3.3 Aircraft Lead-Acid & Nickel-Cadmium Storage Batteries

3.3 Aircraft Lead-Acid & Nickel-Cadmium Storage Batteries

Aircraft storage batteries serve three critical functions: providing emergency electrical power during in-flight generator failures, supplying high-rate starting current for auxiliary power units (APUs) and main engines, and dampening DC bus voltage transients. Aviation maintenance technicians service two distinct secondary chemical battery systems: Lead-Acid and Nickel-Cadmium (Ni-Cad).

1. Lead-Acid Battery Chemistry & Construction

An aircraft lead-acid battery comprises multiple galvanic cells connected in series within an impact-resistant polypropylene or hard rubber case. Each cell consists of interleaved positive and negative plate groups separated by porous insulating sheets.

Cell Chemistry & Materials

• Positive Plate Group: Active material is Lead Dioxide ($\text{PbO}_2$), distinctively dark chocolate brown.
• Negative Plate Group: Active material is pure porous Spongy Metallic Lead ($\text{Pb}$), slate gray.
• Electrolyte Solution: A dilute aqueous solution of Sulfuric Acid ($\text{H}_2\text{SO}_4$) and distilled water ($\text{H}_2\text{O}$), approximately $35%$ acid and $65%$ water by weight in a fully charged cell.

Lead-Acid Chemical Reaction:
  DISCHARGE:
  PbO2 (Pos) + Pb (Neg) + 2H2SO4 (Electrolyte) 
        ---> 2PbSO4 (Both Plates) + 2H2O (Electrolyte) + Electrical Energy

  CHARGE (Reverse):
  2PbSO4 (Both Plates) + 2H2O (Electrolyte) + Charging Energy 
        ---> PbO2 (Pos) + Pb (Neg) + 2H2SO4 (Electrolyte)

Electrochemical Dynamics

1. During Discharge: Both positive and negative plates react with sulfuric acid to form Lead Sulfate ($\text{PbSO}_4$). Sulfuric acid is consumed and water is created, diluting the electrolyte and lowering its specific gravity.
2. During Charge: Applied charging current strips sulfate ions from the plates, reforming $\text{PbO}_2$ on positive plates, spongy $\text{Pb}$ on negative plates, and returning sulfuric acid to the electrolyte, increasing its specific gravity.
3. Nominal Cell Voltage: An open-circuit lead-acid cell produces approximately $2.1\text{V}$ to $2.2\text{V}$. A 12-cell battery provides a nominal $24\text{V}$ rating (charged at $28\text{V}$ bus voltage); a 6-cell battery provides a nominal $12\text{V}$ rating (charged at $14\text{V}$).
4. Gassing: As full charge is reached, charging current electrolyzes water, liberating explosive Hydrogen gas ($\text{H}_2$) at negative plates and Oxygen gas ($\text{O}_2$) at positive plates. Battery sumps and overboard vent tubes exhaust these gases safely overboard.

Valve-Regulated Lead-Acid (VRLA) / Absorbed Glass Mat (AGM)

Modern aircraft frequently utilize Sealed VRLA / AGM batteries. The sulfuric acid electrolyte is immobilized within micro-fiberglass mat separators. AGM batteries are spill-proof in any flight attitude, require no fluid replenishment, and feature internal recombinant catalysts that recombine $99%$ of generated hydrogen and oxygen back into water.

2. Lead-Acid Specific Gravity Testing & Temperature Correction

Specific gravity (SG) is the ratio of the weight of a given volume of electrolyte to the weight of an equal volume of pure distilled water ($1.000$ at $4^\circ\text{C}$). Because pure sulfuric acid has a specific gravity of $1.835$, electrolyte specific gravity serves as a direct, reliable indicator of state of charge in lead-acid batteries.

Temperature Compensation Rule per AC 43.13-1B

Hydrometers are calibrated for an electrolyte reference temperature of $80^\circ\text{F}$ ($26.7^\circ\text{C}$). Liquids expand when heated (becoming less dense) and contract when cooled (becoming denser):

• High Temperature ($>80^\circ\text{F}$): Hydrometer float sinks deeper, giving a falsely low reading. ADD $0.004$ ($4\text{ gravity points}$) for every $10^\circ\text{F}$ ABOVE $80^\circ\text{F}$.
• Low Temperature ($<80^\circ\text{F}$): Hydrometer float floats higher, giving a falsely high reading. SUBTRACT $0.004$ ($4\text{ gravity points}$) for every $10^\circ\text{F}$ BELOW $80^\circ\text{F}$. $\text{Corrected SG} = \text{Observed SG} + \left(\frac{\text{Electrolyte Temp }(^\circ\text{F}) - 80}{10} \times 0.004\right)$

State-of-Charge Interpretation Table

Cell Variation Airworthiness Limit: If the temperature-corrected specific gravity between any two cells in the same lead-acid battery differs by more than $0.050$ ($50\text{ gravity points}$), an internal short or sulfated cell exists, and the battery must be removed from service and replaced.

