Definition
The condition of a storage battery that has been discharged to a level well below its normal operating range, often to the point where its terminal voltage drops sharply and the chemical reactions inside the cells begin to damage the plates. Repeated deep discharges shorten battery life and can leave a battery unable to hold a useful charge.
Plain English
A battery that has been run down much further than it should be — past the point where it can be safely recharged without harm.
Context Anchor
Seen in aircraft battery maintenance, electrical system troubleshooting, and charging instructions.
Derivation
‘Deep’ here means ‘far down’ — far below the normal discharge level. ‘Discharge’ comes from Old French descharger, ‘to unload.’ A deep discharge is the battery being unloaded of its energy far past the safe stopping point.
Why Pilots Care
Deep discharge shortens battery life and can cause sudden failure during engine start or emergency power use.
Analogy
Like draining a car fuel tank until it is empty instead of refilling at a quarter tank.
Intuition Check
Deep discharge does not mean a normal low battery. It means the battery has been drained below its safe lower limit.
Example Sentence 1
Leaving the master switch on overnight caused a deep discharge of the battery, and it would no longer turn the starter the next morning.
Example Sentence 2
Repeated deep discharge forced replacement of the aircraft battery ahead of its normal service interval.