From Eagle Eye Power Solutions:

The Eagle Eye HGD-3000 Hydrogen Gas and Smoke Detector with (optional) Silent Intrusion Alarm is designed for unattended battery installations or remote shelters containing gassing lead acid batteries and charging systems...

Lead-acid batteries on charge emit hydrogen gas after reaching the 80% recharge point. When mixed with enough air, hydrogen becomes a highly flammable gas that can ignite easily with just a spark, destroying equipment and harming personnel. The lower explosive level (LEL) for hydrogen is 4.1% by volume. 

Hydrogen Gas Detection
Attach the HGD-3000 to the wall, ceiling, or optional junction box using the mounting holes at the top and bottom of the detector box. The HGD-3000 will turn on an exhaust fan when hydrogen gas levels reach 1% and will alarm at 2%. This alarm consists of a local 80db horn, a flashing red LED, and a dry contact switch closure for remote alarming.

Smoke Detection
The photoelectric smoke sensor detects minute combustion products from smoldering wire insulation, battery cases, and other material. When smoke is detected, a distinctive alarm is emitted from the 80db horn and a separate dry contact switch is activated. At this time, the exhaust fan is inhibited in order to deny the fire increased oxygen from outside air.

Temperature, Loss of Power, & Intrusion
During normal operation and in the absence of an alarm condition, an internal thermostat will turn on the exhaust fan at a preset temperature to reduce heat buildup in the room. Loss of power to the unit will also generate a dry contact alarm. Additionally, an optional infrared intrusion alarm will trigger a silent dry contact switch.

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