3. Nickel-Cadmium (Ni-Cad) Battery Chemistry & Operation

Nickel-Cadmium (Ni-Cad) batteries deliver superior high-rate discharge current for turbine engine starting, maintain stable terminal voltage under heavy loads, and operate reliably across extreme temperature ranges ($-40^\circ\text{C}$ to $+60^\circ\text{C}$).

Construction and Electrochemistry

• Positive Plate Group: Sintered nickel porous plaque impregnated with Nickel Oxyhydroxide ($\text{NiOOH}$) in charged state / Nickel Hydroxide ($\text{Ni(OH)}_2$) in discharged state.
• Negative Plate Group: Sintered nickel porous plaque impregnated with metallic Cadmium ($\text{Cd}$) in charged state / Cadmium Hydroxide ($\text{Cd(OH)}_2$) in discharged state.
• Separators: Multi-layer barrier consisting of woven continuous nylon fabric for mechanical separation and cellophane / microporous polypropylene membrane to block cadmium dendritic migration.
• Electrolyte: A $30%$ by weight solution of Potassium Hydroxide ($\text{KOH}$) in distilled water, with a specific gravity of approximately $1.240$ to $1.300$.

Ni-Cad Chemical Reaction:
  DISCHARGE:
  2NiOOH (Pos) + Cd (Neg) + 2H2O (Water) 
        ---> 2Ni(OH)2 (Pos) + Cd(OH)2 (Neg) + Electrical Energy

  CHARGE (Reverse):
  2Ni(OH)2 (Pos) + Cd(OH)2 (Neg) + Charging Energy 
        ---> 2NiOOH (Pos) + Cd (Neg) + 2H2O (Water)

CRITICAL DIAGNOSTIC PRINCIPLE: Hydrometer vs. State of Charge

In a Ni-Cad cell, potassium hydroxide ($\text{KOH}$) does not chemically react with or enter into the active plate materials during charge or discharge; it functions strictly as an ionized electron transfer medium (hydroxyl $\text{OH}^-$ ion conductor).

• Consequently, the specific gravity of Ni-Cad electrolyte does NOT change significantly between full charge and complete discharge!
• A hydrometer CANNOT be used to evaluate the state of charge of a Ni-Cad battery!
• State of charge is determined exclusively by performing a controlled constant-current capacity discharge test on a calibrated battery test bench per manufacturer maintenance manuals (C-rate discharge down to an endpoint cutoff of $1.0\text{V}$ per cell; the battery must deliver $\ge 80%$ of rated ampere-hour capacity to remain airworthy per 14 CFR Part 43 / AC 43.13-1B).

Water Replenishment Protocol

Water is consumed only through electrolysis during overcharge.

• MANDATORY PROCEDURE: Distilled or demineralized water must ONLY be added at the end of a constant-current charge cycle, while the battery is still connected to the charger, fully charged, and actively gassing.
• Adding water to a discharged or partially charged Ni-Cad battery will cause violent electrolyte spewing and overflow during subsequent recharging, because electrolyte is drawn deep into the sintered plate pores during discharge and expelled into the cell headspace during charge.

4. Ni-Cad Thermal Runaway Hazard & Mitigation

The Mechanism of Thermal Runaway

Ni-Cad batteries exhibit a negative temperature coefficient of internal electrical resistance: as cell temperature rises, internal electrical resistance decreases.

1. When charged from an aircraft constant-voltage DC bus (e.g., $28\text{V}$ DC), an overheated cell (caused by heavy engine starting cycles or high ambient temperatures) experiences a reduction in internal resistance ($R_{int}$).
2. By Ohm's Law ($I = \frac{E_{bus} - E_{batt}}{R_{int}}$), lower resistance causes the battery to draw significantly higher charging current from the bus.
3. Increased current produces higher $I^2R$ internal heating, which further raises cell temperature.
4. Higher temperature further reduces resistance, establishing an escalating positive feedback loop: Thermal Runaway.
5. Thermal runaway results in electrolyte boiling, violent release of toxic caustic KOH vapor, separator meltdown, catastrophic fire, and cell explosion.

Thermal Runaway Escalation Loop:
  +--> High Cell Temperature ---> Decreased Internal Resistance (R) --+
  |                                                                   |
  +--- High I²R Internal Heat <--- Increased Bus Charging Current (I) -+

Protective Systems & Constant-Current Charging

• Temperature Monitoring: Internal thermistors and thermal switches mounted between center cells monitor temperature and illuminate cockpit annunciators ("BATTERY OVERHEAT" or "BATT TEMP > 140°F / 60°C").
• Automatic Disconnect: Dedicated battery chargers automatically disconnect the battery from the charging bus when cell temperature exceeds safe limits ($>140^\circ\text{F} / 60^\circ\text{C}$) or if charging current fails to taper.
• Constant-Current vs. Constant-Voltage Charging: Ni-Cad shop servicing is conducted exclusively with constant-current chargers, which hold charging current steady regardless of temperature-induced resistance changes, completely preventing thermal runaway.

