There are some trailer brake away devices that use a small 12v sealed 4 or 7 amp hour battery to provide backup power to the brakes if the vehicle gets disconnected. These will include a small battery charging circuit to prevent overcharging for the battery as well as a “sense” to only charge the batteries when the vehicle is running preventing the drive battery from going flat. This is usually accomplished with a relay or solenoid that will trigger when there is power to the other trailer lighting (search battery isolation relay). As always correct fusing must be provided to prevent faults in the trailer from effecting the drive vehicle as well as other precautions.

However, solar charge controllers are so inexpensive these days on Amazon that I’ve converted my cargo trailer and work truck to contain a 100W panel and a sealed lead acid battery with a small solar charge controller and I can’t go back. It’s worry free and self contained from the rest of the vehicle. Nothing more frustrating then troubleshooting trailer wiring or tail lights just to find it’s something to do with the charger or something silly…

I wouldn’t bother with DC-DC battery chargers unless you want to be charging larger batteries and then I think it would just be pointless since the trailer harness can only supply so much current usually somewhere between 5-15A