surfaces cannot contribute to the chemical processes that occur during charging and discharging.
Caution! Note! Check the electrolyte level regularly and top up with distilled or deionized water to the indicated max. marking. Never use tap water!
Deep discharging
The acid density in a deeply discharged battery is very low (nearly all the sulfuric acid has been consumed and almost pure water remains. There is a high risk that the battery will be damaged beyond repair by freezing at relatively mild temperatures.
The table shows the freezing point of electrolyte in relation to the degree of charge of the battery.
A deeply discharged battery can also hydrogenate .
Hydrogenation
If the discharge is extremely deep, ultimately all the sulfuric acid will be consumed and only water will remain in the electrolyte.
Where lead sulphate is more soluble in water than in sulfuric acid, some of the lead sulphate in the plates will fall into the electrolyte. When the battery is charged, lead will fall on to the negative plates and separators. Lead gathers in small spots on the surface. This can cause short-circuits. This is known as hydrogenation.
Incorrect charging
Incorrect charging may result in permanent damage to the battery. Incorrect charging may be, for example, charging using a current/voltage that is so strong that the temperature of the electrolyte increases or that the gas development is too powerful.
Increased electrolyte temperature
If charging occurs with an extremely high current, the temperature of the electrolyte will increase considerably as the battery begins to reach full charge. Excessive temperature may damage the materials in the battery and increase the risk of short-circuits.
Intensive gas development
If the gas development during charging is extremely intensive, some of the particles may be forced loose from the active materials on the plates. The plates suffer wear, reducing the service life and capacity. Short-circuits may occur as released particles drop to the bottom of the cell container or cross to the opposite plate.
To ensure optimal performance, always charge batteries according to Volvo's instructions.
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!
Sulfation
Lead sulphate is formed on the plates during discharge. Normally small crystals are formed, which then revert to lead and lead oxide when the battery charges.
In certain circumstances during discharge, large insoluble crystals of lead sulphate may be formed. These crystals may form an insulating layer on the plates. This reduces the effective surface of the plates, reducing the contact between the active materials on the plates and the electrolyte. As a result, the capacity of the battery reduces considerably. This is called sulfation and is a result of a battery being left standing for a long period (in excess of two weeks) at a low charge, or because the battery has been under charged repeatedly.
The plates always expand slightly during discharge. If the discharge is very slow, the expansion may be so great that the plates deform or crack. Such damage is permanent and the battery cannot be used again. If a battery has undergone excessive sulfation, it may be possible to renovate the battery by charging the battery slowly using a very low current.
Regular maintenance charging will prevent sulfation.
Corrosion
Batteries may corrode in hot conditions, particularly in countries with hot climates. At high temperatures, the grille in the cell plates corrodes and becomes porous. The connections between the plates may also corrode. The result is a reduction in conductivity and therefore capacity.
Vibrations
The battery is subjected to vibrations if the car is driven on uneven surfaces. Such vibrations can place stress on the inner connections in the battery and cell units, resulting in wear and damage to the battery. This is a rare occurrence in modern cars however.