Sealed lead acid battery – Terminologies

by: Alexandra M.

For AGM-batteries, the pocket separator consists of a thin fibre-glass mat (AGM = Absorbed Glass Mat).

Active material
The active material consists of lead oxide in the positive plates and porous lead in the negative plates. The active materials react with the sulphuric acid in the electrolyte during charging and discharging according to the following chemical reaction:

PbO2 + Pb + 2 H2SO4 = 2 PbSO4 + 2 H2O

Ampere (A)
Unit for electrical current. Abbreviated to A.

Ampere hours (Ah)
Unit of measurement used to measure the ability of the battery to store electricity or the capacity. The storage capacity is obtained by multiplying the outflowing current in amperes by the discharge time in hours. Ampere hours are abbreviated to Ah.

A battery that supplies 3 amperes for 20 hours provides 3 A x 20 h = 60 Ah.

Battery acid
Another name for electrolyte. See Terminology:Electrolyte .

Battery water
Battery water is distilled or deionized water and must be used to top up the battery if the electrolyte level is low. Regular tap water must not be used because it contains substances which may damage the battery.
For maintenance-free batteries as well as sealed batteries (AGM) it is not possible to fill battery water.

Abbreviation of Cold Cranking Amperes. The Terminology:Cold cranking amperes of a battery is expressed using the CCA value.

Electro-chemical current producing unit in a battery. Consists of a package with positive and negative plates, separators and electrolyte enclosed in a shell. A fully charged lead battery with a stand-by voltage of 12.72 V has six cells.

In a battery, a cycle consists of a discharge phase and a charge phase.

An industry norm. Abbreviation of Deutsche Industrie-Norm.

Electrical current
A current is a flow of electrodes moving in a cable. It can be compared with a stream of water. Electrical current is measured in amperes (A).

An electrode is the place where the chemical reactions during discharge and charge take place. A cell consists of at least one positive and one negative electrode. During discharge, lead oxide is converted to lead sulphate at the positive electrode. At the negative electrode, porous lead is converted to lead sulphate. During charging, the reactions are in the opposite direction; lead dioxide and porous lead are reformed. Electrodes also transport current. In a battery, the electrodes are normally called plates.

In a lead battery, electrolyte consists of concentrated sulphuric acid and distilled water. Electrolyte is a liquid that conducts current and adds hydrogen and sulphate ions to the electro-chemical reaction during charging and discharging:

PbO2 + Pb + 2 H2SO4 = 2 PbSO4 + 2 H2O

A type of plastic insulator enclosing each positive plate in the cell. The roll of the separator is to separate the positive and negative plates from each other and to catch particles that detach from the positive plate to avoid short-circuits.
For AGM-batteries, the pocket separator consists of a thin fibre-glass mat (AGM = Absorbed Glass Mat).

A lead alloy frame construction on each plate in the battery. The grid functions as a carrier of the active materials, lead oxide and porous lead, which contribute to the electro-chemical processes during charging and discharging. The grid also conducts the current in the cell.

The capacity is the ability of a fully charged battery to provide a constant current over a certain time and is given in ampere-hours (Ah). The time taken to discharge depends on the intended use of the battery. For starter batteries one refers to 20 hours capacity (K20). This refers to the amount of current the battery can provide over 20 hours at an ambient temperature of +25°C (+77°F) without the pole voltage falling below 10.5 V.

Example: A battery with a listed capacity of 70 Ah should be able to provide a current of a maximum 3.5 A (3.5A x 20h = 70 Ah) for 20 hours.

A destructive chemical reaction when metal is broken down in an aggressive chemical environment. Sulphuric acid, for example, is aggressive against iron. The iron breaks down and corrodes. The battery terminal is vulnerable to corrosion if it is not maintained correctly.

A short-circuit is when the current takes a short cut where the resistance is less and therefore does not go via the intended route. A short-circuit can be caused for example by particles detaching from the plates in the cells. Eventually these deposits can be so large that a conductive connection is made between the two plates so that the current can flow between the two plates instead. A short-circuit in a cell can fully discharge a battery and render the battery useless.

