Vacuum cleaner; battery; engine on or off?

I bought this little 60W vacuum cleaner for my car and it plugs only in the lighter socket in the car. So the question is directed in terms of saving my car battery from dying. Should I vacuum with the engine on or with the engine off (and the key turned to Contact)?

PS
Not sure if it is called 'Contact' but by that I mean the stage when they key is turned just before the next turning stage of ignition.

Hello again soewhaty.

Your vacuum cleaner will draw around 5 amps from the battery. If you have a good battery which is fully charged by the alternator from reasonable length journeys then the battery should stand a few minutes vacuuming. 10 minutes use at 5 amps is about 1 amp hour, roughly 3-4% of the batteries capacity.

The ignition position is usually called the accessory position.

The C3 also has the added protection of 'Eco Mode'. It will turn things off to save the battery when the engine isn't running. I'm not sure if this applies to the power/lighter outlet though.

Thanks for the reply! Well I just tested it and when the key was in and half-way turned (what I previously called a Contact stage) the vacuum cleaner worked. It also worked when the engine was on, obviously. But it did not work when the key was not plugged in. So my question was - in order to save battery life should I do my vacuuming while the engine is running or while it is not? What's the right thing? It says on the vacuum cleaner cover to do it while the engine is off, but some forums on the net suggest the opposite, so ...