Truck power to trailer

Yes, Looks good now.

waynefi wrote:
OK, I tried a couple of more experiments. The trailer has been plugged in for weeks.

Unplug, open the battery cutoff, let it sit for a few minutes. Voltage at battery 12.9. Connect the truck, close the battery cutoff, voltage at battery 13.7. Voltage at truck battery 14.1, so I still have a 0.4 drop.

The time I got 13.2 at the trailer battery I didn't measure the voltage at the truck battery. I had just been driving the truck, so if it's charger voltage drops down after being driven a while, it could have been lower. The 13.8 measurement was earlier, while I didn't have any connection at all.

A later experiment, with the battery disconnected. At truck battery 13.88. At trailer battery terminals with nothing on but the propane detector, 13.83. Turn on the bathroom fan, it drops to 13.69. So there is still a fair amount of resistance there.

Those numbers are much better! All looks good now IMO.

OK, I tried a couple of more experiments. The trailer has been plugged in for weeks.

Unplug, open the battery cutoff, let it sit for a few minutes. Voltage at battery 12.9. Connect the truck, close the battery cutoff, voltage at battery 13.7. Voltage at truck battery 14.1, so I still have a 0.4 drop.

The time I got 13.2 at the trailer battery I didn't measure the voltage at the truck battery. I had just been driving the truck, so if it's charger voltage drops down after being driven a while, it could have been lower. The 13.8 measurement was earlier, while I didn't have any connection at all.

A later experiment, with the battery disconnected. At truck battery 13.88. At trailer battery terminals with nothing on but the propane detector, 13.83. Turn on the bathroom fan, it drops to 13.69. So there is still a fair amount of resistance there.

Still 0.2 volt increase seems like a high resistance connection. I would remove the battery cable again and see what the voltage is running a few lights.

What bothers me is that 13.2 when connected to 13.8. I would normally consider that to mean the battery was low on being connected --maybe even 50% SOC.

The OP says it was 13.0 to start with, so that does not compute. IMO stick an hydrometer in it to remove all doubt. I would at least go camping near where there is a battery store, or else take along a spare. But again, I do not have a good picture of what is going on.

You can have high voltage, but where it does not hold that high on loading. The OP seems to have a clue, so this is just me being a worry wort.

200 watts of solar and a good MPPT controller (SunSaver is one) will solve lots of problems. I almost never let the vehicle connect to charge the camper anymore. The solar going to the MPPT charge controller, which has 4 stages of charging, and monitors the battery, is a much more effective charging system than your alternator. Your batteries will have a longer life.
Plus the solar will charge as long as there is sun, no more relying on vehicle power or plugging it into a wall and using the on-board low-tech camper charger.
But I did install heavy gauge, direct run, cables on another selectable switch and on its own heavy-duty (big gauge) trailer plug. I can charge much quicker from the truck to camper now if I so choose.
Your current truck wiring for the trailer and the plug setup is just to going to cut it.

BFL13 wrote:
Yes, the 7-pin charging is at low amps, so not much gets done while driving. IMO something is not quite right about the voltage numbers, but hard to tell. Nothing to stop going camping whatever it might be.

I agree that it's not a big problem. Coming back from our last trip, we had three days of dry camp for one night, drive 300 miles. So if I could get even 5 amps of charging for a 6 hour drive, that would replace most of what we used. (We don't use a lot).

With 12-gauge wire, the most that this connection can accomplish.....

Is slow down the rate of discharge of the trailer battery bank.

The statement is derived from actually doing it. Many repairs of customer's systems.

A 10% to 20% reduction in drawdown would be fair. And that depends on the electrical load in the trailer.

Yes, the 7-pin charging is at low amps, so not much gets done while driving. IMO something is not quite right about the voltage numbers, but hard to tell. Nothing to stop going camping whatever it might be.

BFL13 wrote:
Good work. However, note that the full battery should read 12.7ish not connected. The truck is at 13.8. When connected the full trailer battery would be say 13.7ish (allowing for line loss).

If it only goes up by 0.2v that means it is not full.

The battery voltage went up from 13.0 to 13.2. It had been on a charger recently, so 13.0 isn’t too surprising. I’m interpreting the 13.2 to mean that there is still resistance in the 7 pin. or wires to it.

The reason I’m trying all this is to figure how much charging I get from a day of towing. The answer seems to be “not very much”, but that it depends a lot on how clean the connectors are.