Definition
A two-sided rocker switch installed in many light aircraft that combines the master battery switch and the alternator switch into a single control. One half (commonly labeled BAT) connects the battery to the aircraft's electrical bus; the other half (commonly labeled ALT) brings the alternator online to charge the battery and supply electrical loads in flight. The two halves are typically hinged together so both can be turned on with one motion, but each side can also be operated independently when needed.
Plain English
It is a single switch with two halves. One half turns on the battery, the other half turns on the alternator. You can flip both at once, or use just one side if you need to.
Context Anchor
Seen on the cockpit switch panel during preflight, engine start, electrical checks, and electrical problem procedures.
Derivation
Called a 'rocker' switch because it pivots back and forth like a rocker — pressing one end down lifts the other. The two-sided design lets the battery and alternator share one physical control while still functioning as separate electrical circuits.
Why Pilots Care
It is the single point of control for all aircraft electrical power; improper use can drain the battery or leave critical systems without power.
Analogy
It is like a two-part main power switch: one side connects the battery, and the other side lets the airplane’s generator help carry the electrical load once the engine is running.
Intuition Check
“Master” does not mean it controls every system directly; here it means the main switch for electrical power. “Rocker” does not mean the switch is unstable; it describes the tilting motion of the switch.
Example Sentence 1
Before engine start, the pilot pressed both sides of the master/alternator rocker switch to ON, bringing the battery and alternator online together.
Example Sentence 2
During shutdown the pilot moved the master/alternator rocker switch to the off position to prevent battery drain after engine stop.