4.2 Lead-Acid & Nickel-Cadmium Batteries

4.2 Lead-Acid & Nickel-Cadmium Batteries

Quick Answer: Lead-acid aircraft batteries use PbO₂ / Pb plates with dilute sulphuric acid (H₂SO₄) and about 2 V per cell. Nickel–cadmium (Ni–Cd) batteries use nickel and cadmium electrodes with potassium hydroxide (KOH) electrolyte and about 1.2 V per cell. Care and safety follow the chemistry: acid versus alkali, gassing, temperature, and correct charging.

Section 4.1 established primary versus secondary cells. This section drills the two secondary chemistries that dominate classic Module 3 and aircraft maintenance teaching: lead-acid and nickel–cadmium. Exam stems love swapping electrolytes, plate materials, and nominal voltages—memorise the pairs, then understand why hangar procedures differ.

Lead-Acid Cells

Plates and electrolyte

In a charged lead-acid cell:

• The positive plate is primarily lead dioxide (PbO₂).
• The negative plate is primarily spongy lead (Pb).
• The electrolyte is dilute sulphuric acid (H₂SO₄) in water.

Separators keep plates from shorting. The container resists acid attack. On discharge, both plates tend toward lead sulphate (PbSO₄) and the electrolyte becomes more dilute (specific gravity falls). On charge, the process reverses: sulphate converts back toward PbO₂ and Pb, and specific gravity rises as acid is restored to the electrolyte.

You do not need every stoichiometric coefficient for Module 3, but you must recognise:

• Acid electrolyte → H₂SO₄, not KOH.
• Specific gravity of the electrolyte is a traditional state-of-charge indicator for flooded lead-acid cells (temperature-corrected).
• Nominal open-circuit voltage ≈ 2.0 V per cell (loaded and temperature conditions vary).

Aircraft lead-acid packs

A common 24 V nominal aircraft lead-acid battery uses 12 cells in series (12 × 2 V). Some light-aircraft systems use 12 V batteries (6 cells). Always count from chemistry: volts ≈ (cells in series) × (≈2 V).

Flooded cells may require periodic electrolyte level checks with approved water only—never add acid as a routine top-up unless a specific maintenance instruction says so after a spill or repair procedure. Valve-regulated / sealed designs reduce watering but still need correct charge profiles and temperature attention.

Lead-acid safety (exam + hangar)

PPE, neutralisation procedures per the maintenance manual, forced ventilation when charging flooded batteries, and insulated tools are not optional culture—they are how you keep chemistry inside the cell instead of on the hangar floor.

Nickel–Cadmium (Ni–Cd) Cells

Plates and electrolyte

In nickel–cadmium secondary cells:

• The positive electrode system is based on nickel compounds (commonly described as nickel oxyhydroxide / nickel oxide hydrate forms depending on state).
• The negative electrode system is based on cadmium.
• The electrolyte is aqueous potassium hydroxide (KOH)—an alkaline electrolyte.

Unlike lead-acid, the KOH electrolyte’s specific gravity does not provide a simple SOC hydrometer story comparable to sulphuric acid cells. State of charge is assessed by voltage behaviour under controlled charge/discharge, capacity checks, and maintenance data—not by “acid gravity.”

Nominal cell voltage ≈ 1.2 V. Therefore a 24 V nominal Ni–Cd aircraft battery typically uses about 20 cells in series (20 × 1.2 V = 24 V). That cell-count difference versus lead-acid is a classic Module 3 discriminator.

Why Ni–Cd appears on aircraft

Ni–Cd packs are valued for robust high-rate performance, wide temperature capability in many designs, and long service life when maintained correctly. They demand disciplined charging, ventilation or thermal monitoring as fitted, and strict control of cell balance and deep-discharge recovery procedures specified by the manufacturer.

Ni–Cd safety and care

Side-by-Side Chemistry Table

Charging Ideals (Module 3 Level)

Both chemistries need a controlled DC source that respects current limits, voltage limits, and temperature. Overcharge of flooded lead-acid increases gassing and water loss. Overcharge or excessive temperature on Ni–Cd risks cell damage and thermal problems. Aircraft systems use dedicated battery chargers / generator control / battery charge contactors as designed—never improvising a “shop car charger” onto an aircraft battery without approved procedure.

Worked voltage check:

• 12 lead-acid cells × 2.0 V ≈ 24 V nominal pack.
• 20 Ni–Cd cells × 1.2 V ≈ 24 V nominal pack.
• Same aircraft bus voltage class; different chemistry and cell count.

If a stem says “electrolyte KOH, about 1.2 V per cell,” answer Ni–Cd. If it says “H₂SO₄, about 2 V per cell, lead dioxide and lead,” answer lead-acid.

Aircraft Battery Care Themes

Regardless of chemistry, Module 3 expects you to recognise good practice:

1. Keep it clean and dry on the exterior—conductive contamination across terminals is a short waiting to happen.
2. Observe polarity on connection; reverse connection can destroy the battery and connected equipment.
3. Use the correct charger profile for the battery type and aircraft system.
4. Watch temperature—capacity and life both depend on it; never ignore overheat indications.
5. Respect venting and compartment integrity—gases and leaks are containment problems.
6. Follow the AMM / CMM for capacity checks, deep-cycle reconditioning (where approved), and reject criteria.

Lead-acid flooded maintenance historically includes specific-gravity sampling and watering with distilled/demineralised water as permitted. Ni–Cd maintenance emphasises deep-cycle capacity testing, electrolyte level where design allows, and cell replacement when imbalance or capacity fails limits. Do not cross-apply procedures blindly.

Other Alkaline Cells (Brief)

Module 3 may mention other alkaline-family cells for classification:

• Alkaline–manganese primary cells (common consumer cells) use alkaline electrolyte but are primary—not aircraft main batteries.
• Nickel–metal hydride (Ni–MH) secondary cells also use alkaline electrolyte ideas and roughly 1.2 V/cell, but aircraft type-specific approvals and procedures still rule; do not assume Ni–MH equals Ni–Cd in thermal or charge behaviour.
• Older nickel–iron cells are alkaline curiosities in textbooks more than line aircraft batteries.

Exam use: if the stem stresses KOH and rechargeable nickel chemistry, think alkaline secondary family; if it stresses disposable zinc alkaline consumer cells, think primary. The aircraft main-battery comparison pair remains lead-acid (acid) versus Ni–Cd (alkaline).

Exam Scenario Mindset

A typical CAAS trap swaps electrolytes: “Ni–Cd battery with sulphuric acid electrolyte”—false. Another trap assigns 2 V to Ni–Cd or 1.2 V to lead-acid—false. A third claims hydrometer specific gravity is the standard SOC tool for Ni–Cd exactly as for flooded lead-acid—false. Lock the table, then apply aircraft care language: acid burns and hydrogen for lead-acid; alkali burns, cadmium handling, and thermal discipline for Ni–Cd.