The working process of the power unit occurs when the crankshaft rotates, switching to increased load mode only after pressing the accelerator. At idle, the engine performs the function of recharging energy consumers, say, lighting equipment, a stove or an audio system. However, this does not mean that the battery is charging at this moment.
In fact, the drive does not have enough engine speed to keep itself in good shape. And if you thresh at idle at sub-zero temperatures, then after you need to start the engine again, this may not happen. Yes, the battery will run out and you will end up in a puddle. To avoid this, you should keep the engine speed above the norm set for idle mode. Or simply drive the car before turning it off.
What should the idle speed actually be? Firstly, stable, not floating and jumpy. In addition, without the manifestation of detonation of the power unit. Secondly, sufficient for optimal operation of not only the battery, but also the generator, oil pump and cooling system. Finally, not excessively high, nor, in fact, extremely low. To be specific, to ensure normal performance algorithms, the idle speed should be at 1000 rpm. When warming up, naturally, this indicator can increase, and with increasing temperature it can fall. In both cases, it is insignificant.
As for engine speeds while driving, the optimal range is considered to be from 2500 to 4500 rpm for naturally aspirated engines and from 2100 for turbocharged units. In other words, the engine is most beneficial when operating at 50-70% of peak torque. In the winter season, due to the duration of warm-up and increased dynamic and climatic loads, experts do not recommend revving the engine above 2500 rpm while driving.