Yes, a faulty alternator can absolutely drain your car's battery even when the car is off. While a properly functioning alternator should not cause a drain, a malfunctioning diode inside it can create a "parasitic drain," allowing current to flow backward from the battery through the alternator, slowly depleting it. This is a common cause of a dead battery overnight.
A healthy electrical system has a very small, acceptable parasitic drain (typically 50 milliamps or less) to power things like your car's clock and security system. A bad alternator can cause a drain significantly higher than this. The most common failure is within the diode trio, which is responsible for converting the alternating current (AC) produced by the alternator into direct current (DC) for the battery. When a diode fails, it can act like a closed circuit, creating a constant path for battery power to leak to ground.
How to Diagnose a Parasitic Drain from the Alternator:
- Perform a Parasitic Draw Test: Use a multimeter set to measure amps (in the 10A range) to check the total drain on the battery with the car off. If it's high (e.g., over 0.1A or 100mA), you need to find the source.
- The Alternator Test: With the multimeter still connected and the car off, simply disconnect the main power cable from the back of the alternator. If the multimeter reading immediately drops to a normal level (under 50mA), the alternator is the culprit.
| Common Symptom | Possible Cause | Typical Measured Drain (if faulty) |
|---|
| Car starts fine but dies overnight | Faulty Alternator Diode | 0.3A - 2.0A (300 - 2000 mA) |
| Battery warning light is faintly glowing when off | Severe Diode Failure | 1.0A+ (1000 mA+) |
| Smell of burning rubber/electrical from alternator | Internal Short Circuit | Varies, often very high |
| Normal Acceptable Drain (for clocks, ECU memory) | Healthy Vehicle Systems | 0.02A - 0.05A (20 - 50 mA) |
If you suspect your alternator is causing the drain, it's best to have it tested by a professional mechanic. Driving with a faulty alternator can lead to a dead battery and leave you stranded.
