Trailers with electric brakes have a battery backup connected to a break away system. If the trailer comes unhooked a cable (tied to the tow vehicle) pulls and engages the system and the brakes engage on the trailer.I've never heard of a trailer brake battery?
This system keeps an unhooked trailer from freewheeling down the road.
Now that LED lighting is so prevalent the manufacturers have hooked the interior lights up to that battery instead of drawing power from the tow vehicle.
This is very nice for three reasons:
Using the interior lights cycles the battery (that is charged by the tow vehicle) which before was never used unless the breakaway system was engaged. This extends the battery life greatly.
If your interior lights dim you know the battery is going bad or not charging. The system is part of the inspection process so you can address any issues before you get the trailer inspected.
The trailer doesn't have to be hooked to the tow vehicle for the interior lights to work. This is really nice since the old system would draw straight from the tow vehicle's battery and could kill it. Some vehicles required the key to be on for the lights to get power which really sucked.
The battery and break away connector look like this: