In most cases your car is being charged by the alternator and cars are sophisticated enough to not overcharge your battery. Water is typically lost during overcharging of a car battery. This may occur if you use a car battery charger that is not sophisticated with a built in float mode and ambient temperature sensor. Battery capacity changes according to the ambient heat it is subjected to. Colder temperatures decrease battery capacity while hotter temperatures increase battery capacity. As a battery is heated up from overcharging, it will allow more current to flow into the battery. If the charger does not take ambient temperature into consideration it could generate too high of a charge voltage causing excessive current to flow into a battery once it is fully charged. This excessive current will cause decomposition of water in the electrolyte resulting in premature aging of the car battery.
Furthermore, excessive overcharging could lead to thermal runaway where the battery continues to heat up and capacity continues to increase so the charger is able to push more current into the battery. This could destroy a lead-acid car battery in a few hours.
Remember if you use a car battery charger, be sure it has a float mode and an ambient temperature control sensor so it will not over charge the battery.