<Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> In the absorbed glass mat design, or AGM for short, the separators between the plates are replaced by a glass fibre mat soaked in electrolyte . There is only enough electrolyte in the mat to keep it wet, and if the battery is punctured the electrolyte will not flow out of the mats . Principally the purpose of replacing liquid electrolyte in a flooded battery with a semi-saturated fiberglass mat is to substantially increase the gas transport through the separator; Hydrogen or Oxygen gas produced during overcharge or charge (if the charge current is excessive) is able to freely pass through the glass mat and reduce or oxidize the opposing plate respectively . In a flooded cell the bubbles of gas float to the top of the battery and are lost to the atmosphere . This mechanism for the gas produced to recombine and the additional benefit of a semi saturated cell providing no substantial leakage of electrolyte upon physical puncture of the battery case allows the battery to be completely sealed, which makes them useful in portable devices and similar roles, additionally the battery can be installed in any orientation, though if it is installed upside down then acid may be blown out through the over pressure vent . </P> <P> To reduce the water loss rate calcium is alloyed with the plates, however gas build - up remains a problem when the battery is deeply or rapidly charged or discharged . To prevent over-pressurization of the battery casing, AGM batteries include a one - way blow - off valve, and are often known as "valve regulated lead--acid", or VRLA, designs . </P>

What gas does a lead acid battery give off