Battery Types Volvo XC70 I рестайлинг

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 fiberglass 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 fiberglass 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 86

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 sulfuric acid (H 2 SO 4 ) which is a mixture of concentrated sulfuric acid and distilled or agonized 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 87

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 88

The Process During Discharge

1. Negative plate: Pure lead is converted to lead sulfate
2. Electrolyte: The sulfuric acid is converted to water
3. Positive plate: Lead oxide is converted to lead sulfate
4. Power consuming components.

During discharge, the lead in the negative plate is converted to lead sulfate (PbSO 4 ).

The lead dioxide (PbO 2 ) in the positive plate is also converted to lead sulfate. During the discharge process, sulfuric 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 89

The process during charging

1. Negative plate: Lead sulfate is converted to pure lead
2. Electrolyte: Water is converted to sulfuric acid
3. Positive plate: Lead sulfate 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 sulfate (PbSO 4 ) in the negative plate is converted back to pure porous lead (Pb) and the lead sulfate (PbSO 4 ) in the positive plate is converted to lead dioxide (PbO 2 ).

Water (H 2 O) is consumed during the charging process. Sulfuric acid (H 2 SO 4 ) is formed. The density of the electrolyte increase as the amount of sulfuric 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 90

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 agonized 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 91

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 sulfate. 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 92

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 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/cm 3 . 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/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.