how long to let car run to charge battery

[JesseLynn]

Simply idling your car to recharge a dead battery is an inefficient and often ineffective method. For a significantly drained battery, you might need to idle the engine for at least 30 minutes to an hour to provide enough charge for a successful restart. However, driving the car is far superior, as the engine runs at higher RPMs, allowing the alternator to charge the battery much faster. A 20-30 minute drive on a highway is typically the recommended solution.

The time required depends heavily on several factors: the battery's existing state of charge, the capacity and health of the alternator, the battery's age, and the electrical load from accessories like headlights and air conditioning. A modern alternator can typically output between 40 to 150 amps, but this maximum output is only achieved at higher engine speeds, not at idle.

The primary purpose of idling is to replenish the small amount of charge used to start the engine, not to recharge a battery that has been deeply discharged. If your battery is repeatedly dead, it may be failing and unable to hold a charge, indicating a need for professional testing or replacement. Relying on idling to fix a chronic battery issue can lead to being stranded.

The table below outlines estimated charging times under different scenarios, assuming a standard 48Ah (Amp-hour) car battery and a healthy 100-amp alternator.

Scenario	Engine State	Estimated Time for Meaningful Charge	Key Considerations
Post-Starting Recharge	Idling	5-15 minutes	Replenishes the small burst of energy used for ignition.
Lights Left On (Partially Drained)	Highway Driving	20-30 minutes	Higher RPM allows alternator to operate at peak efficiency.
Completely Dead Battery	Idling	60+ minutes (Not Recommended)	Extremely slow, puts strain on alternator, may not work.
Deeply Discharged Battery	Professional Charger	4-12 hours	The only safe and reliable method for a "dead" battery.
With Major Accessories On (A/C, Headlights)	Idling	Charge rate may not exceed drain	Electrical load can prevent any net charging at idle.

For a reliable and battery-friendly charge, using a dedicated battery maintainer or trickle charger is always the best practice.

[MacJude]

Look, if you just left your dome light on overnight and the car is struggling to start, a good 20-minute drive should get you back on track. But if the battery is totally dead—like, no lights on the dashboard—idling for an hour might not even cut it. You're better off getting a jump start and then driving it. Honestly, if it dies more than once, just get the battery tested. It's probably time for a new one.

[MacMatteo]

I learned this the hard way. Idling does almost nothing. The alternator needs the engine to be spinning faster to produce real power. After a jump start, don't just sit in the driveway. Take it for a spin on a main road for a solid half-hour. This lets the alternator work properly. If you're just idling with the AC on, you might be using more power than you're putting back in.

[MacOlivia]

Think of it like this: your alternator is a water pump. At idle, it's just trickling. When you're driving, it's like opening the faucet all the way. A dead battery needs a full faucet. A short drive is the fastest way to get a decent charge back into it. Idling for a long time is wasteful, bad for the engine, and not great for the environment. It's a last-resort, temporary fix, not a solution.

[EmmettRose]

The key factor is alternator output, which is significantly lower at idle RPM (around 500-800 RPM) compared to cruising RPM (over 2000 RPM). While a 15-minute idle might recover the charge used for starting, a deeply discharged battery requires high-amperage charging that idling cannot provide. This is why a jump-start followed by a sustained drive is the standard procedure. Continuous reliance on the alternator for deep-cycle charging can shorten its lifespan. For battery health, a dedicated charger is always recommended.