My new sony ericson w850i

My new sony ericson w850i is having to be charged every other day even though sometimes I haven't used it for more than 1 or 2 text messages,when I plug the charger in (the one that came with it in the box) it says on screen "optimising charging)which I thought meant topping up but the battery indicator has gone to nothing and if I try to call someone it warns me that the battery is low.
I've only had this phone for 2 weeks and hope I don't have to send it back.
Any help would be appreciated.
Thank you.
Al