Originally Posted by nbhybrid
you are talking about 2 different systems. Brakes- to stop the trailer come in electric and hydraulic. Hydraulic are for boat trailers primarily because of the water. Electric brakes do not use an on board battery. They get their power solely from the tow vehicle.
the other system is the break-away switch to activate the brakes. This does require an on board battery because the premise of it is that you are disconnected from the TV.
Her Pup I believe does not require a break -away by law because of the weight. Likely why the dealer didn't install a battery.(maybe the OP can come back on to confirm).
If she doesn't have a break-away I would either not install a battery at all or install a true deep cycle for boondocking not a standard battery which I assume she bought
lots of info here, but a few things i can tell you: the regular brakes, and the breakaway brakes, use two different actuation systems. electric brakes require a battery to activate the breakaway, because, as you say the assumption is that the trailer is not connected to the TV.
hydraulic breakaway systems (mine at least) do not require a battery. there is a master cylinder in the tongue that is normally activated by the weight of the trailer. in the case of a breakaway, the breakaway cable activates the brakes by pulling (compressing) the master cylinder. my hydraulic system does not require a battery.
i don't know the specs of the original trailer in question, but to assume that because it has no battery it has no breakaway system is incorrect. in fact, if it is hydraulic brakes, she likely has a mechanical breakaway system as i've described.