Battery - Design and Function (Convertible): Other Volvo C70 I

A battery works as follows

The battery consists of several cells. Schematically, a battery can be said to be made up of two electrodes, one positive and one negative. These electrodes are different materials, which are submerged in a container filled with electrolyte, often called battery acid. As a result of the different electrode materials, there is an electrical charge between the positive and negative electrodes. There is a chemical reaction between the electrodes and the electrolyte causing electrical energy to be generated. If the positive and the negative poles of the electrodes are connected to form a closed circuit, a current flows through the circuit.

In a lead accumulator, the positive electrode consists of lead oxide and the negative electrode of porous lead. The electrolyte consists of diluted sulphuric acid (H 2 SO 4 ) which is a mixture of concentrated sulphuric acid and distilled or deionized water.

Battery types

In principle, lead batteries can be divided into two different categories

1. Open batteries
2. Sealed batteries

Open batteries

In principle, open batteries have free-hanging plates in the battery box, most often insulated from each other by the negative plate being located in a small separator bag of polyethylene. The electrolyte is highly viscous and can flow freely between the plates.

A maintenance-free battery is normally an open battery where the design has reduced the gas and also has made the battery box so airtight that any gases stay in place. A lot of acid means that the fluid will last for its service life without topping up.

Sealed batteries (recombination batteries)

For sealed batteries the battery box is designed as a pressure vessel with relief valves and go under the name VRLA (Valve Regulated Lead Acid), that is, valve-controlled lead-acid batteries.

If you shut in the acid and hydrogen gas then these can react with each other and be reformed to water. This is called recombination and is a good way to almost completely eliminate fluid losses. It is not possible to shut in and recycle everything, but it can be reduced significantly. The sealed batteries can be divided into two groups

1. Gel batteries
2. AGM-batteries

Gel batteries

A gel battery has a few substances, often silicone compounds, added in the electrolyte so that it gels. Thus there is no free-floating acid that can run out of the battery.

Gel batteries have good capacity, but due to the slightly higher resistance in the low viscosity acid, they may be limited as start batteries. They can handle deep cycle discharges very well, that is, when you slowly discharge the batteries a lot several times.

Gel batteries are very rugged and are often used, e.g., in cleaning machines and golf carts.

The small volume of electrolyte in the battery and that it is low viscosity makes the battery sensitive to dehydration that an overcharge with resulting gas generation could entail.

AGM-batteries

In AGM-batteries the electrolyte is kept in place by the separator paper, consisting of a fibreglass mat (AGM = Absorbed Glass Mat), working as a "sponge". The capillary forces in the separator means that electrolyte ends up in the right place. The batteries can be built with very thin separators, which keeps down the internal resistance. This means that you can get high power from a small volume, making it perfect as start battery.

The fibreglass mat is quite thin, which makes for a small electrolyte volume in the battery. This makes the battery sensitive to dehydration that an overcharge with resulting gas generation could entail.

Properties (a few typical properties)

Scheme 19

Battery

1. Handle (on certain battery models)
2. Plugs (not maintenance-free as well as AGM-batteries)
3. Positive and negative posts.

The battery is in a plastic container and has six internal chambers, one for each cell. These chambers are not connected to each other. This means that the electrolyte level can drop in one chamber without affecting the level in the other chambers. The cells are connected in series by a sealed joint between each chamber.

On the top of some "open" batteries, there is a cover with six plugs, one for each cell. The plugs can be opened to check electrolyte level and for topping up with battery water. Maintenance-free batteries as well as AGM-batteries do not have these plugs and therefore cannot be topped up with battery water.

The battery is filled with electrolyte. The electrolyte consists of diluted sulphuric acid (H 2 SO 4 ) which is a mixture of concentrated sulphuric acid and distilled or deionized water. Electrolyte is often referred to as battery acid. Electrolyte has a density of 1.28 g/cm 3 when the battery is fully charged.

A battery is built up of two or more cells connected in series depending on which terminal voltage is desired. An open car battery consists of six cells and the terminal voltage of a fully charged battery is 12.72 V, that is 2.12 V per cell. For AGM-batteries, the max. terminal voltage of a fully charged battery is 12.93 V.

The AGM-batteries' container is provided with a safety valve. The purpose is to protect the battery against too high pressure inside the battery. Too high pressure can occur in case of incorrect charging, e.g., with too high voltage.

Note. The battery is damaged if the safety valve has been activated.

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.

Scheme 20

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.

Scheme 21

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 (PbSO 4 ).

The lead dioxide (PbO 2 ) in the positive plate is also converted to lead sulphate. During the discharge process, sulphuric acid (H 2 SO 4 ) is consumed while water (H 2 O) 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/cm 3 . The density of the electrolyte in a fully discharged battery is 1.10 g/cm 3 .

Scheme 22

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 (PbSO 4 ) in the negative plate is converted back to pure porous lead (Pb) and the lead sulphate (PbSO 4 ) in the positive plate is converted to lead dioxide (PbO 2 ).

Water (H 2 O) is consumed during the charging process. Sulphuric acid (H 2 SO 4 ) is formed. The density of the electrolyte increase as the amount of sulphuric acid increases.

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.

Scheme 23

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 (O 2 ) and hydrogen (H 2 ). 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.

Note. Make sure that ventilation is good.

Scheme 24

Example of self-discharge (for open battery type) depending on battery temperature and discharge time

1. A. Acid density in g/cm 3
2. B. Number of days that the battery was not under load
3. 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 .

Scheme 25

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)

1. A. Stand-by voltage in V
2. B. Acid density in g/cm 3
3. C. State of charge, SOC, in %
4. D. Variation in the stand-by voltage with the state of charge
5. 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/cm 3 . 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/cm 3 at +25°C (+77°F). The density of the electrolyte in a fully discharged battery is 1.10 g/cm 3 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.

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.

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.

The freezing point of the battery in relation to the degree of charge

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.

Scheme 26

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.

AGM-battery

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

See: Separator

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

PbO 2 + Pb + 2 H 2 SO 4 = 2 PbSO 4 + 2 H 2 O

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.

Example

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

Battery acid

Another name for electrolyte. See 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.

CCA

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

Cell

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.

Cycle

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

DIN

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).

Electrode

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.

Electrolyte

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

PbO 2 + Pb + 2 H 2 SO 4 = 2 PbSO 4 + 2 H 2 O

Separator

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).

Grid

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.

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.

Short-circuit

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.

Charging

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 PbSO 4 + 2 H 2 O --> PbO 2 + Pb + 2 H 2 SO 4

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.

Plate

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.

SAE

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

See Separator .

Self-discharge

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

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/cm 3 . 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/cm 3 . The density of the electrolyte in a fully discharged battery is 1.10 g/cm 3 .

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

Discharge

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

PbO 2 + Pb + 2 H 2 SO 4 --> 2 PbSO 4 + 2 H 2 O

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.

Volvo Battery Analyser 3100

A tool used to test and troubleshoot batteries of open type.

Note. Not intended for AGM-batteries.

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.