Definition
A type of sealed lead-acid battery cell in which the sulfuric acid electrolyte has been thickened into a gel by adding silica. The gel immobilizes the electrolyte so it cannot spill, allowing the battery to be operated and stored in any position without leaking.
Plain English
A battery cell where the acid inside has been turned into a thick jelly instead of being a free-flowing liquid. Because the acid can't slosh or spill, the battery is safer and can be mounted in almost any position.
Context Anchor
Seen in aircraft battery discussions, backup power equipment, and maintenance information for sealed batteries.
Derivation
From 'gel' (a thickened, jelly-like substance) and 'cell' (one of the individual electrochemical units that make up a battery). The name simply describes the physical state of the acid inside — it's been gelled rather than left as a liquid.
Why Pilots Care
Gelled-cell batteries are sealed, maintenance-free, and resistant to vibration and attitude changes, making them common in general-aviation aircraft.
Analogy
Think of the difference between water in a cup and gelatin in a cup. If the cup tips, the water runs out easily, but the gelatin tends to stay together.
Intuition Check
Do not read “gelled” as meaning the battery is soft or weak. It means the liquid inside the cell has been thickened so it stays in place.
Example Sentence 1
The emergency attitude indicator is powered by a small gelled cell battery mounted behind the instrument panel.
Example Sentence 2
Before engine start the pilot verified that the gelled cell battery showed a full charge on the voltmeter.