Get the most out of your battery by making sure it is maintained correctly. Even a sealed battery requires maintenance. Starting a bike in the winter and letting it idle, or even short 20 to 30 minute rides will not keep your battery charged. Frequent rides in the day, on and off such as police duty will not keep the battery charged.
The recommended practice regardless of season is to keep the battery on a maintenance charger anytime the machine is parked. The detail here is to use the appropriate type of charger for a specific type of battery.
Most quality battery chargers and tenders will have labels or information with them that will indicate the type or types of battery they will work correctly with. Use of an incorrect type of charger or tender can damage the battery.
Common motorcycle batteries are referred to by three different descriptions found in modern motorcycles. They are the Lead-Acid, Absorbent Glass Mat (AGM), and Gel batteries.
The Lead Acid Battery; Still popular, the fluid level needs to be maintained. Fluid evaporates through the normal charging process. Because it is a wet cell battery a by-product of charging is hydrogen gas that is expelled through the vent tube. Hydrogen gas is very explosive and very corrosive. Take the necessary precautions when working with and around this type of battery!
Maintenance Free Batteries: Many of these are known as “Valve Regulated Acid Batteries” (VRLA). Motorcyclists often refer to these as Absorbent Glass Mat (AGM) or Gel batteries, the two basic categories of VRLA batteries.
Sealed or maintenance-free batteries are often more costly but can offer advantages over the lead-acid type.
The AGM battery: The construction of this type allows it to produce more current than a lead-acid type of the same physical size, and normally is more resistant to vibration. The AGM battery will hold it’s charge longer and is more resistant to freezing. Most AGM batteries will require a charging system output of 14 volts, making them a poor choice for older machines. Over-charging can damage an AGM battery!
The Gel Battery: The Gel Battery is very good at holding a charge, and can be run to low voltages and recharged without loss of much capacity. They don’t produce as much power as the AGM or Lead-Acid of the same physical size. Over-charging can damage a Gel Battery!
Most motorcycle batteries should last 48 months. Often failure prior to this can be traced to incorrect or a lack of maintenance. Correct installation and preparation procedures and periodic checking and cleaning of the terminals are part of proper maintenance.
Modern machines impose great electrical demands. Even when the machine is turned off there are electrical demands on the battery beyond static discharge. Use the correct charger or tender for your battery and extend the life of the battery beyond the expected 48 months.

Reference: http://www.motorcyclecruiser.com/tech/1102_crup_motorcycle_battery_basics/index.html