2.1 Lead-Acid and Nickel-Cadmium Batteries

Chemical Operation of Primary and Secondary Cells

In aviation maintenance engineering, batteries are not merely accessories; they are safety-critical components that provide emergency power for flight instruments, radio communications, and engine starting. Understanding the fundamental chemistry, construction, and operating principles of these DC sources is a core requirement for EASA Part-66 Module 3.

Primary vs. Secondary Cells

Chemical cells are broadly classified into two categories based on the reversibility of their internal chemical reactions:

1. Primary Cells: In these cells, the chemical reaction that produces electrical energy is irreversible. Once the active materials on the electrodes are consumed, the cell is discharged and cannot be recharged electrically. Examples include zinc-carbon, alkaline, and primary lithium batteries. In aircraft, primary cells are typically limited to low-power, non-critical systems, such as emergency locator transmitters (ELTs), clocks, and backup power for memory circuits.
2. Secondary Cells: The chemical reactions in secondary cells are electrically reversible. By passing an external charging current through the cell in the direction opposite to the discharge current, the active chemical materials are restored to their original state. These are commonly known as storage batteries or accumulators. The two primary secondary cell chemistries utilized in aviation are lead-acid and nickel-cadmium (Ni-Cd).

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Lead-Acid Batteries

Lead-acid batteries are widely used in general aviation and some commercial aircraft. They are reliable, have low internal resistance, and can supply high currents for engine starting.

Construction

An aircraft lead-acid battery is typically assembled in a rugged, acid-resistant case, divided into individual cells. Each cell contains a set of positive and negative plates arranged alternately and kept apart by porous separators (usually made of microporous plastic or fiberglass).

• Positive Plate (Anode during discharge): Consists of a grid made of a lead-alloy (such as lead-calcium or lead-antimony) pasted with lead dioxide ($PbO_2$), which has a chocolate-brown color.
• Negative Plate (Cathode during discharge): Consists of a similar lead grid pasted with spongy metallic lead ($Pb$), which is slate gray.
• Electrolyte: A solution of sulfuric acid ($H_2SO_4$) and distilled water ($H_2O$).

Chemical Operation (Charge and Discharge)

The chemical operation of a lead-acid cell is governed by the double-sulfate reaction.

Discharge Reaction

When a load is connected across the terminals, spongy lead from the negative plate and lead dioxide from the positive plate react with the sulfuric acid. This reaction produces lead sulfate ($PbSO_4$) on both plates and releases water ($H_2O$) into the electrolyte:

$PbO_2 + Pb + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$

• Plate Changes: Both plates are converted to lead sulfate ($PbSO_4$), which is a whitish substance. This is known as sulfation.
• Electrolyte Changes: Sulfuric acid is consumed and water is produced. Consequently, the concentration of acid decreases, lowering the density (specific gravity) of the electrolyte.
• Voltage: The nominal voltage of a lead-acid cell is approximately $2.0\text{ V}$ to $2.1\text{ V}$ under no load.

Charge Reaction

When an external DC charging source is applied, current is forced into the positive terminal. This reverses the chemical reaction, converting the lead sulfate back into lead dioxide on the positive plate and spongy lead on the negative plate, while regenerating sulfuric acid:

$2PbSO_4 + 2H_2O \rightarrow PbO_2 + Pb + 2H_2SO_4$

• Plate Changes: The white lead sulfate is cleared from both plates, restoring them to chocolate-brown lead dioxide and slate-gray spongy lead.
• Electrolyte Changes: Sulfuric acid is regenerated, increasing the specific gravity of the electrolyte.

Specific Gravity and State of Charge

The state of charge of a lead-acid battery is directly proportional to the specific gravity of its electrolyte. Specific gravity is measured using a hydrometer, which measures the relative density of the liquid compared to pure water ($1.000$).

• Fully Charged: Specific gravity is typically between $1.275$ and $1.300$ at $80^\circ\text{F}$ ($26.7^\circ\text{C}$).
• Fully Discharged: Specific gravity drops to approximately $1.150$ or lower.

Temperature Correction

Because the density of liquids changes with temperature, hydrometer readings must be corrected to a standard temperature of $80^\circ\text{F}$ ($26.7^\circ\text{C}$). The standard correction factor is:

• Add $0.004$ to the hydrometer reading for every $10^\circ\text{F}$ ($5.56^\circ\text{C}$) above $80^\circ\text{F}$.
• Subtract $0.004$ from the hydrometer reading for every $10^\circ\text{F}$ ($5.56^\circ\text{C}$) below $80^\circ\text{F}$.

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Nickel-Cadmium (Ni-Cd) Batteries

Nickel-cadmium batteries are favored in large commercial and military aircraft because they maintain a relatively constant terminal voltage during discharge, perform well under extreme cold temperatures, and have a longer service life than lead-acid batteries.

Construction

A Ni-Cd battery cell consists of positive plates containing nickel oxyhydroxide ($NiOOH$) and negative plates containing metallic cadmium ($Cd$).

• Plates: Plates are usually constructed by sintering carbonyl nickel powder onto a nickel-plated steel grid, creating a highly porous structure. The active materials are then chemically deposited into the pores.
• Separators: The plates are separated by a multilayer barrier of nylon and cellophane to prevent physical contact while allowing ion transfer.
• Electrolyte: An aqueous solution of potassium hydroxide ($KOH$) and distilled water (typically around $30%$ $KOH$ by weight). A small amount of lithium hydroxide ($LiOH$) is often added to improve capacity and cell life.

