If you are sure that the existing battery is in good health then it seems likely that there is a fault elsewhere in the electrical system with a drain from a component which is not switching off as it should. It is difficult to diagnose faults such as this yourself, but one thing you can try is to sit in the driver's seat, lock the vehicle, wait two or three minutes and check if any lights are illuminated or components such as the radio or air conditioning are still operational. However it may be that you need the expertise of an automotive electrician, who will test the battery and alternator and will be able to check for parasitic drains.