I looked into this a bit when I was questioning whether  the power wire on a new 7-blade plug was large enough. I found multiple posts on various websites and forums that explained that it is really the equivalent of a trickle charge coming from the tow vehicle. Given the voltage drops along the way, it will not fully charge a trailer battery. It should be viewed as a way to maintain rather than recharge the battery. This is what I've read. 

What has been people's experience? I haven't driven much with a discharged battery so I don't have personal experience.