Try charging the battery fully, remove the earth cable from the battery, measure the voltage across the battery, then leave it uncoupled for a couple of days and check the voltage again. Ideally the first reading would be about 12.8 volts and the second about 12 - 12.4v if the second reading is under 12v then the battery most likely is on it's way out. If the second reading is good, above 12v, then there may be a drain when connected to the vehicle, (a hidden interior light on, a door switch not operating correctly, etc and don't forget the alarm system will drain the battery quite quickly, 4 or 5 days if not run regularly). Easiest way if it's possible keep the battery topped up every week with a charger if you're not using the vehicle. Good luck!
PS you don't need a special stop/start battery.
After thought, don't forget to reset the windows after disconnecting the battery.