Characteristics of the Lead-Acid Battery

Mechanical Structure

The basic lead-acid battery has changed little in concept since the 1880's although improvements in materials and manufacturing methods continue to bring improvements in energy density, lifetime, and reliability.

A battery consists of a combination of electro-chemical primitive cells. Cells are the building blocks of which batteries are constructed. A primitive cell normally consists of the following principal components:

a positive electrode (anode) that receives electrons from the external circuit when the cell is discharged, a negative electrode (cathode) that donates electrons to the external circuit as the cell discharges, an electrolyte which provides a mechanism for charge to flow between positive and negative electrodes, and a separator or spacer which electrically isolates the positive and negative electrodes. In some designs, physical distance between the electrodes provides the electrical isolation and the separator is not needed. Since such a primitive cell will generate a voltage of about 2V, a battery will be assembled from a series connection of different primitive cells to obtain the required battery voltage. For commercial batteries, the components of the primitive cells and the cell connectors will be mounted and packed in a solid package for proper and save handling.

The mechanical structure of lead-acid batteries consist of flat pasted plates (the electrodes) immersed in a pool of a sulphuric acid and water solution (the electrolyte). The plates are packed together in a comb-like structure. Two such plate sets (a positive and a negative) from the basic cell block of the battery. Such a basic cell block thus consisting of a parallel connection of primitive cells. The plates itself consist of a metal grid (current collector) used for the electrical connection, pasted with lead oxide (PbO2 for the positive plates or lead (Pb) for the negative plates. This lead coverage is the chemical active layer of the cell. In order to increase this active surface, the lead and lead oxide coverage is given a porous structure.
Plate thickness (of the positive plate) is related to lifetime of the battery because of a factor called "positive grid corrosion". Over time, the positive plate gets gradually eaten away, so eventually the complete material is consumed. The corroded material is deposed as sediment at the bottom of the battery. Thicker plates are directly related to longer life. Staring batteries typically have plates about 1mm, while deep-cycle batteries may have plates of about 6mm thick.

A complete battery consists of a series connection of a number of basic cell blocks. A 12V battery will contain 6 basic cell blocks. The electrolyte is a sulphuric acid - water solution (typically 30% acid -70% water at full charge). During the lifetime of the battery, and especially during recharging some water may evaporate from the electrolyte. Therefore regular water addition is required for most forms of wet-cell batteries. Newer low-maintenance types come with excess electrolyte that is calculated to compensate for water loss during a normal lifetime.

Chemistry of the Lead-Acid Battery

The heart of the lead-acid battery is a reversible electro-chemical reaction, that can produce a voltage difference between the involved metal electrodes. The basic construction has been shown before; the lead-acid battery is made up of plates (electrodes): the negative electrode (cathode) is covered with lead (Pb) and the positive electrode (anode) is covered with lead oxide (PbO2). Both electrodes are immersed in a 35% sulphuric acid and 65% water solution (H2SO4 + H2O). In the electrolyte the sulphur-oxide crystals H2SO4 are split into free H+ and HSO4- ions.

Discharging

During the discharge process, the positive electrode receives electrons from the external circuit. These electrons will react with the active materials of the positive electrode in a "reduction" reactions that continue the flow of charge through the electrolyte to the negative electrode. In this reduction reaction, PbO2 of the active material will be converted to PbSO4 absorbing HSO4- and H+ from the electrolyte and electrons from the external load. As a by-product water is produced.
At the negative electrode, an "oxidation" reactions between the active materials of the negative electrode and the charge flowing through the electrolyte results in surplus electrons that can be donated to the external load closing the electrical loop. In this oxidation reaction, the Pb active material is oxidized to PbSO4 absorbing HSO4- from the electrolyte and giving H+ ions to the electrolyte while loading electrons to the negative electrode.

Remember that the system is electrically closed: for every electron generated in an oxidation reaction at the negative electrode, there is an electron absorbed in a reduction reaction at the positive electrode. At the same time H+ ions are generated in the electrolyte at the negative electrode and absorbed at the positive electrode. So for the external electron flow through the load, there is an internal opposite H+ flow through the electrolyte. The electrolyte is always a part of the electrical circuit!

