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Sealed lead acid (SLA) batteries only outgas an appreciable amount when overcharged/overdischared. Vented instead of sealed types can also be used indoors but require their own ventilation system. The majority of UPS systems use SLA batteries indoors.

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As long as you don't keep the battery in an airtight container you're generally good (vents on the battery container, room has ventilation)

But is it safe to inhale the gases?

Its Hydrogen, the concern is explosion https://www.ccohs.ca/oshanswers/safety_haz/battery-charging....

> Overcharging a lead acid battery can also lead to the generation of hydrogen sulfide, which can cause harm to workers if exposed.

The concern is also the toxicity of H2S. Most SLA chargers are poorly designed and will overcharge any battery that isn't in mint condition.

Agreed that cheap "trickle" chargers with unregulated voltage can cause overcharging if left on for too long (days to weeks on larger batteries, as their current is usually under 1A)

Better to invest in a "float" charger with a regulated output and automatic shutoff. As with many subjects, if you go the cheapest route you pay with time or safety risk.

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