JD, Electrical and Legal questions get the most responses from about any topic on here, I enjoy them and try to help when I can, so FWIW heres my take, which is worth what you paid for it ZERO lol
Even if its NOT how I may do it, I'm basing my answer on YOUR requirements:
"Is there a way to keep the batteries charged and not have to unhook the light plug when I sue the winch or lift???"
MY RESPONSE YES by using a toggle switch or an isolation relay, see below
"I am not going to run heavy battery cable to the trailer from the truck batteries."
MY RESPONSE If you ONLY want the truck to charge the trailer batteries while driving (say 20 to 30 amps max) and NOT rely on high current or the truck battery to assist or be in parallel with the trailer batteries, a 20 to 30 amp circuit and NOT 50 to 100 amp circuits and breakers etc will suffice
1) The FIRST thing I would do is (if not already) upgrade the charge wire circuit on all your trucks FOR 30 AMP CHARGING CAPACITY to the trailer batteries. I would use a 30 amp circuit breaker at the truck battery and run 10 Gauge wire back to the trailer plug. (HOWEVER, if an isolation relay is used as described below in Para 3, the charging circuit runs from the truck battery to and through the relay and then back to the trailer plug.) NOTE If you don't want to upgrade and only have say 20 amp rated wiring back to the plug, then use an appropriate rated breaker such as a 20 amp etc.
2) In order that heavy trailer loads don't trip that 30 amp charging circuit breaker above, you could use a 30 amp rated SPST On/Off (Open/Close) toggle switch in the circuit from the truck to the trailer batteries and simply turn it OFF when you're going to operate the trailer. That prevents the 30 amp relay in the charging circuit from tripping.
3) Another method to accomplish the above WITHOUT having to remember to throw the switch, is similar to how my RV is wired with an isolation relay between my engine and RV house Batteries (yours would be to isolate truck from trailer batteries when truck isn't running). You only need a continuous duty 30 amp minimum rated 12 VDC coil NORMALLY OPEN isolation relay. Its wired with its 12 VDC activation feed to the relays coil from a truck circuit that is hot ONLY when the truck is on/running. That way when the truck is started the relay closes so the trucks alternator and battery circuit can charge the trailer batteries, but when the truck is turned off the isolation relay opens so the truck and trailer batteries aren't tied together and no relays will trip open.
My RV isolation relay looks like an old Ford type starter relay except its 100% duty rated (you cant use a normal starter relay, theyre NOT rated for continuous duty) and is at least 100 amp capacity. They are a fairly cheap and easy way to isolate truck from trailer or house batteries and are simply wired so they are normally open when truck isn't running but closed when it is so the charging circuit can charge the trailer batteries. A switch is easier and cheaper and all you have to do is turn it off when using the trailer.
YOUR MONEY YOUR CHOICE even if I prefer the slightly more expensive isolation relay so you never have to worry about it or to tell Billy Bob or Bubba to turn it off (and then back on) when using the trailer!!!!!!!!
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Today's Featured Article - Harvestin Hay: The Early Years (Part 2) - by Pat Browning. The summer of 1950 was the start of a new era in farming for our family. I was thirteen, and Kathy (my oldest sister) was seven. At this age, I believed tractor farming was the only way, hot stuff -- and given a chance I probably would have used the tractor, Dad's first, a 1936 Model "A" John Deere, to go bring in the cows! And I think Dad was ready for some automation too. And so it was that we acquired a good, used J. I. Case, wire tie hay baler. In addition to a person to drive th
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