Design and Function [4 of 4]

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 maximum marking. Never use tap water!

Deep discharging
The acid density in a deeply discharged battery is very low (nearly all the sulphuric 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 sulphuric acid will be consumed and only water will remain in the electrolyte.
Where lead sulphate is more soluble in water than in sulphuric 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.