Motorcycle batteries don't last forever because though the chemical reaction going on inside them are reversible, they're not 100% efficient. Each time the battery is charged a few lead sulfate molecules refuse to change back to lead and sulfuric acid. As time goes by, more and more sulfates build up on the plates, and the battery loses its ability to take and hold a charged and must be replaced.
Better-quality conventional batteries sometimes use chemicals intended to reduce sulfation and water consumption. My own experience with these chemical composition does reduce maintenance and water consumption, but they still should be checked at regular intervals.
Maintenance-free batteries use newer technology to eliminate water loss and the need to periodically replenish water lost during normal operation of the battery. These newer technologies involve electrolyte that has been absorbed by porous glass mat between the plates in each cell. One of the terms you may see in battery catalogs or advertising is absorbed glass mat (AGM), which functions without vent caps of a vent hose. Batteries release oxygen at the positive plate and hydrogen at the negative plate during charging. In a conventional battery, these gases are vented into the atmosphere during charging. That's what the vent caps and vent hose are for. However, a process called gas recombinant technology (GRT) eliminates water loss be recombining the oxygen and hydrogen molecules to form water. Some people refer to maintenance-free batteries as sealed batteries. This is actually a misnomer because a totally sealed battery could be hazardous if the recommended charging rate were exceeded. Another term you may see in battery catalogs or advertising is valve-regulated lead acid (VRLA). The valve mentioned I at the topmost part of the battery and is so small that many people don't even know it's there. The valve on average motorcycle battery is about 12.7mm wide and slightly more than 0.508mm high. It's not really a vent, it's a check valve, and some batteries have two valves as a safety precaution in case one becomes clogged or stuck closed. If too much pressure builds up inside the battery, the valve opens to release the excess pressure, and then closes to avoid getting air into the battery. This is important because if the oxygen present in air entered the battery it would upset the balance of hydrogen and oxygen in the battery and contribute to premature battery failure.
The common reason why too much pressure builds up inside the battery is too high a charging rate. IF charging current is too high, oxygen and hydrogen are released faster than they can recombine back into water. The resulting gas pressure is released to avoid an explosion but after the check valve closes the battery now has two problems. First, it's thirsty because it has lost some water. Second, there may be a partial vacuum in the battery because of the action of the check valve. It's a bad sign if a battery looks sucked in. In cases where there's an extreme difference in battery pressure and atmospheric pressure, plates can be shorted out within a cell, which means it's time to go shopping for a new battery for your motorcycle.
Although maintenance-free motorcycle batteries don't need water and have no vent tube, they still need some care if the rider wants maximum life and reliability. Look for cracks in the case on all four sides of the battery. A battery that bulges or looks sucked in, should be replaced. The only other maintenance you need to do is keep the battery terminals and cables clean and properly tightened: keep the outside of the case clean to reduce self-discharge across dirt and grime, and keep battery hold-down hardware properly tightened to reduce battery vibration.