My first trailer, I bought of them new, had a small dry rechargeable sealed battery. After several years, the break away pin had never been pulled, I removed the battery for the winter and checked the battery and found it was holding it's charge.
The 20' trailer with the system to charge the same type battery. I don't where it gets the power to charge the battery, I'm guessing it comes from the travel lights. I haven't tried to figure it out.
All I know is there is a very slow drain to the battery. I don't use the trailer very often, same with the first one. I just have to remember to unhook the battery, and hook back up when in use.
Dusty