The most common reason for a battery to be discharged is from taking a lot of short, low-rpm trips. It takes about 10-20 minutes of running over 3-4k rpm to recharge the battery from a start. If you go take a bunch of 5 minute trips down the road (start, run 5 minutes, stop, start, run 5, stop, start, run 5, etc.) you can wear down the charge on your battery quickly. If you take longer trips, charging should be covered. Not riding the bike for a long period is the most common way to kill a battery, so you should use a battery charger to help maintain it, especially if you have a lead-acid type battery. I put my bike on the charger once a month to help give it a good and thorough charge.
Battery chargers are relatively inexpensive. You can get a good one for under $25. It's better to have one and not need it, than need it and not have it. Sort of a cheap insurance policy, just in case. Also having a cheap multimeter is a good way to check your voltage and for other electrical task.
See this thread for more info;
Motorcycle Battery & Battery Charging Tips
