Yes, but very inefficiently. At idle, a car's engine runs at a low RPM (typically 600-1,000 RPM), which causes the alternator to produce only a minimal electrical charge. This small output is primarily used to power essential systems like the engine control unit and fuel injection. If you are trying to recharge a significantly depleted battery, idling is an ineffective method and can even be detrimental over time. For a meaningful charge, you need to drive the car, as engine RPM increases to 1,500-2,000 RPM or higher, allowing the alternator to generate sufficient amperage.
The core issue is the alternator's output curve. Its charging capacity is directly tied to engine speed. At idle, the alternator might only produce enough power to break even with the car's electrical demands, leaving little to no surplus for the battery. This is especially true if electrical loads are high, such as when using headlights, air conditioning, or a powerful sound system. In these scenarios, idling can actually drain the battery further.
For context, here's a comparison of typical alternator output under different conditions:
| Engine State | Approximate Alternator Output (Amps) | Primary Electrical Loads | Net Effect on Battery |
|---|
| Idling (600-1,000 RPM) | 20 - 40 A | Engine ECU, Fuel Pump, Lights | Minimal to no charge; potential drain with accessories on |
| Cruising (1,500-2,500 RPM) | 60 - 120 A (or more) | All vehicle systems | Significant, positive charge |
| High RPM (3,000+ RPM) | Max output (e.g., 150 A) | All vehicle systems | Maximum charging rate |
The most effective way to recharge a healthy battery is a 30-minute drive on a highway or open road. This sustained higher RPM allows the alternator to work efficiently. If your battery is consistently dead, the problem may not be a lack of charging; it could be a failing battery, a faulty alternator, or a parasitic drain—where a component draws power even when the car is off. In these cases, idling the engine is merely a temporary workaround, not a solution. For a completely dead battery, using a dedicated battery charger is the safest and most effective method to restore it to full health.
