Certainly sounds like your battery is no good. I know you said it was new.

You should get it load checked....... either battery is no good, or you have a parasitic drain. Make sure there is nothing getting left on. Common places to look, light in vanity mirror back of sun visor. Lights under hood. Power seats sometimes will cause the drain.

First you need to figure out what circuit the drain is on. You will need a multi meter.

Google parasitic drain for some how to videos.  

Unless you drive it for a while, or leave it on charger until fully charged, it might continue to do the same thing.

 

Get it checked out while you still have some kind of warranty in place. At least then, you will know if it is the battery. There is always a chance there is parasitic drain on the battery, but I would start with the battery.

 

 

firstly I dont think leaving the key in the ignition is a good idea anyway with modern cars, you never know if they go properly "to sleep" of if somethings stay active. 

anyway, I´ve found that hooking the C4 up to a "smart" charger, eg CTEK, prevents any nasty surprises with cars which aint driven very often. my convertible often sits for some weeks till I ve got time AND the weather is fine... the battery is kind of small for such a big engine anyway, my 3ltr Merc Coupe got a similar size. then, modern cars (by which I mean cars that simply go "off" when turning the key) do have sufficient residual power requirements so that the battery is unlikely to be weak after 3-4 weeks. In the C4, the engine ECU will remain on (but asleep, hopefully) as the ECU memory needs to be powered, the VATS anti theft system needs to monitor if the doors are opened without the key (I did once....and the car panicked as required and I had to rush indoors to get the key...)

battery in my 1992 Merc is weak after 4 weeks, in my (modern) Harley after 3 weeks. so... get a charger.

cheers

Wolfram