As the discharge process continues, the electrolyte loses sulphur and the active materials become depleted absorbing this sulphur as as PbSO4. As the sulphur concentration is the electrolyte decreases, the chemical reactions slow down until finally, the battery is no longer capable of supplying electrons to the external load. At this point the electrolyte is primary water and the battery is discharged.
Both electrodes are now covered with PbSO4. This PbSO4 tends to crystallize, building a hard crystalline coating, which is hard to break down and this may destroy the electrodes. Therefore discharged batteries should be recharged as soon as possible to avoid this sulphation of the electrodes. If the battery is recharged, the sulphur is returned to the electrolyte and the electrodes are chemically converted back into lead oxide (anode) and lead (cathode).

Recharging

The chemical reactions in the lead-acid cell are reversible. By reversing the flow of electrons i.e. by putting electrical current in rather than taking it out, the chemical reactions are reversed to restore active material that had been depleted.

Chemical Reactions

Discharge		Charge
Positive Electrode Anode (+)	Negative Electrode Cathode (-)	Positive Electrode Anode (+)	Negative Electrode Cathode (-)
PbO2 + HSO4- + 3H+ + 2e- PbSO4 + 2H2O	Pb + HSO4- PbSO4 + H+ + 2e-	PbSO4 + 2H2O  PbO2 + HSO4- + 3H+ + 2e-	PbSO4 + H+ + 2e- Pb + HSO4-
Overall Cell Reaction		Overall Cell Reaction
Pb + PbO2 + 2H+ + 2HSO4- 2PbSO4 + 2H2O		2PbSO4 + 2H2O  Pb + PbO2 + 2H+ + 2HSO4-

Electrical Characteristics

A battery is an accumulator for electric energy: it can be "loaded" (charged) with energy when e.g. the engine is running (alternator) and can later release the energy previously stored in it's chemical contents. The efficiency of this charge-discharge cycle depends on the quality of he electrolyte-electrode system of the battery and on the efficiency of the charger system.

Two parameters are commonly used to describe battery performance: voltage and capacity. In simple terms, the voltage is the force driving each of the electrons coming out of a battery and the capacity is the number of electrons that can be obtained from a battery.

• The voltage of a battery cell is determined by the chemical materials used in it. The reduction and oxidation reactions mentioned above, each produce a fixed potential. The sum of the reduction and oxidation potentials determines the voltage of the cell.
  
  The discharge reaction at the positive electrode for a lead-acid cell has a "potential" of 1.685 Volt. The reaction at the negative electrode has a potential of 0.356 Volt. So the overall voltage of a lead-acid cell is 2.04 Volt. This value is known as the standard electrode potential. Other factors, such as the acid concentration can also effect the voltage of a lead-acid cell. The typical open circuit voltage of commercial lead-acid cells is around 2.15 Volt.
  
  The voltage of any battery cell is determined by the cell chemistry. Using other chemical materials different cell voltages may be obtained. Whereas lead-acid cells have about 2.0 Volt, Nickel-cadmium cells have a cell voltage of about 1.2 Volt, and the cell voltage of lithium cells may be as high as 4 Volt.
  Moreover, cells can be connected together in series (cascaded) so that their cell voltages can be accumulated. This way lead-acid batteries with nominal voltages of 2V, 4V, 6V, etc. can be assembled.

• While the voltage of a cell is fixed by its chemistry, cell capacity is variable depending on the quantity of active materials it contains. Capacity is given as the Ampere-Hour product. The most common definition of capacity is the Ampere-Hour product obtained if the battery is discharged at an ambient temperature of 25°C with a constant load current over a time span of 20 hours without the cell voltage dropping below 1.75V. The capacity value obtained in this way (C-20) is called "nominal capacity". The actual amount of energy that can be recovered from a battery substantially depends on the ambient temperature, on the load with which the battery is discharged and on the age of the battery.
  Individual cells may range in capacity from fractions of an ampere-hour to many thousands of ampere-hours. A typical capacity value for a starting battery is 75Ah.

