How much charge can idling for half an hour provide to the battery?

[SanRiley]

Idling for half an hour can charge the battery by about twenty percent. Automatic battery charging: Whether the car is in motion or idling, the battery will automatically charge. The engine directly drives the generator via a belt, so once the engine starts, the electricity generated by the alternator undergoes rectification and voltage regulation before directly charging the car battery. However, if the battery is severely depleted to the point where it cannot start the engine, a jump-start may be required. Precautions for idling charging: Avoid using other power-consuming devices during idling charging. When the car battery is low and being charged by idling, although the current is low, it is sufficient. But if high-power devices like air conditioning, high beams, or car audio systems are used during charging, they may exceed the alternator's maximum output power. This could overdraw the already low battery, leading to permanent damage.

[LaKaleb]

I remember old car enthusiasts often said that idling to charge the battery isn't very effective. When I was younger, I tried idling by the roadside for half an hour to top up the battery, but the charge level barely moved. The reason is simple: at idle, the engine speed is too low, resulting in weak alternator output—typically just 20-30 amps. After half an hour, you might gain 1-2 amp-hours at most, which may only offset the power used to start the car, with little extra to spare. In winter, it's even worse—this trickle of power barely keeps the engine running. In contrast, driving for just ten minutes at over 1,000 rpm doubles the alternator output, making charging far more efficient. My advice? Don’t rely on idling for meaningful charging—it wastes fuel, increases emissions, and raises the risk of engine carbon buildup. If your battery is truly dead, opt for professional charging equipment—it’s safer and far more effective.

[SanBrody]

From a technical perspective, idling charging is actually quite limited. I've seen in some materials that for ordinary sedans, the alternator speed during idling is typically below 1,000 RPM, with output current generally ranging between 20-50 amps. However, the actual charging current is significantly affected by battery voltage and internal resistance. At most, only 1-3 ampere-hours can be charged in half an hour, equivalent to about 5-10% of the battery's total capacity, and this assumes the battery is half full. If the battery is aged or deeply discharged, this value could be even lower or ineffective. Truly efficient charging requires the engine to operate at higher speeds, such as during driving when the RPM exceeds 1,500, where output can increase to over 100 amps. In daily practice, I've found this charging method is more suitable for maintaining charge rather than recovering a depleted battery. Long-term reliance on it can easily damage engine health and lead to increased maintenance costs.

[AubreyLee]

Idling for half an hour to charge the battery? The effect is almost negligible. I've encountered similar situations multiple times, like trying to charge while waiting in a parking lot, with almost no change in battery level. The reason is that the alternator output is too low at idle. For a regular car, it's already good if it can replenish the power consumed during startup in half an hour. The actual amount might only be one or two ampere-hours, far less efficient than a few minutes of high-speed charging while driving. Considering battery lifespan, I don't recommend doing this, especially for older cars which are more prone to issues. It's more worthwhile to pay attention to regular maintenance, like turning off all electrical devices before parking to avoid power drain, or using a home charger for periodic maintenance. Simply put, idling to charge isn't a good idea—it wastes fuel and money with little effect.