You do not need to disconnect the battery from the car to charge it. However, if it does not have an explicit maintainer function then I would not leave it permanently connected to the battery. Without knowing the model and vintage of your charger it is not possible to advise on its suitability as a battery maintainer.
If it is only a charger and you are not using the car semi regularly, do not use the charger for battery maintenance. If the car is going to be parked for a several of weeks then disconnect the negative battery clamp instead to prevent discharge. However, if you ran your battery absolutely flat, chance are the battery is now on deaths door step. With a parts store OEM sized AGM battery, I can disconnect my battery all winter or park my car for 2+ weeks leaving the battery connected and have no problem starting it