There are two types of VRLA batteries,
Gel Cells and AGM.
Gel cells add silica dust to the electrolyte, forming a thick putty-
AGM, short for "absorbed glass mat", batteries feature Fibreglass mesh between the battery plates which serves to contain the electrolyte..
VRLA batteries are prone to thermal runaway which can cause sudden and large scale failure
Both designs offer advantages and disadvantages compared to conventional batteries, as well as each other
At high overcharge currents, electrolysis of water occurs, expelling Hydrogen and Oxygen gas through the battery's valves. This is why slow charging is much better for these batteries.
Many modern motorcycles use AGM batteries to reduce likelihood of acid spilling during cornering, vibration, or after accidents, and for packaging reasons.
Battery life and performance -
Cold weather is also hard on the battery. The chemistry does not make the same amount of energy as a warm battery. A deeply discharged battery can freeze solid in sub zero weather. If you have tried to start your bike and failed make sure you charge the battery as soon as possible.