Chemical Operation

The simplified chemical reaction for discharge and charge is:

$2NiOOH + Cd + 2H_2O \rightleftharpoons 2Ni(OH)_2 + Cd(OH)_2$

Key Electrolyte Characteristic

Unlike lead-acid batteries, the potassium hydroxide ($KOH$) electrolyte in a Ni-Cd battery does not enter into the chemical reaction to change its chemical composition. It acts purely as an ionic conductor.

• Specific Gravity: The specific gravity of the $KOH$ electrolyte remains virtually constant (around $1.24$ to $1.30$) during both charge and discharge.
• Testing Charge State: Because specific gravity does not change, a hydrometer cannot be used to determine the state of charge of a Ni-Cd battery. Instead, the state of charge is determined by performing a measured discharge capacity test in a battery shop or by checking individual cell voltage under a heavy load.
• Voltage: The nominal voltage of a Ni-Cd cell is $1.2\text{ V}$ (compared to $2.0\text{ V}$ for lead-acid).

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Safety and Hazards

Working with aircraft batteries presents significant safety hazards. Technicians must understand these risks and the corresponding prevention methods.

1. Gassing (Explosion Hazard)

During the final stages of charging, both lead-acid and Ni-Cd batteries undergo electrolysis of water, releasing hydrogen ($H_2$) at the negative plates and oxygen ($O_2$) at the positive plates.

• Risk: Hydrogen and oxygen form a highly explosive mixture. Even a small spark (e.g., from disconnecting a live charger or loose terminal connection) can cause a violent explosion.
• Mitigation: Charging rooms must be well-ventilated. Chargers must be turned off before connecting or disconnecting batteries.

2. Corrosive Electrolyte Burns

• Lead-Acid: Sulfuric acid is a strong acid that can cause severe skin burns and destroy clothing.
  • Neutralizing Agent: Sodium bicarbonate (baking soda) solution mixed with water.
• Ni-Cd: Potassium hydroxide is a strong alkaline (base) substance, which is highly corrosive to skin and eyes.
  • Neutralizing Agent: Boric acid solution, vinegar, or citric acid.

3. Cross-Contamination

Lead-acid and Ni-Cd chemistries are chemically incompatible.

• Risk: Acid fumes or residue from a lead-acid battery will neutralize the alkaline electrolyte of a Ni-Cd cell, ruining it. Conversely, alkaline fumes will destroy a lead-acid battery.
• Mitigation: EASA regulations require that lead-acid and Ni-Cd batteries be serviced in separate, dedicated rooms with isolated ventilation systems, tools, and test equipment (including hydrometers and clothing). Never mix tools or servicing equipment between the two types of batteries.

4. Thermal Runaway (Ni-Cd Specific)

Thermal runaway is a dangerous phenomenon that can occur in Ni-Cd batteries during constant-voltage charging. Under constant-voltage conditions, if a cell's temperature rises, its internal resistance drops. This resistance drop causes the charging current to rise ($I = V/R$), generating more Joule heating ($P = I^2 R$), which further raises the temperature. This feedback loop escalates rapidly until the electrolyte boils and the cell is destroyed. Modern aircraft use constant-current chargers, temperature monitoring sensors, and thermal cutouts to prevent this hazard.

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Worked Exam Scenarios: Specific Gravity Temperature Correction

Scenario: A technician uses a hydrometer to measure the specific gravity of a lead-acid cell and obtains a reading of $1.270$. The temperature of the electrolyte is measured to be $100^\circ\text{F}$. What is the temperature-corrected specific gravity of the cell, and what is its state of charge?

Step-by-step Solution:

1. Identify the deviation from the standard temperature ($80^\circ\text{F}$): $\text{Deviation} = 100^\circ\text{F} - 80^\circ\text{F} = +20^\circ\text{F}$
2. Calculate the correction factor. For every $10^\circ\text{F}$ above $80^\circ\text{F}$, we must add $0.004$ to the reading: $\text{Correction} = \left(\frac{20}{10}\right) \times 0.004 = +0.008$
3. Apply the correction to the observed hydrometer reading: $\text{Corrected Specific Gravity} = 1.270 + 0.008 = 1.278$
4. Interpret the result: A corrected specific gravity of $1.278$ indicates the battery is fully charged. Without this correction, the reading would have falsely suggested the cell was slightly undercharged.

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Worked Exam Scenarios: Cell Count for Nominal Voltage

Scenario: An aircraft system requires a nominal DC bus voltage of $24\text{ V}$. How many cells must be connected in series if a lead-acid battery is used, compared to a nickel-cadmium battery?

Step-by-step Solution:

1. For a lead-acid battery, each cell has a nominal voltage of $2.0\text{ V}$: $\text{Number of cells} = \frac{24\text{ V}}{2.0\text{ V/cell}} = 12\text{ cells}$
2. For a nickel-cadmium battery, each cell has a nominal voltage of $1.2\text{ V}$: $\text{Number of cells} = \frac{24\text{ V}}{1.2\text{ V/cell}} = 20\text{ cells}$ (In practice, aircraft Ni-Cd batteries are typically composed of 19 or 20 cells in series to achieve the nominal 24V system voltage, allowing for terminal voltage variations under load).