Hi - I just bought a 2004 Honda Civic Hybrid. Very often when I park the car and shut off the engine, there is a humming noise which indicates by the sound of it that the battery is still running. When I restart the engine and shut it down again, it turns off. This is especially a worry in parking lots with other engines running as it's pretty hard to detect. I'm new to hybrids, so am I doing or not doing something to cause this? Thanks Much!

Is the humming sound coming from the front of the car where the radiator is? On a lot of cars (this isn't a hybrid-only thing) the radiator fan will continue to run for a few minutes after you shut the car off, especially if it's hot out or you've been driving a while and the engine is hot. If you were to open the hood of the car when you hear this sound, you'd probably see the fan rotating. It usually turns off after a couple of minutes.

If that's not the sound you're referring to, I don't know what it might be.

http://www.greenhybrid.com/compare/m.../car/5679.html

I concur with Gairwyn. I have read that the HCHI radiator fan will run after the car is off.

Jess

Can you clarify: does the sound seem to be coming from the battery area, behind the rear seat back?

I believe the first gen Civic Hybrid's Integrated Motor Assist battery has a cooling fan, assuming it's the same as second generation ('06 and onward). Maybe that fan can run after the engine is shut down, if temperatures are high enough?

I agree with the other possibility: it could be the engine compartment radiator fan.

Another noise you might hear coming from the rear after shut down, particularly in hot weather: the gas cap will sometimes vent, if there is excessive air pressure in the gas tank. This will go on for a minute or two at most.

I have the same car as you, same model, same year. That sound is normal.

It's the cooling fan working to cool down the electrical components (such as the motor) in your '04 HCH. It's quite loud, but it's not your battery, and it should shut off by itself within 1-2 minutes.

Based on how long you've been driving, and how hot the engine and electric motor are, the fan will turn on for a specified amount of time after you shut off your engine.

If you turn the engine on and off again to stop that fan, you will be interfering with your car's cooling systems, which will not be good for the longevity of your electric motor and other systems.