Bad service

I bought a car 3 weeks ago. Ive been promised that yhe car has been for a varlet, not the case. Now today the car warns be of my battery that is low. Took the car to the nearest battery center and they told me i need a new battery. Spoke to the DP about it, they have all sorts of excuses for example its winter... Point being, the battery was never checked properly otherwise this would not have been an issue 3 weeks later. Another car my brother wanted to testdrive had to be charged before he could drive it which shows that these cars do not start in order to charge the battery. Ive been told there is nothing they can do. Disgusting service.