Does Running the Air Conditioner While the Engine Is Off Damage the Car?
3 Answers
Running the air conditioner while the engine is off can damage the car for the following reasons: 1. If the air conditioner is not turned off before shutting down the engine, the compressor will start simultaneously during the next ignition, increasing the load on the motor; 2. Using the air conditioner when the engine is cold can harm the engine; 3. Turning off the air conditioner about 2 minutes before shutting down the engine allows the cold air in the ducts to be discharged in time, preventing mold and bacterial growth. Car air conditioners have two modes: internal and external circulation. In external circulation mode, a fan draws air from outside the car into the interior, meaning there is an airflow channel between the outside and inside of the car. Even if the fan is not turned on, air will still flow into the car while driving, replenishing fresh air inside. In internal circulation mode, the airflow channel between the inside and outside of the car is closed. Without the fan, there is no airflow circulation. When the fan is on, the air drawn in comes only from inside the car, effectively preventing external dust and harmful gases from entering the car.
The air conditioning cooling function cannot be used after the engine is turned off because the compressor relies on the engine belt to operate. However, the blower fan can still be turned on, as it is entirely powered by the battery. I once encountered a similar situation where I turned on the fan to blow natural air while waiting in the car after turning off the engine, and also charged my phone and listened to music. After about 30 to 40 minutes, the car couldn't start at all. After being towed to a repair shop, it was found that the battery had suffered deep discharge, leading to sulfation of the plates, and the entire battery had to be replaced at a cost of over 1,000 yuan. Now, I always advise my friends not to use electrical devices for more than 15 minutes after turning off the engine, especially for older cars with smaller battery capacities, as they are more prone to power loss. If you really need to wait for a long time, it's better to start the engine briefly to let the air conditioning work for a few minutes—this keeps you cool and doesn’t harm the battery.
One summer day, I arrived half an hour early to pick up my child from school. Thinking to save fuel, I turned off the engine but kept the air conditioning on. To my surprise, it wasn't cooling at all—only the fan was blowing natural air. By the time I picked up my child, the battery was completely dead, and even the dashboard lights were extremely dim. I had to ask a neighboring car owner for a jump-start to get going. The mechanic explained that once the engine is off, the battery relies solely on its stored power, just like a smartphone battery, which depletes with use. A standard lead-acid battery can suffer from sulfation if deeply discharged, and more than three deep discharges can essentially render it useless. Now, I always keep an emergency jump starter in my car just in case. It's advisable to limit using just the fan to under 10 minutes after turning off the engine, especially if you're also using the audio system or charging devices. Regularly testing your battery's health can help avoid the embarrassment of a breakdown.
