4.1 Primary & Secondary Cells

4.1 Primary & Secondary Cells

Quick Answer: A primary cell is a one-shot chemical energy converter that is not designed to be recharged. A secondary cell stores energy chemically and can be recharged by forcing current through it in reverse. Cell EMF is set by the electrode–electrolyte chemistry; capacity (Ah) is set mainly by how much active material is available and how the cell is used.

CAAS SAR-66 Module 3 topic 3.5 DC Sources of Electricity starts with the electrochemical cell—the fundamental building block of aircraft batteries and many portable DC supplies. Generators and alternators produce bulk power in flight, but batteries still provide engine start, APU start, emergency DC bus support, and ground power when generators are offline. If you confuse primary with secondary, or mix EMF with capacity, later battery-care and series/parallel questions become guesswork.

What a Cell Is

An electrochemical cell converts chemical energy into electrical energy (and, for secondary cells, can convert electrical energy back into chemical energy during charge). A battery is an assembly of two or more cells connected to obtain a required voltage and capacity. In hangar language people often say “battery” for a single multi-cell unit; on the exam, keep the cell-versus-battery distinction clear when the stem uses both words.

Essential parts of a cell

During discharge, chemical reactions at the electrodes create a potential difference. Electrons leave the negative terminal, travel through the external load, and return at the positive terminal. Inside the cell, ions move through the electrolyte to keep charge balance. Outside: electron current. Inside: ionic current. Both are required for continuous current.

Primary Cells

A primary cell is manufactured with chemical reactants in a charged state. As you draw current, those reactants are consumed. When the useful chemistry is exhausted, the cell is discarded (or recycled environmentally)—it is not designed for routine recharging. Attempting to recharge many primary types can cause leakage, rupture, or fire.

Typical primary examples (familiarisation)

• Zinc–carbon and alkaline–manganese dry cells (common AA/AAA shop cells).
• Some specialised lithium primary cells used in emergency equipment where long shelf life matters.

Primary cells matter on Module 3 mainly as the contrasting category: non-rechargeable chemical sources. Aircraft main batteries are secondary systems, but primary cells still appear in emergency beacons, some portable instruments, and ground-support tooling.

Construction notes for dry primary cells

A classic dry cell places a zinc can (negative electrode) around a paste electrolyte and a central carbon/manganese-dioxide positive electrode structure. The “dry” label means the electrolyte is immobilised as a paste, not that moisture is absent. The case provides mechanical strength and often forms one electrode.

Secondary Cells

A secondary cell (storage cell / accumulator) is designed so that passing a charging current reverses the discharge reactions and restores active materials. You can charge and discharge many times within the manufacturer’s cycle life, provided voltage, current, temperature, and electrolyte limits are respected.

Why aircraft use secondary cells

Aircraft electrical systems need a reusable DC reservoir that can:

1. Deliver high cranking or start current for short periods.
2. Accept recharge from engine-driven generators, APU generators, or ground power.
3. Support essential buses if generation is lost until procedures restore power or the aircraft is secured.

That mission profile matches lead-acid and nickel–cadmium (Ni–Cd) secondary batteries (covered in Section 4.2), not disposable primary packs.

Construction shared ideas

Secondary cells still need two electrodes of different materials, an electrolyte matched to the chemistry, separators, and a case. Compared with small dry primaries, aircraft secondary cells emphasise:

• Large plate area for high current.
• Controlled venting or recombination design for gas management.
• Temperature and electrolyte monitoring provisions on many installations.
• Rugged mounting for vibration and attitude changes.

Basic Chemical Action (Discharge View)

You do not need full redox equations for every commercial chemistry on Module 3, but you must understand the pattern:

1. At the negative electrode, a material is oxidised and releases electrons to the external circuit.
2. At the positive electrode, a material is reduced by accepting electrons from the external circuit.
3. The electrolyte transports ions so the electrodes do not rapidly polarise to a useless state.
4. The open-circuit EMF is characteristic of the electrode couple and electrolyte, not of how big the plates are.

Key exam separation:

• Chemistry → sets volts per cell (EMF class).
• Quantity of active material / usable plate area / design limits → sets ampere-hour capacity and current ability.

Doubling plate area does not double EMF. It roughly increases the charge the cell can deliver and usually improves the ability to supply high current with less voltage sag—capacity and internal-resistance behaviour, not a new open-circuit voltage chemistry.

Capacity: Ampere-Hours (Ah)

Capacity expresses how much charge a cell or battery can deliver under stated conditions. The common aviation and Module 3 unit is the ampere-hour (Ah).

If a battery delivers a steady I amperes for t hours until it reaches a defined end voltage,

Capacity ≈ I × t (ampere-hours)

Worked capacity examples

1. A cell supplies 5 A for 4 hours to its end point. Capacity used ≈ 5 × 4 = 20 Ah.
2. A 40 Ah battery is discharged at 10 A. Idealised endurance ≈ 40 / 10 = 4 hours (real life is shorter at high rates—see Peukert-type behaviour in advanced texts; Module 3 wants the basic I × t idea).
3. A battery delivers 2 A for 30 minutes. Time = 0.5 h, so Ah drawn ≈ 2 × 0.5 = 1 Ah.

Conditions that change usable capacity

Nameplate Ah is therefore a rated figure under a specified discharge regime, not a promise under every hangar condition.

State of Charge vs Capacity Rating

Do not confuse:

• Rated capacity (Ah): design/label quantity under stated test conditions.
• State of charge (SOC): how fully charged the battery is right now (often expressed as a percentage).
• Terminal voltage under load: affected by SOC, temperature, and internal resistance (Section 4.3).

A 40 Ah battery at 50% SOC is not a 20 Ah “different battery chemistry”—it is the same battery half full. Its open-circuit EMF may still be near nominal while loaded terminal voltage sags more when internal resistance is high or temperature is low.

Primary vs Secondary Comparison Table

Aircraft Maintenance Framing

On a CAAS Module 3 paper, a stem might ask: “Which statement describes a secondary cell?” Correct direction: chemical action is reversible by charging. Another stem might ask whether increasing plate area raises EMF—answer no; it primarily increases capacity / current capability. A third might give current and time and ask for Ah—multiply, watching hours versus minutes.

Keep three habits:

1. Classify the cell as primary or secondary from the rechargeability clue.
2. Separate EMF (chemistry) from Ah (active material and use conditions).
3. Remember discharge needs both external electron flow and internal ion flow.

Those habits prepare you for lead-acid versus Ni–Cd details, series/parallel Ah–voltage arithmetic, and internal-resistance voltage drop in the next sections.