Car airconditioning problem....?

The other day I went to start my car and the battery was pretty much dead... so we had it replaced and the car starts up better than ive ever heard... ( its a 2004 holden barina btw) except now im noticing when i drive and brake at a red light or whatever while the air con is running... the engine seems to 'drop out' a bit... almost feels like the car is close to completely konking out... but it doesnt... so i tried it without the aircon... and no problem.... could it be the new battery... or nothing related to that at all...??? ive never had a drama with this car really... air con has always been great...