Well it's there to hold the battery and protect it against the elements
You need a battery when you drive along the road as it will activate the trailer brakes if ever there is a TT/TV separation (through the safety link)
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2014 JayFlight 28BHBE (replacing a 2009 Kodiak 25QS)
Started off with 2011 Sierra SLT CrewCab 4x4
Now pulling with a 2018 F150 XLT Special Edition EB 3.5
1200/12,000 Equal-I-Zer Hitch
And in the event the battery is in an enclosed compartment (like in a fifth wheel basement) the battery box at the top should be connected to a vent hose to the outside, and to an open vent at the bottom to admit air flow - this allows hydrogen gases (which are corrosive and explosive) to harmlessly vent to the outside.
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There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.