A belt driven device that creates electrical current for maintaining the charge state of the battery and the electrical requirements for the vehicleChecked 2026-08-14
Alternator and charging system
The belt-driven generator that keeps the battery charged and runs the car's electrics while the engine is running.
Also called: alternator · charging system · generator · voltage regulator · rectifier
What it means
Definition
DENSO, which manufactures them, defines the alternator as a belt driven device that creates electrical current for maintaining the charge state of the battery and the electrical requirements for the vehicle. It produces alternating current, so a rectifier inside it — a set of diodes, in DENSO's description — changes that output to the direct current the car uses, and a voltage regulator holds the level steady. The battery's own job is narrower: DENSO calls it an electrical storage device used to supply electrical current for starting a vehicle.
Why a buyer cares
What it changes
Battery and alternator are two parts with one symptom between them, and a car that starts on the forecourt has only demonstrated the battery. Which of the two is at fault is a measurement, not an impression.
In practice
Where you meet it
The battery-shaped warning light on the cluster, and invoices naming an alternator, a rectifier, a voltage regulator or a battery replacement.
Two things that look alike
Not the same as
The battery starts the engine; the alternator supplies the car once it is running and puts back what starting took out.
Read next
Related terms
Where it shows up
Related problem files
Wear item
Charging system: battery and alternator
The alternator charges the battery while the engine runs and powers everything electrical. Both are consumable parts with a service life, and on a used car at least one of them has usually been replaced already — or is due.
Safety recall
A replacement battery that can move and short out
The original battery fits. A replacement one may not — and if it moves in the tray it can touch the hold-down bracket and short. Toyota recalled 1,853,568 cars for it. This is a used-car problem by construction: it can only exist on a car whose battery has been changed, which after ten years is most of them.
Where this comes from
Sources
This is an electrical component in the alternator that contains sets of diodes which are used to change the alternator output from alternating current (AC) to direct current (DC).Checked 2026-08-14
DENSO's page also prints a voltage range for a charging-system test. It is not repeated here: the reading is taken at the battery with the engine running, which is a workshop measurement rather than something the entry can hand a reader as a check.