KEEP BATTERY CHARGED WHILE USING THE NORCOLD 3-WAY ON 12V

A point to remember is that the alternator charge is designed and controlled in order to properly charge and maintain your vehicle battery.  The tow vehicle charge system knows and cares nothing about the trailer battery.  When you ask the voltage to be higher in order to charge the trailer battery, you are stressing your vehicle battery as it is then being over charged.

The exception to this is turning on the vehicle lights which increases the vehicle amp draw causing the electrical system to increase voltage to maintain the vehicle battery at it's desired voltage.  The only certain way to fully charge and condition a trailer battery from a tow vehicle is to use an external boost system as described in the very first post.

Hey all,
This is proving to be an interesting thread with lots to unpack and think about.  The differences in vehicle design and battery management are an ongoing issue that will no doubt prove to be a subject of ongoing research.  
That said, I don't mean to add to the obfuscation here but there are some basics that need to be looked at as well.  First, assuming  there is a direct battery connection between the TV (tow vehicle) battery and the 7 way plug, that means that whenever the trailer plug is connected there is a parallel circuit connecting the two batteries, whether the engine is running or not.  It's possible that on some vehicles, when the ignition is off the trailer battery circuit is disconnected but for this discussion anyway, that's neither here nor there.  Assuming there's a parallel circuit connecting the two batteries, the batteries will work to even each other out.  The higher voltage battery will push to charge the lower voltage one until they are of matched voltage.  The closer they are voltage-wise, the longer that will take.  Nevertheless, they will push/pull until they match voltage.  As I said, this will happen regardless of whether the engine is running or not.  If the TV is running, the ecm will sense the (approximate) average voltage of both batteries.  In layman's terms, if the TV battery is 14 volts and the trailer battery is 12 volts the ecm will see about 13 volts and respond accordingly. Regardless of what kind of vehicle it is, the ecm is smart, but not THAT smart.  It (the ecm) cannot see separate voltages for the two batteries, only the average of both.  This, again, assumes they are wired in parallel and not run through a battery management system such as a smart controller.  
What I've not seen discussed here that needs to be is the very basics of circuitry.  If you have a 10 gauge wire going out to charge your trailer battery, then you need "at least" a 10 gauge ground wire coming back through the trailer connector.  Most trailer connectors (in my experience) have quite small 16 or maybe 14 gauge wires going out to the trailer.  This includes the ground wire.  Remember that all of the voltage out to the trailer for lights, refrigerators, charge lines etc, must run back through the same single ground wire to the TV.  If you have a 16 or even a 14 gauge ground wire in your trailer connector, then when the lights are on, that's about all the capacity that ground wire has.  There are a lot of lights on a trailer and it's a long way back there.  Now, in addition to the lights and other stuff, you add the current requirements of a 20 or 30 amp charging circuit and you're asking too much of the ground circuit.  You've got 6 wires going out hot to the trailer and all of that potential voltage (amperage) must return on 1 ground wire.  If your lights take a maximum of, say, 12 amps, then you add 20 amps from the charging line, you need a ground wire with a minimum capacity of 32 amps.  It stands to reason that the total potential capacity of the ground wire must be equal to the total capacity of all the hot wires combined.  If you limit the capacity of the ground circuit, you limit the amperage going out to the trailer.  The voltage, as seen with a meter or gauge, will be the same or nearly so between the TV and trailer but the amperage available will be drastically less.  It's the same as filling two 1 gallon jugs of water from the same pump, but one hose is 1/2" and the other is 1/8".  The pressure (voltage) will be the same in both hoses but the volume (amperage) will be very different.  The larger diameter hose will fill its jug first every time.  
It's safe to say that many folks are experiencing problems charging because the trailer ground is too small for the capacity of the combined load of lights and charging circuit. This is easy to check without doing a whole bunch of wiring.  Just add a jumper wire (10 gauge) that bolts from the trailer frame to the TV frame.  I suspect you'll see a dramatic reduction in charge time. 

I have run 10 gauge wire from the trailer plug to the battery for BOTH my charge line and my trailer ground (dedicated ground) on: My Toyota Rav4, my Jeep Cherokee and my Jeep Wrangler and have never experienced problems with my trailer battery draining while towing.  I leave my refrigerator on all the time, run electric brakes and have a full compliment of lights.  

WilliamA

Edit: Fixed. @MuttonChops, your link is goofy. The "http://" prefix is duplicated, I think. 

I have to ask, what's it for? 12v to 12v...?

ChanW said:

This thread is very interesting. Thanks for the detailed explanations from all you smart folks. I've always had a rudimentary understanding, or 'just enough to be dangerous', as they say.

I'll have to check our Tacoma for increased voltage with the headlights on.

It might explain why I couldn't seem to get a handle on keeping up with the three-way fridge. It seemed to keep up fine sometimes, and not keep up other times. Could be I simply didn't have the headlights on! (even though our 7-pin supply and ground are 10ga).

Agreed. I am going to try with the headlights, on, too on my GC. Usually use the Automatic Lights feature but it might make a difference to turn on the lights.