All batteries self discharge every month, hooked up or not. If you leave your battery sitting around there is a very real risk it will never work again, or never work that good again. Batteries hate sitting. It is not a warranty if you didn't keep the battery in use, either by your driving habits, or using a battery charger or maintainer on it.
Vehicle alternators are not battery chargers. While they do put back the energy just used to start your vehicle, they are not meant to recharge a battery that was already low, and is now even lower from starting your vehicle. This is hard on the alternator and increases the risk of you needing to replace it. Also, if you do a lot of short trips in city driving, this doesn't give the alternator enough time to actually charge the battery. This can kill a battery in a hurry. A longer drive at highway speeds is much better for the charging system.
The best thing you can do for your batteries when your vehicle is in storage is to keep them on a battery maintainer, or at the very least charge them once a month with a battery charger. Driving around or starting the car and letting it run for 20 minutes is not the way to go.
If it isn't a maintenance free battery, you must check the water levels as well. Do it with each oil change. Just make sure there is about a 1/2 inch of water above the lead plates inside. Battery acid never evaporates, only the water does. Keeping them topped up with distilled water is critical to extending the life of the battery.
Batteries aren't magic. There is science in each one of them. It is up to you to understand this science.
We are always available with expert advice and information to help you keep your battery from being murdered.