Great - Glad it worked...
Each brake magnet will pull a good 3 Amps DC current with 12.3VDC applied from your trailer battery ... for single axle that would be 6AMPs total... a double axle will be 12AMPs DC current... Won't take too long to drain the battery...
Also the electric brakes are not designed to be left in the ON position for long time periods as well...
Since you got it this way you need to really check to see if the Electric Brakes are working now... They may be blown now or the previous owner did something to disable the 12.3VDC going to the brake Magnets...
Hopefully it is all working when being towed. It is DOT SAFETY REQUIREMENT to have a working Brake setup in case the trailer becomes disconnected from the tow vehicle while being towed on public highways. If something happens and the Insurance Company figures out the brakes were not in working order you may get stuck with the bill or worse yet someone gets hurt...
Roy Ken