4.3 Series/Parallel Cells, Internal Resistance, Thermocouples & Photocells

4.3 Series/Parallel Cells, Internal Resistance, Thermocouples & Photocells

Quick Answer: Series cells add voltage; Ah stays the same. Parallel matched cells keep voltage; Ah adds. Loaded terminal voltage falls by I × r_internal. Thermocouples make a small EMF from heat at a dissimilar-metal junction; photocells respond to light. Neither replaces the aircraft battery for bulk energy.

Topic 3.5 closes by asking you to combine cells correctly, predict voltage sag from internal resistance, and recognise two important non-battery DC transducers: thermocouples and photocells. These ideas feed later DC-circuit work (topic 3.6) and appear directly on Module 3 MCQs.

Series Connection of Cells

Cells in series are connected positive-to-negative in a chain so the same current passes through every cell.

Rules (identical cells)

• Total EMF / voltage: E_total = E₁ + E₂ + … + Eₙ (voltages add).
• Capacity (Ah): remains approximately the Ah of one cell (they all carry the same current for the same time).
• Internal resistances: add: r_total = r₁ + r₂ + … + rₙ.

Worked example — series voltage and Ah

Four identical cells, each E = 1.5 V, capacity 2 Ah, are connected in series.

• E_total = 1.5 × 4 = 6.0 V
• Capacity = 2 Ah (not 8 Ah)

Aircraft link: twelve lead-acid cells of ~2 V in series → ~24 V battery; twenty Ni–Cd cells of ~1.2 V in series → ~24 V battery. Series is how we reach bus voltage.

Polarity warning

If one cell is inserted reverse in a series string, it subtracts its EMF and may be driven into damaging reverse charge when current flows. Series strings demand correct polarity and reasonably matched cells.

Parallel Connection of Cells

Cells in parallel have like terminals connected together: all positives joined, all negatives joined.

Rules (identical, equal-voltage cells)

• Voltage: remains approximately one cell’s voltage (E_total ≈ E_cell).
• Capacity (Ah): adds — Ah_total = Ah₁ + Ah₂ + …
• Current capability: increases because load current can split among branches.
• Internal resistances: combine in parallel (effective r falls if identical cells share load).

Worked example — parallel voltage and Ah

Three identical cells, each 12 V, 40 Ah, connected in parallel:

• Voltage ≈ 12 V
• Capacity ≈ 40 + 40 + 40 = 120 Ah

Worked example — series–parallel mix

You need 24 V and higher capacity than one series string. Build two series strings of lead-acid cells for 24 V, then parallel the strings:

• Each string: 12 × 2 V = 24 V, say 40 Ah
• Two strings in parallel: 24 V, 80 Ah

Series sets voltage; parallel sets Ah. Mixing unequal voltages in parallel causes circulating currents—Module 3 expects matched cells.

Comparison Table: Series vs Parallel

Internal Resistance and Terminal Voltage

Every real cell has internal resistance (r or r_int) caused by electrolyte, plates, separators, and connections. When current I is drawn, part of the EMF is dropped inside the cell.

V_terminal = E − I × r_internal (discharge, simple model)

Where:

• E = open-circuit EMF (ideal source voltage)
• I = load current
• r_internal = internal resistance
• V_terminal = voltage measured at the terminals under that load

Worked internal-resistance examples

1. E = 12.0 V, r = 0.05 Ω, I = 40 A (heavy load). Drop = 40 × 0.05 = 2.0 V. V_terminal = 12.0 − 2.0 = 10.0 V.

2. Same battery at I = 5 A: drop = 0.25 V; V_terminal = 11.75 V. Light loads sag less.

3. Aged battery: E = 12.0 V, r = 0.2 Ω, I = 40 A. Drop = 8.0 V; V_terminal = 4.0 V — may fail to crank even though open-circuit voltage looked “almost OK.”

Exam implications

• High cranking current makes internal drop obvious—hence voltage falls during start.
• Cold temperatures often raise effective internal resistance and reduce available capacity.
• On charge, a simple model becomes V_terminal = E + I × r (applied voltage must overcome EMF and internal drop).

Always ask: is the stem quoting open-circuit EMF or loaded terminal voltage?

Thermocouples

A thermocouple is a junction of two dissimilar metals (or metal alloys) that produces a small EMF when the junction temperature differs from the reference—Seebeck effect. It is a heat-to-electricity transducer, not a battery substitute.

Construction and materials

• Two wires of different thermoelectric materials joined at the hot (measuring) junction.
• The other ends form the cold (reference) junction connection to the measuring instrument.
• Common Type examples in industry (familiarisation): Type K (chromel–alumel), Type J (iron–constantan), Type T (copper–constantan). Aircraft engine temperature systems use approved material pairs and harnesses—not random workshop wire.

Operation

1. Heat the measuring junction (exhaust gas, cylinder head, turbine inlet probe, and so on).
2. A small DC millivolt-range EMF appears related to the temperature difference between junctions.
3. Instruments compensate for cold-junction temperature and scale millivolts to temperature readout.

Module 3 points: dissimilar metals + temperature difference → EMF; polarity depends on which metal is which; output is small (sensing), while aircraft power still comes from generators/batteries.

Aircraft framing

Thermocouples appear in EGT/TIT/CHT-style temperature measurement and similar monitoring. They generate a usable signal from heat; they do not energise a 28 V bus.

Photocells

A photocell (photoelectric cell) converts light into an electrical response. Depending on type, light may generate a voltage/current (photovoltaic behaviour) or change conduction (photoresistive / photoemissive families in older texts). Module 3 wants the principle: radiant energy → electrical effect.

Operation outline

• Photons strike a photosensitive material.
• Electrons are liberated or carrier behaviour changes, producing a current or voltage related to illumination.
• Circuits measure that signal for light detection, flame sensing, or control inputs.

Aircraft / maintenance framing

Photocells and related light sensors support indication and control tasks (for example light-operated switches or sensing applications in equipment). Solar photovoltaic panels can produce useful power in other industries, but on Module 3 photocells sit with thermocouples as energy-conversion principles and sensors—not as the primary DC source architecture of a transport aircraft electrical system.

Sensors vs Batteries: Keep the Categories Clean

Exam Scenario Drill

Scenario A: “Four 1.2 V, 10 Ah cells in series.” Answer: 4.8 V, 10 Ah.

Scenario B: “Four 1.2 V, 10 Ah cells in parallel.” Answer: 1.2 V, 40 Ah.

Scenario C: “E = 28 V, r = 0.1 Ω, I = 50 A.” Terminal voltage = 28 − 5 = 23 V.

Scenario D: “Device using two dissimilar metals generating EMF when heated.” → thermocouple.

Scenario E: “Device whose electrical output depends on incident light.” → photocell.

Master the arithmetic first—series/parallel mistakes are pure giveaway marks—then attach internal-resistance sag and the two sensor principles. That completes syllabus 3.5 DC Sources of Electricity for CAAS SAR-66 Module 3.