Cold cranking amperes
A measurement of the ability of the battery to start the car. The cold cranking amperage is expressed as a CCA value. Volvo batteries are usually marked with a CCA value according to the SAE norm. The cold cranking amperes according to the SAE norm is defined as the current that a fully charged battery can supply at a temperature of -17.8 °C (0 F) for 30 seconds without the voltage dropping below 7.2 V. The higher the cold cranking amperes value, the better the starting capacity of the battery. In certain markets the batteries are marked according to the DIN norm.

The process by which a battery is supplied with energy by a charger or the generator (GEN) in the car. During charging, the lead sulphate in the plates is converted back to pure lead on the negative plates and to lead oxide on the positive plate. Water is consumed, sulphuric acid is formed and the density of the acid increases.

This reaction can be written:
2 PbSO4 + 2 H2O → PbO2 + Pb + 2 H2SO4

State of charge, SOC
The state of charge 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. This is the “State of Charge”, SOC.

Direct current, DC
An electrical current that only travels in one direction in an electrical cable. A battery supplies direct current during discharge and must be recharged (from the generator (GEN) or an external battery charger) with direct current in the opposite direction to current during discharge.

The plates in a battery function as electrodes. Each cells consists of several positive and negative plates. These plates are welded together in groups by plate straps. The plate is made up of a grid with an external layer of active material. This active material contributes to the electrochemical process during charging and discharging. The grid is constructed of a type of a lead alloy which functions as a conductor of 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.

Plug (not maintenance-free as well as AGM-batteries)
On top of the battery is a cover with six plugs, one for each cell. The plugs can be opened when checking and topping up the electrolyte.

Reserve capacity, RC
The reserve capacity, RC, is measured in minutes and is, according to the SAE norm, the time taken to discharge a fully charged battery at 25 A to a voltage of 10.5 V at a temperature of +25°C (+77°F). The reserve capacity is the length of time the battery is able to supply necessary components with current when the generator (GEN) in the car is not functioning. Volvo batteries are marked with the RC value according to the SAE norm.

An industry norm. SAE is an abbreviation of “Society of Automotive Engineers”.

A battery that is not in use will discharge itself over time. This is know as self-discharge and is quicker in hot environments. The battery must be maintained by recharging at regular intervals to avoid excessive discharge.

Sulfation is where, in some circumstances, large insoluble lead sulphate crystals are formed on the plates as the battery discharges. This reduces the capacity of the battery. Note that the formation of small soluble lead sulphate crystals is normal during discharge.

The plates always expand 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 must be discarded. This is an excessive form of sulfation. The longer the sulfation process continues, the more difficult it is to save the battery.
Sulfation is a result of a battery being left standing for a long period at a low charge, or the battery has been under charged repeatedly. Regular charging of the battery will prevent sulfation.

Acid density
The unit showing the amount of sulphuric acid in the electrolyte and is a measurement of the battery voltage and charge status. The density of the acid is measured in g/cm3. The higher the value of the acid density (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. The density of the electrolyte in a fully discharged battery is 1.10 g/cm3.

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

The process where a battery is under load and gives off current. During discharge, the lead on the negative plate and the lead oxide on the positive plate are converted to lead sulphate. Sulphuric acid is consumed and water is formed, reducing the density of the acid.

This reaction can be written:
PbO2 + Pb + 2 H2SO4 → 2 PbSO 4 + 2 H2O

Stand-by voltage
The stand-by voltage is the voltage measured from an unloaded battery after approximately 2 hours discharging or charging. It is important that the battery is left unloaded for a longer period so that the concentration of sulphuric acid has time to distribute evenly in the electrolyte. The measured voltage is then a good indication of the charge status of the battery.

Stand by current consumption
The stand by current is the current used by the car when the key is removed (such as the current for the clock, anti-theft alarm and remote control locking).

Volt (V)
Unit for electrical power. Abbreviated to V.

Surface charging
The concentration of sulphuric acid is higher at the plates than in the electrolyte in a battery that has recently been charged. This is because the sulphuric acid formed at the plates during charging has not had time to spread into the rest of the electrolyte. If the stand-by voltage of the battery is measured directly after charging, a higher value will be obtained, giving an incorrect reading of the charge status of the battery. This is called surface charging. If the battery is left without load for a while the concentration of the sulphuric acid will even out. It is therefore important to leave the battery without load for at least 2 hours before measuring the stand-by voltage of the battery, allowing time for the concentration of the sulphuric acid to even out in the electrolyte. The measured voltage is then a good indication of the charge status of the battery.