will idling charge a car battery

[JocelynMarie]

Yes, idling your car does charge the battery, but it is an extremely inefficient and inadequate method. The car's alternator, which is the component responsible for generating electricity, is driven by the engine via a serpentine belt. At a typical idle speed of 600-800 RPM (Revolutions Per Minute), the alternator spins too slowly to produce its full output. It might only generate 30-50 amps, which is often just enough to power essential systems like the fuel injection, ignition, and basic electronics, leaving little to no surplus for charging the battery. This is especially true if you are using power-hungry accessories like the air conditioning or heated seats.

For a battery that is significantly drained—say, from leaving the interior lights on overnight—idling for 15-20 minutes is unlikely to provide a meaningful charge. You would need to drive the car for a sustained period at higher RPMs (e.g., a 30-minute highway drive) to allow the alternator to operate at a much higher output, often exceeding 100 amps, to effectively recharge the battery.

Relying on idling as a primary charging method can actually be detrimental. It leads to parasitic drain, where the charge going in is less than the power being consumed. Furthermore, frequent short trips with long idling periods prevent the battery from ever reaching a full state of charge, which can cause sulfation (a buildup of lead sulfate crystals) that permanently reduces the battery's capacity and lifespan.

Scenario	Estimated Idling Time for Meaningful Charge	Key Consideration
Minor Drain (e.g., radio on for an hour)	10-15 minutes	May be sufficient, but a short drive is better.
Moderate Drain (e.g., dome light left on overnight)	30-60 minutes	Inefficient; a drive is strongly recommended.
Severe Drain (near-dead battery)	Multiple Hours	Highly impractical and may not work at all; use a battery charger.
With Accessories On (A/C, headlights, defroster)	Effectively Never	The alternator's output at idle may not even cover the power draw.
Modern Stop-Start System	Designed for it	These systems have heavy-duty batteries and sophisticated management to handle frequent cycling.

The most reliable way to charge a healthy battery is to drive the car. If the battery is consistently low, the underlying issue could be a failing alternator, a parasitic draw from a malfunctioning component, or an old battery that needs replacement.

[Saul]

It charges it, but barely. Think of it like trying to fill a swimming pool with a garden hose that's only dripping. When your engine is just idling, the alternator isn't spinning fast enough to do much good. If the battery is really dead, you're better off getting a jump start and then taking a good 20-minute drive on the highway. That's what actually gets the charge flowing properly.

[DiNova]

As a mechanic, I see this misconception all the time. Technically, yes, idling produces a charge. However, the charging rate is minimal. On a modern car with all its electronics, the alternator at idle might only produce enough juice to run the engine computer, fuel pump, and A/C. If your battery is weak, idling often adds more wear and tear than meaningful charge. The best practice is to drive the vehicle. The higher engine RPM from driving spins the alternator faster, generating the necessary amperage for a proper recharge.

[CarmenAnn]

I learned this the hard way after a few too many dead batteries from just sitting in my driveway listening to the radio. Idling is a slow trickle charge at best. If you've completely drained the battery, idling for a short while might give it just enough power to start the car again, but that's it. You haven't fixed the problem. To truly recharge it and avoid being stranded tomorrow, you need to get the car out on the road. The engine needs to be under a real load.

[AlexisLynn]

The simple answer is yes, but with major caveats. The charging system is designed to replenish the battery under normal driving conditions, not at idle. At 800 RPM, the alternator's output is a fraction of its capacity. For a battery that is only slightly discharged, a prolonged idle might help. For a deeply discharged battery, the internal resistance is so high that the low current from idling is ineffective. This is why a dedicated battery charger is the recommended solution for a full recovery, as it provides a controlled, multi-stage charge that restores health.