Lead Acid vs Nickel Cadmium Battery

Both Lead Acid and Nickel Cadmium (Ni-Cd) batteries are the most common types of battery used on an aircraft. Both of them are secondary batteries, that means they can be charged and discharged several time or the chemical reaction can be reversed.

They differ in the following way

Positive Plate (charged state)

Lead Acid : Lead Peroxide Ni-Cd : Nickel oxyhydroxide/Nickel oxides

Negative Plate (charged state)

Lead Acid : Pure Lead Ni-Cd : Cadmium

Positive Plate (discharged state)

Lead Acid : Lead Sulphate Ni-Cd : Nickel hydroxide

Negative Plate (discharged state)

Lead Acid : Lead Sulphate Ni-Cd : Cadmium hydroxide

Electrolyte

Lead Acid : Sulphuric Acid Ni-Cd : Potassium Hydroxide

Note:

Sulphuric acid solution consist of 30% sulphuric acid and 70 % distilled water by volume

Potassium Hydroxide solution consist of 30% potassium hydroxide and 70 % distilled water by weight.

Electrolyte Specific Gravity (SG)

Lead Acid : 1.25 to 1.27 Ni-Cd : 1.24 to 1.32

Note:

The SG value changes with the state of the Lead Acid battery it falls to about 1.10 for a fully discharged battery. The Lead acid battery charge or discharge state can be stated by measuring the SG of the electrolyte.

The SG value does not change with the state of the Ni-Cd battery, it remains constant. Hence, battery charge or discharge state of Ni-Cd can't be stated by measuring the SG of the electrolyte.

Open Circuit Voltage (OCV)

Lead Acid : 2.1 V Ni-Cd : 1.28 V

Closed Circuit Voltage (CCV)

Lead Acid : 2.0 V Ni-Cd : 1.20 V

For further reading about Aircraft Batteries

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