Here are some points to consider when deciding whether to idle or drive to charge a car battery:

Driving:
Alternator Charging: When the engine is running while driving, the alternator generates electricity, which charges the battery.

Higher RPMs: Driving typically involves higher engine RPMs than idling, which can result in faster charging as the alternator spins faster.

Effective for Longer Distances: Driving serves a dual purpose if you need to charge the battery and get somewhere.

Engine Warm-Up: Driving allows the engine to warm up to its optimal operating temperature, which can benefit overall engine health.

Idling:
Low RPMs: Idling typically involves lower engine RPMs, meaning the alternator may not generate as much electricity as during driving.

Slow Charging: Due to the lower RPMs, charging the battery while idling may take longer than charging it while driving.

Less Fuel Consumption: Idling consumes less fuel than driving, which might be beneficial if you try to conserve fuel.

Convenient for Short Periods: If you only need to charge the battery briefly, idling might be more convenient than driving around.

Considerations:
Battery Condition: If the battery is severely depleted, driving might be more effective as it can provide a higher charging rate.

Environmental Impact: Idling for extended periods can contribute to air pollution, so consider the environmental impact when deciding between idling and driving.

Safety: Always prioritize safety when deciding whether to drive or idle to charge the battery. If the vehicle is unsafe to drive due to battery issues or other reasons, idling may be the only option.

In summary, driving is generally more effective for charging a car battery due to higher engine RPMs and the continuous generation of electricity by the alternator. However, idling can be a viable option for short-term charging or when fuel conservation is a concern. Ultimately, the best choice depends on the specific circumstances and needs of the situation.