if the battery is OK, then in an hour the motor will not discharge the ventilation even in the cold - if you plan to leave it on for example overnight, then it will discharge when the engine is not running, then it will heat up just a moment, in ten minutes it will blow completely cold (the engine itself needs to be warm for two hours, but the point is that the coolant does not circulate and the radiator that passes through it the air for ventilation / heating) those modern smart cars still switch off running appliances after a while, whether you want or You don't want another thing is independent diesel heating, there's a extra deal