Methane recharge station
From Halopedia, the Halo wiki
The Unggoy homeworld, Balaho, is a methane-rich planet, and as such, they breathe methane gas. Thus, when they are in an oxygen-rich atmosphere, like that of the Halo installations or Earth, they need to wear a methane gas tank. When Unggoy are in battle, they may run low on their supply of methane, so they must have a ready supply nearby if required. These tanks contain the ready supply of methane gas the Unggoy need.
The device can refill the methane tanks of four Unggoy simultaneously. It is highly explosive and leaks gas when it is about to explode.
A battlefield methane tank in use during the Fall of Reach.