I'm getting rid of my flatbed and installing my winch and tool box on my brother in laws brand new trailer.  This new trailer has a break away battery system for the brakes and according to the description, charges through the trailer plug.  I had the winch battery setup to charge through the plug on my old trailer but it was the only battery.  Can I just tap into the new trailer harness and run a charge wire to the winch battery so both get charged?  I have a feeling it won't be that simple, but that's why I'm here.  I'll verify the break away is charged by the tow vehicle as I know not all do.  Would it perhaps just be easier to get a small solar charger for the winch battery to keep it maintained?

See if this helps

 

 

It turns out my BIL still had a weather proof solar charger he used to keep his old tractor battery charged.  He sold the tractor so I put the charger on the trailer for now.  It's parked in an open field, so sunlight all day long.  The plug hangs just outside the battery box so we can unplug it and go.  If he want's it wired into the trailer, I figure he can do it since he is much more experienced in the electrical aspect than I am. 

 

I finished installing the winch, tool box and battery box, everything should be good to go next time we need it.  I like this trailer more than mine, it's much lighter.  Mine was about 3500lbs empty, I think this one is about 2k, can hardly tell it's there when empty. 

Yes you can charge them together, but I would personally remove the brake only battery and just run one single deep cycle battery for the winch and brakes. 

 

With a solar charger I would make sure it doesn't overcharge and shorten the battery life. That will likely happen without a charge controller. 

 

 

That's not a bad idea.