Re: [A/S] 7 way Plug

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Trapper,
  You should use either a solid-state isolator in your truck or a relay. 
I personally prefer the latter, for several reasons, but it's important 
to use one that's heavy enough. If the trailer battery is "low" when the 
relay energizes, the contacts can weld. A typical scenario is to back 
the truck to the trailer, hitch up, connect the umbilical cable, then 
shut off the truck while completing the 'getting ready to go' chores. 
Unbeknownst to you, the relay contacts welded, and the trailer pulls the 
truck battery low enough that you won't get started.

The problem with the isolators is that they must carry all the current 
from the alternator, for both the truck AND the trailer. They cause a 
slight voltage drop, which can cause inadaquet charging to both batteries.

I use a unit that looks like a Ford starter relay ("solenoid") that can 
handle over 100 amps and has a continous-duty coil. It's connected 
directly to the truck battery through a 40-amp auto-reset circuit breaker.

My connector is wired according to specs; even though the round pin 
appears to be able to handle more current than the others, the ground 
wire is still a flat-blade connection. Equal current flows through both 
'hot' and 'ground', so it makes no difference. If you want to seriously 
charge your trailer batteries, you need a LOT heavier wire anyway, it 
should be #4 welding cable and a forklift battery connector, seperated 
from the standard umbilical. The wire in the latter, and the associated 
wiring in the truck and trailer simply isn't heavy enough to do a good 
job. The slight voltage drop fools the alternator regulator into 
thinking the batteries are fully charged, so the alternator shuts down 
too early.

                                    <<Jim>>