Batteries are assembled connecting cells electrically together to obtain the required voltage and capacity. Connecting cells together in series will increase the battery voltage, whereas connecting cells (of equal voltage) in parallel will increase the capacity of the resulting battery. With a suited combination of parallel and serial cell connections almost any specific battery voltage and battery capacity can be obtained.

Cells used for battery assemblies should always be identical. Mixing cells of different chemistry or different size may be hazardous and should be avoided.

In operation the remaining capacity of a battery is commonly expressed in percentage of the nominal capacity and referred to as "State-of-Charge" (SoC). For a specific type of battery, the State-of-Charge, the electrolyte concentration and the cell Voltage are closely related. As the battery is discharged the concentration of sulphur in the electrolyte decreases, resulting in a decrease of specific weight. Some batteries allow access to the electrolyte for maintenance. For such batteries the specific weight (density) of the electrolyte can be measured.

The following table summarizes the relation between "State of Charge", Specific Weight of the electrolyte and Battery Voltage for a 12V lead-acid battery system:

Notice:
Sulphation of batteries starts when the specific weight of the electrolyte falls below 1.225 kg/l or when the voltage measures less than 12.5V. Sulphation hardens the battery plates, reducing and eventually destroying the ability of the battery to store electric energy.

Practically this means that if durability of a lead-acid battery system is an issue:

• only about 25% of the nominal capacity of the system should be used (up to 50% for deep-cycle battery types),

• whenever possible, the battery system should be kept in a fully charged state.

In a fully charged battery the electrolyte consists of about 35% sulphuric acid (H2SO4 + H2O) and 65% water. This mixture will freeze at about -60°C. In a discharged state, the sulphuric acid is largely absorbed in the battery plates and the electrolyte is basically water. In this state the electrolyte will freeze at about -20°C. If the electrolyte is frozen it looses conductivity resulting in a complete - and usually permanent - battery failure.

Energy Density

Although the active mass of a lead-acid battery typically has an energy density of about 160Wh/kg, the energy density of practical assemblies are only in the range of 25 to 45 Wh/kg. This is due to limitations in active mass utilization (40% to 60%) and the weight of electro-chemical and mechanical accessories needed to encapsulate the aggressive chemicals of the electrolyte and to make the battery practical and save to handle.

For 12V lead-acid batteries the energy density typically will be in the range of 3.5 Ah/kg. But keep in mind that these are nominal values and that in order to prevent systematic sulphation of the battery system this value will be only about 1 Ah/kg in practical applications.

Temperature Effects on Batteries

Battery capacity strongly depends on the battery temperature: capacity is reduced as temperature goes down and increases as temperature goes up. If the nominal rating is 100% at +25°C (room temperature), then the capacity is reduced to around 50% for a battery temperature of -25°C whereas at +50°C the capacity increases to about 120%.

In contrast to battery capacity, the battery lifespan increases with lower temperatures. At an average working temperature of -25°C battery capacity is reduced to 50% compared to a temperature of +25°C, but battery life will increase to about 160% of the lifespan at room temperature. This is providing the same cycle depth is used, because battery life is also directly related to how deep the battery is cycled. If a 50% cycle depth is taken as reference, reducing the cycle depth to 25% will double the lifespan, whereas increasing the cycle depth to 80% will half the battery's lifespan. But is does not make much sense using large battery systems to limit the cycle depth to less that 10%. This is because at very shallow cycles, the Lead-DiOxide tends to build up in clots on the positive plates rather than as a thin film, leading to local sulphation.

Also the battery charging voltages (absorption voltage and float voltage) change with temperature. For AGM batteries the absorption voltage will vary from about 2.74 volts per cell (16.4 V for a 12 V battery) at -40°C to 2.30 volts per cell (13.8V for a 12V battery) at +50°C. This is why a good battery charger will have a temperature compensation, with a temperature sensor thermally connected to the battery.

Finally, the self-discharge rate of lead-acid batteries strongly depends on temperature. This is especially an issue when a battery is stored or left at rest for a while. An increase of 15°C storage temperature will approximately double the self-discharge rate. Therefore, unused batteries should be kept dry and cool.