LEAD ACID BATTERY

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Also termed as Dry charged or Flooded or wet battery is a secondary type of cell assembled with electrodes that are fully charged and dried. The electrolyte is added to the battery when it is placed in service, which means b

attery life begins when electrolyte is added.

Characteristics

• Electrolyte: Sulphuric Acid (30%) and water (70%), therefore it is dilute.

• Positive electrode: Lead Peroxide

• Negative electrode: spongy lead

• Specific Gravity checked by: Hydrometer

• Specific gravity (values might vary)

  • Charged: 2.2 – 1.275

  • Discharged: 1.8 – 1.1

An aircraft storage battery consists of 6 to 12 lead acid batteries connected in series.

Therefore, the OCV (Open Circuit Voltage) of the 6 cell battery is approximately 12 volts and that of a 12 cell battery is 24 volts.

Construction

Each positive plate is always positioned between two negative plates, there are always one or more negative plates than positive plates. Between the plates are porous separators that keep the positive and negative plates from touching each other and shorting out the cell. The separators have vertical ribs on the side facing the positive plate.

This construction permits the electrolyte to circulate freely around the plates.

In addition, it provides a path for sediment to settle to the bottom of the cell. The settling of the sediment is called stratification (mentioned below).

Each cell is seated in a hard rubber casing through the top of which are terminal posts and a hole into which is screwed a non-spill vent cap. The hole provides access for testing the strength of the electrolyte and adding water.

When the battery is on charge, the oxygen generated at the positive plates escapes from the cell. Concurrently, at the negative plates, hydrogen is generated from water and escapes from the cell. The overall result is a gassing of the cells and water loss.

The vent plug permits gases to escape from the cell with a minimum of leakage of electrolyte, regardless of the position the airplane might assume.

Therefore, flooded cells require periodic water replenishment.

Charge

• Process: Constant Voltage

• Charging voltage: at or below 14.4 voltage for 12 volt battery and 24.5 volt for 24 volt battery.

• Charge Rate: 50% – 90%

Caution: Always connect positive terminal before connecting negative terminal

In the fully charged state, the negative plate consists of lead, and the positive plate is lead dioxide. The electrolyte solution has a higher concentration of aqueous sulfuric acid, which stores most of the chemical energy.Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which bubbles out and is lost. The design of some types of lead–acid battery allows the electrolyte level to be inspected and topped up with pure water to replace any that has been lost this way.

Discharge

Discharge Rate: 50% – 90%

Caution: Always unplug negative terminal followed by positive terminal

In the discharged state both the positive and negative plates become lead sulfate and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water.

The discharge process is driven by the pronounced reduction in energy when 2 H+(aq) (hydrated protons) of the acid react with O2− ions of PbO2 to form the strong O-H bonds in H2O. This highly exergonic process also compensates for the energetically unfavorable formation of Pb2+(aq) ions or lead sulfate.

The release of two conducting electrons gives the lead electrode a negative charge. As electrons accumulate they create an electric field which attracts hydrogen ions and repels sulfate ions, leading to a double-layer near the surface. The hydrogen ions screen the charged electrode from the solution which limits further reaction unless charge is allowed to flow out of the electrode.

In practice, a lead–acid cell gives only 30–40 watt-hours per kilogram of battery, due to the mass of the water and other constituent parts.

Corrosion

Corrosion of the external metal parts of the lead–acid battery results from a chemical reaction of the battery terminals, plugs, and connectors.Corrosion on the positive terminal is caused by electrolysis, due to a mismatch of metal alloys used in the manufacture of the battery terminal and cable connector.

White corrosion is usually lead or zinc sulfate crystals. Aluminum connectors corrode to aluminum sulfate. Copper connectors produce blue and white corrosion crystals.

Corrosion of a battery’s terminals can be reduced by coating the terminals with petroleum jelly or a commercially available product made for the purpose. If the battery is overfilled with water and electrolyte, thermal expansion can force some of the liquid out of the battery vents onto the top of the battery.

This solution can then react with the lead and other metals in the battery connector and cause corrosion.The electrolyte can seep from the plastic-to-lead seal where the battery terminals penetrate the plastic case.

Acid fumes that vaporize through the vent caps, often caused by overcharging, and insufficient battery box ventilation can allow the sulfuric acid fumes to build up and react with the exposed metals.

Stratification

Sulfuric acid has a higher density than water, which causes the acid formed at the plates during charging to flow downward and collect at the bottom of the battery. Eventually the mixture will again reach uniform composition by diffusion, but this is a very slow process.

Repeated cycles of partial charging and discharging will increase stratification of the electrolyte, reducing the capacity and performance of the battery because the lack of acid on top limits plate activation.

The stratification also promotes corrosion on the upper half of the plates and sulfation at the bottom.Periodic overcharging creates gaseous reaction products at the plate, causing convection currents which mix the electrolyte and resolve the stratification. Mechanical stirring of the electrolyte would have the same effect. Batteries in moving vehicles are also subject to sloshing and splashing in the cells, as the vehicle accelerates, brakes, and turns.