SkyDrill

When to add water to a nickel-cadmium battery

Aviation Mechanic General (AMG) practice question 39 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.R2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic is servicing a vented nickel-cadmium aircraft battery whose electrolyte level appears low. At what state of charge should water be added?

  1. AImmediately after the battery is discharged
  2. BOnly when the battery is fully charged
  3. CAt any time, as long as distilled water is used and the caps are removed
Show the answer and explanation

Correct answer: B. Only when the battery is fully charged

  • A — incorrect At the end of discharge the plates have absorbed electrolyte and the level is low; adding water then would cause overfilling when the level rises on recharge.
  • B — correct During discharge the plates absorb electrolyte and the level drops; on recharge the level rises and is highest at full charge, so water is added only when the battery is fully charged.
  • C — incorrect Because the electrolyte level changes with state of charge, the timing matters; water is added only at full charge.

BACKGROUND

In a nickel-cadmium cell the electrolyte is a potassium hydroxide solution and the typical open-circuit cell voltage is about 1.25 volts. Toward the end of the charging cycle, and during overcharge, the cells emit gas as water in the electrolyte decomposes, so some water is used and must be replaced. During discharge the plates absorb a quantity of the electrolyte; on recharge the level rises, and at full charge the electrolyte is at its highest level. For this reason water should be added only when the battery is fully charged. The manufacturer's service instructions should always be consulted.

STUDY TIP

Think of the electrolyte level as a state-of-charge gauge: judge and top it up only at full charge.

KEY POINTS

  • Ni-Cad electrolyte level falls on discharge and rises on charge.
  • Add water only when the battery is fully charged.
  • Gassing during charge consumes some water.

SOURCES

  • FAA-H-8083-30B ch12 "Operation of Nickel-Cadmium Cells"
Practice the Aviation Mechanic General (AMG) test →Timed 60-question exam, by ACS area, or review your misses. Free.
MORE ON FUNDAMENTALS OF ELECTRICITY AND ELECTRONICS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.