Ni-Cad Deep-Discharge Cell Reconditioning

Repeated shallow cycling causes cadmium crystalline agglomeration, resulting in apparent capacity loss ("memory effect") and cell voltage imbalance. Technicians perform periodic deep discharge reconditioning:

1. Discharge battery at rated current down to $1.0\text{V}$ per cell.
2. Connect individual shorting clips (resistors or jumper wires) directly across each cell terminal for $12$ to $24\text{ hours}$, pulling cell voltages to exactly $0.0\text{V}$.
3. Equalize all cells to an identical discharged state, followed by a complete constant-current recharge.

5. Battery Shop Safety & Maintenance Segregation

Because lead-acid and Ni-Cad batteries utilize chemically incompatible active electrolytes, strict FAA facility segregation standards apply per AC 43.13-1B Chapter 11.

Chemical Contamination Hazards

• Sulfuric Acid (Lead-Acid): Highly acidic ($ ext{pH} < 1$). Acid fumes or residue in contact with a Ni-Cad battery will permanently neutralize the alkaline potassium hydroxide electrolyte and destroy porous nylon/cellophane separators.
• Potassium Hydroxide (Ni-Cad): Highly alkaline / caustic base ($ ext{pH} > 13$). Alkaline fumes in contact with lead-acid cells will neutralize sulfuric acid, forming potassium sulfate and destroying lead plates.

FAA Shop Maintenance Isolation Rules

1. Physical Room Separation: Lead-acid and Ni-Cad servicing must take place in completely separate, dedicated rooms equipped with dedicated, non-interconnected exhaust ventilation systems. If isolated rooms are unavailable, work areas must be physically separated by sealed partitions.
2. Tool and Equipment Isolation: Dedicated tools, hydrometers, syringes, torque wrenches, distilled water containers, and PPE must be color-coded and strictly segregated. Never use a lead-acid hydrometer on a Ni-Cad battery, or vice versa.

Neutralizing Agents and Cleaning

6. Worked Numerical Examples

Example 1: Hydrometer Temperature Correction (High Ambient Temperature)

Problem: During an annual inspection on a flightline in summer, a technician measures the electrolyte specific gravity of a lead-acid battery cell as $1.268$ with an electrolyte temperature of $105^\circ\text{F}$ ($40.6^\circ\text{C}$).

• Calculate the temperature-corrected specific gravity and evaluate the battery state of charge.

Solution:

1. Calculate the temperature difference above the $80^\circ\text{F}$ reference: $\Delta T = 105^\circ\text{F} - 80^\circ\text{F} = 25^\circ\text{F}$
2. Calculate the correction factor ($0.004$ per $10^\circ\text{F}$): $\text{Correction} = \frac{25^\circ\text{F}}{10^\circ\text{F}} \times 0.004 = 2.5 \times 0.004 = +0.010$
3. Add correction to observed reading: $\text{Corrected SG} = 1.268 + 0.010 = 1.278$
4. Evaluation: Corrected specific gravity of $1.278$ falls in the $1.275\text{--}1.300$ range, confirming the cell is $100%$ fully charged.

Example 2: Hydrometer Temperature Correction (Cold Ambient Temperature)

Problem: In an unheated hangar during winter, a lead-acid battery electrolyte hydrometer reads $1.285$ at an electrolyte temperature of $30^\circ\text{F}$ ($-1.1^\circ\text{C}$).

• Calculate the temperature-corrected specific gravity.

Solution:

1. Calculate temperature difference below $80^\circ\text{F}$: $\Delta T = 80^\circ\text{F} - 30^\circ\text{F} = 50^\circ\text{F}$
2. Calculate correction factor: $\text{Correction} = \frac{50^\circ\text{F}}{10^\circ\text{F}} \times 0.004 = 5.0 \times 0.004 = 0.020$
3. Subtract correction from observed reading: $\text{Corrected SG} = 1.285 - 0.020 = 1.265$
4. Evaluation: The cell is approximately $80%$ charged (not fully charged as the uncorrected $1.285$ reading suggested).

Example 3: Ni-Cad Battery Capacity Test Calculation

Problem: A $24\text{V}$, $40\text{ Ampere-hour (Ah)}$ rated aircraft Ni-Cad battery undergoes an annual shop capacity test at a constant $1\text{C}$ discharge rate ($40.0\text{ Amperes}$). The battery sustains current until the first cell drops to the $1.0\text{V}$ cutoff threshold at $51\text{ minutes}$ ($0.85\text{ hours}$).

• Calculate the delivered capacity and determine if the battery meets the FAA $80%$ airworthiness minimum requirement.

Solution:

1. Calculate delivered ampere-hour capacity: $\text{Delivered Capacity} = I \times t = 40.0\text{ A} \times \left(\frac{51}{60}\text{ h}\right) = 40.0\text{ A} \times 0.85\text{ h} = 34.0\text{ Ah}$
2. Calculate percentage of rated capacity delivered: $\% \text{ Capacity} = \left(\frac{\text{Delivered Ah}}{\text{Rated Ah}}\right) \times 100 = \left(\frac{34.0\text{ Ah}}{40.0\text{ Ah}}\right) \times 100 = 85.0\%$
3. Evaluation: Because $85.0% \ge 80.0%$, the battery satisfies the FAA airworthiness capacity threshold.