Unless you keep the battery topped up all the time, it will erode the plates and the battery will eventually go bad. To keep my battery in good shape I use a BatteryTender and I keep it plugged in while it sits in the garage. I just mounted mine under the seat and out of sight and have it wired into the battery so all I have to do is plug my bike in to keep the battery topped off. A Battery Tender will keep a battery in good shape for years... well worth it in my opinion.
Also, a Battery Tender and a charger are different. A Battery Tender does charge up a battery, but when the battery is fully charged it doesn't keep trying to charge. Instead it goes into a maintenance mode or "float" mode where it only puts a bit more charge into the battery as it loses charge from sitting there. A lot of chargers don't have those maintenance modes or float modes so they have to be used differently. There's more info out there on the web about it, so you might want to look into it some more as well, but I hope that helps.