the.com/alternator

means A generator in a vehicle's engine that converts mechanical energy into alternating current to charge the battery and power the electrical system while the engine runs.

from Straightforward machine-age naming: 'alternate,' from Latin 'alternare' (to do by turns, from 'alter,' meaning 'other'), plus the agent suffix '-or.' Early car generators produced direct current, but engineers found alternating current generators more efficient and durable, so by the mid-20th century the 'alternator' — literally, the thing that alternates — took over under the hood, its output later rectified into DC for the battery's sake.