With regular use, the Spyder should keep its battery charged. Many people, especially in cold climates, keep their Spyder plugged in to a Battery Tender (or equivalent) when not in use, however. A fully charged battery is less likely to sulfate or fail prematurely. Your Spyder could have a variety of problems. First, it is common for the battery to not be connected firmly, resulting in a poor charge. Loose connections can also happen from changes in weather, or due to vibration. Second, there could be corrosion or oxidation from sitting so long...the connections and terminals should be cleaned. Finally, a battery that is charged, then left idle, can readily get weak or fail. Your battery was probably installed, then left in the Spyder on the floor. It could have become sulphated, where it would not take a charge well.
The first thing to do, if you do not have a Battery Tender, is to check and clean the terminals. This means the terminals at the battery, under the Tupperware, not the charging terminals under the seat. While you are there, you may want to attach the Battery Tender harness....if you choose to purchase a charger. The second thing to do (or first if you already have a Tender) is to fully charge the battery. The light on the Battery Tender should indicate the state of charge, and should show green when the battery becomes fully charged. If it does not come to full charge with clean, tight teminals, the battery is probably bad. If it repeatedly takes many hours to charge, the battery is probably weak, too. The battery should retain at least 12.7 volts after sitting several hours to overnight...after having been fully charged. When briefly cranking the starter with the throttle fully open, the battery should remain above 10.0 volts. Your dealer can do a more sophisticated load test. If the battery tests bad, it should be replaced.