A VRLA-AGM battery is an electric storage lead-acid battery

Sealed with special compound epoxy and using pressure controlled vent valves.
Starved electrolyte design-acid solution is absorbed in separators.
Using a recombination reaction to prevent the escape of hydrogen and oxygen gases.
Non spillable -can be operated in any position. But, upside-down installation is not recommended.
Maintenance free. But connections must be retorqued and the batteries should be cleaned periodically.

A VRLA-AGM battery uses recombinant technology. The oxygen produced from the positive plates of the battery is absorbed by the negative plates. This suppresses the generation of hydrogen at the negative plates. The recombination of oxygen and hydrogen leads to Water, retaining the electrolyte amount within the battery. Water filling is never required. Battery should never be opened as this would damage the battery with additional oxygen from the air. The warranty will be void if the battery is opened.