Definition
A nickel-cadmium (ni-cad) battery cell whose state of charge differs from the other cells in the battery, causing it to charge or discharge at a different rate. An unbalanced cell can overheat during charging because the other cells reach full charge first and force excess current through it, which in extreme cases leads to thermal runaway.
Plain English
One cell in a battery that is out of step with the others -- holding more or less charge than it should. Because the cells are wired together, the odd one out gets stressed during charging and can overheat.
Context Anchor
Seen during aircraft battery inspection, battery charging, and checks of how long the battery can supply power.
Derivation
Unbalanced means not evenly matched. Cell originally meant a small compartment; in a battery, it means one separate section that works with the other sections to make the battery’s total electrical output.
Why Pilots Care
An unbalanced cell can reduce battery capacity and lead to sudden electrical system problems in flight.
Analogy
It is like a team where one member is much weaker than the others; the whole team is limited by that weak member.
Intuition Check
Unbalanced does not mean the battery is physically tilted or uneven in weight. Here it means one battery cell is electrically different from the others.
Example Sentence 1
The maintenance crew removed the ni-cad battery from service after testing revealed an unbalanced cell that was overheating during charging.
Example Sentence 2
An unbalanced cell caused the battery to fail the load test, grounding the aircraft until it was serviced.