Alternator not charging: battery light on
By Altay Hasanli, auto electrician
Short answer
If the battery light comes on while driving, the alternator isn't charging and the car is running on what's left in the battery; that won't last, and the engine may then stall. The usual culprits are the voltage regulator, brushes, diode bridge or belt. Most of the time the alternator can be repaired rather than replaced.
The most common causes
- 1
Voltage regulator or brushes
A failed regulator or worn brushes stop the alternator charging. The voltage ends up either too low or dangerously high.
- 2
Diode bridge
When one of the diodes fails, the alternator can't deliver full output, the battery slowly drains, and you may hear a whine through the speakers.
- 3
Drive belt or tensioner
A slipping, worn or broken belt doesn't turn the alternator. A squeal on cold starts is the classic sign.
- 4
Wiring, terminals or the control circuit
In modern cars the engine control unit commands the alternator. A wiring or connector fault stops charging even when the alternator itself is fine.
What you can check yourself
- Measure the battery voltage with the engine running: most cars should show 13.8–14.7 V.
- Look at the belt and listen for a squeal.
- Watch whether the lights dim when you switch on the headlights and air conditioning.
When to stop driving
- If the battery light is on, switch off anything you don't need (air conditioning, seat heaters, audio) and drive straight to a workshop.
- If the voltage is above 15 V, the regulator has failed: the battery and control units can be damaged.
- If the belt has snapped, on some cars the water pump stops too: pull over before the engine overheats.
How we find the cause
- 1We measure the charging voltage with and without load.
- 2We check the diode bridge for ripple with an oscilloscope.
- 3We check the alternator's control signal and wiring.
- 4If needed, we strip the alternator and replace the brushes, regulator, diode bridge or bearings.
This guide is general information: one symptom can have several causes, and only a diagnostic shows the exact one.
Services and prices
Computer diagnostics
from 20 AZNCar diagnostics: fault codes and sensors
Wiring and electrical repair
from 10 AZNShort circuits, sensors and control units
Alternator and starter repair
from 60 AZNBattery and charging system check
Battery test, replacement and registration
from 10 AZNLoad test, parasitic drain, replacement and BMS registration
Frequently asked questions
Does the alternator need replacing, or can it be repaired?
Usually a repair is enough: the regulator, brushes, diode bridge or a bearing is replaced. That costs far less than a new alternator.
How far can I drive with a faulty alternator?
It depends on the battery and the load — sometimes only a few kilometres. Switch off what you don't need and take the shortest route to a workshop.
How much does alternator repair cost?
Alternator and starter repair starts at 60 AZN. We give you the exact price after diagnostics and agree it before any work starts.
Other problems
- Car won't start
Starter won't crank, clicks, or the engine won't catch
- Battery keeps dying
The battery goes flat overnight or after a few days parked
- Check Engine light is on
The engine warning light is on or flashing
- Engine shaking or losing power
Shaking at idle, hesitation and loss of power when accelerating
- Electrical faults
Lights, windows or dashboard not working, fuses blowing
- Fuel consumption is up
The tank empties faster than usual with the same driving
- Engine stalls by itself
The engine cuts out while driving, at a stop or right after starting