Definition
The opposition to the flow of electric current that exists within a source of electrical energy itself, such as a battery or generator. It is caused by the resistance of the source's internal components and electrolyte, and it causes the source's terminal voltage to drop as more current is drawn from it.
Plain English
Every battery or generator has some resistance built into its own insides. When you draw current from it, a small amount of voltage is lost inside the source before the electricity ever reaches the wires. The bigger the load, the more voltage you lose inside.
Context Anchor
Seen in aircraft electrical system discussions, especially when checking battery condition, starter performance, voltage drop, or electrical troubleshooting.
Derivation
From Latin internus, meaning 'inward,' and resistere, 'to stand against.' The term simply describes resistance that lives inside the source itself, rather than out in the circuit.
Why Pilots Care
High internal resistance reduces available voltage during engine start or high electrical loads, which can prevent reliable operation of critical systems.
Grounding Statement
Internal resistance is the hidden electrical loss inside a component when it is actually doing work.
Intuition Check
Internal resistance does not mean the part is physically hard to move or remove. It means the part itself resists the flow of electricity from inside.
Example Sentence 1
As the battery aged, its internal resistance increased, causing the bus voltage to sag noticeably during engine start.
Example Sentence 2
As the battery aged, its internal resistance rose and the starter turned more slowly.