The alternator and the charging system: how a car makes its own electricity

The battery does not run the car. It starts the car, and after that the alternator runs everything and puts back what the starter took out. A car with a dead alternator will drive until the battery is flat and then stop — which is why a charging fault so often presents as a car that died on the road, not as a car that would not start.

Dynamo and alternator are not the same machine

Worth separating first, because the Persian word dinam is used for both and the original article describes one while meaning the other.

A dynamo is a DC generator. The magnetic field is fixed to the housing, the current-producing windings spin inside it, and the output is taken from the spinning armature through a commutator and brushes. Everything the brushes touch is carrying the full output current.

An alternator inverts that. The rotor carries a field winding and spins; the stator — the current-producing windings — is fixed to the housing. Nothing spinning carries the output. Brushes are still there, but they feed only the small field current through slip rings, so they last far longer.

Cars stopped using dynamos for a specific reason worth knowing: a dynamo cannot charge at idle. Output depends on speed, and in traffic an engine spends long periods near idle where a dynamo produces less than the car consumes. As electrical loads grew, that became untenable. An alternator makes useful current at idle, which is the whole reason for the change.

The two parts the original never mentions

An alternator produces alternating current, three-phase. A car needs direct current at a stable voltage. Two components bridge that gap, and both are common failure points, so an article about charging that omits them omits the diagnosis.

The rectifier. A bridge of diodes — typically six — converts the three-phase AC into DC. Diodes fail, and they fail in two ways with two different symptoms, covered below.

The voltage regulator. Alternator output would otherwise rise with engine speed without limit. The regulator controls how much current goes to the rotor's field winding, and therefore how strong the magnetic field is, and therefore the output voltage. It holds the system at a target regardless of speed or load.

That target is normally somewhere in the 13.5–14.5 V range on an older car. Note that many modern vehicles use smart charging, deliberately varying between roughly 13.2 and 14.8 V according to battery state, temperature and load — so a voltage that moves around on a late car is not automatically a fault, and judging one by an older car's fixed expectation produces false diagnoses.

The original article follows its dynamo section with several hundred words of general alternating-current theory — RMS values, two-phase and three-phase phase angles, star and delta connections, self-induction. None of it helps anybody diagnose a car, and one paragraph states that a three-phase alternator has two coils at 120 degrees, which is a copy-and-paste from the two-phase paragraph above it. It has been left out rather than corrected.

The warning lamp is part of the circuit

This surprises people and it explains a fault that otherwise makes no sense.

On many alternators the charge warning lamp is not merely an indicator. It is wired in the field circuit and supplies the initial excitation current that gets the rotor's magnetic field started. Without some current in the field there is no field, without a field there is no output, and without output there is nothing to sustain the field.

The consequence: a blown warning lamp bulb can stop the alternator charging at all — and the symptom is a flat battery with no warning light, because the bulb that would have warned you is the failed part.

So when a charging fault comes with no light, check the bulb before condemning anything expensive. And a lamp that glows dimly rather than going out usually means a voltage difference across it — commonly one failed diode.

Testing it

Voltage, which answers most cases

Measure across the battery terminals with a multimeter.

• Engine off: a rested, healthy battery reads around 12.6 V. Below about 12.4 V it is partly discharged; around 12.0 V it is flat.
• Engine running: the voltage should rise to somewhere in the 13.5–14.5 V band and stay there. That rise is the charging system working.
• Load it: headlights, blower, heated rear screen. The voltage should sag a little and hold above roughly 13 V. Falling towards battery voltage under load means the alternator cannot meet demand.

No rise at all means no charging. A rise well above 15 V means the regulator has failed the other way — and overcharging boils a battery and damages electronics, so it is the more urgent fault of the two despite looking healthier on the gauge.

The ripple test, which finds the failure voltage misses

This is the measurement most people never make and it identifies a failing rectifier directly.

Set the meter to AC volts and measure across the battery with the engine running at a fast idle and the lights on. You are looking for AC leaking through a rectifier that should be blocking it.

• Below about 0.1 V AC — healthy.
• Above about 0.5 V AC — a rectifier problem.
• Above 1 V AC — significant diode failure.

A failed diode is worth catching because its symptoms are so easily blamed on other things: dim or flickering lights, a battery that is never quite charged, radio interference, and on modern cars communication faults, because AC riding on the supply upsets modules. DC voltage can look perfectly normal with a failed diode, which is exactly why the AC check exists.

The mechanical half

Before any of the above, look at the drive belt. A glazed or slack belt slips under load, so the alternator turns more slowly than it should exactly when most is being asked of it — undercharging that comes and goes with electrical load. A snapped belt on many engines also stops the water pump, so a charging warning can be the first sign of an overheat about to happen.

Check the wiring too, particularly the heavy output lead and the earth strap between engine and body. A voltage drop test along those paths finds a resistance that a continuity test passes: with the engine running and a load on, measure the difference between the alternator output terminal and the battery positive post. More than a few tenths of a volt is a bad connection, and it will look exactly like a weak alternator — the alternator is producing the right voltage and the cable is losing it on the way.

Alternator or battery?

The two get confused constantly because both produce a car that will not start.

• Battery fine, alternator failed: the car starts normally after a night on charge, then dies while driving, usually with lights dimming first. The warning lamp comes on while running.
• Alternator fine, battery failed: the car struggles to start, especially cold, but runs indefinitely once going. Voltage rises correctly with the engine running.
• Neither, and it is a drain: the car is fine all day and flat in the morning — see finding a parasitic drain.

One caution: a failed alternator will flatten a good battery, and running a battery flat repeatedly ruins it. So a charging fault left long enough produces two faults, and replacing only the alternator leaves a car that still starts badly.

Related codes

Charging faults on modern cars usually surface as P0562 system voltage low or P0563 system voltage high, and the generator-specific codes around P0620–P0622 for the control circuit.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.