Truck Camper Solenoid Question

Thanks for all the info and different view points.

So far, in the short term, it worked great on our last trip. The truck camper battery is now grounded to the frame.

As far as boiling the electrolyte, my charge controller for our solar system on our previous motorhome had the "balance" mode that would ramp up and overcharge to balance the battery bank. I guess the argument against charging the truck camper with the alternator through the truck batteries is that it's possibly in a constant state of overcharge without any way to regulate it.

At idle, the truck camper/truck batteries are at 14.1v after driving for a couple hours. I don't have a remote meter to monitor the voltage while driving, and the dummy gauge in the truck is analog, not digital.

wwest wrote:
Using the tow vehicle's alternator/VR system to recharge a low SOC battery often results in boiling off the battery electrolyte, the better the electrical connection, the more likely the electrolyte will boil.

The alternator charging characteristics are such as to prevent overcharging (and "boiling") of the starter batteries on a long drive, so it is unlikely to do it to house batteries which are essentially connected in parallel to the engine starter batteries (which often consists of two batteries connected directly in parallel anyway.

What can happen if the alternator is relatively small, and the house batteries are very large and discharged, is the alternator will exceed its long-term rated output (which is usually way under the nominal rating) and eventually fail prematurely.

I prefer no to use the chassis as the return conductor for large current circuits because under some circumstances it can cause corrosion of dissimilar metal components if it flows elsewhere than through the steel chassis.

Using the tow vehicle's alternator/VR system to recharge a low SOC battery often results in boiling off the battery electrolyte, the better the electrical connection, the more likely the electrolyte will boil.

Best solution is to run the ground to the negative battery terminal.

wwest wrote:
Better solution...

Wire the trailer power to an 800W HF inverter, power the trailer's inverter/charger with the 120 VAC inverter output.

The inverter will not drop out unless the source voltage drops below ~10.5 volts.

Controlled (re-)charging rate for the trailer battery(s).

Enough AC power for the fridge...

Charging a deeply discharged battery from the trucks FIXED voltage source will seriously shorten battery life.

Ask anyone having charged series 6V deep cycle batteries from an automotive alternator/VR system.

Boils the electrolyte.

The OP has a TC.
Your inverter idea is a sound one but he will need much more than 800 watts if the battery is ever low.
The fridge takes about 350 watts and a 50~60A charger (like a PDI) takes around 1500 watts under full charging current.

Better solution...

Wire the trailer power to an 800W HF inverter, power the trailer's inverter/charger with the 120 VAC inverter output.

The inverter will not drop out unless the source voltage drops below ~10.5 volts.

Controlled (re-)charging rate for the trailer battery(s).

Enough AC power for the fridge...

Charging a deeply discharged battery from the trucks FIXED voltage source will seriously shorten battery life.

Ask anyone having charged series 6V deep cycle batteries from an automotive alternator/VR system.

Boils the electrolyte.

4ga!

Something I ran into. My 4 batteries (group 24 wet cell)were charging fairly slowly while driving, at about 65-70% charge on the Trimetric I was only charging at about 10-15 amps. I think I might be getting a lower charge rate because I was tied int0 the truck batteries and they were at 13-13.3 volts. I might get a better charge rate if I tied to the alternator and picked up the 14+ volts that it has. The multi stage chargers can use higher voltages for charging. I don't have an answer here, maybe someone else does.

-I'm using new 4 gauge.

-Yes, the taillight ground in the camper plug.

-It breaks hot line, and is working well.

-I just added a 4 gauge ground to frame tonight.

Thanks for the advice.

Hmm made me think of questions more than answers-
The ground wire should be at least wire size of the charge line. You didnt mention if you upgraded charge wire size.
By tailight ground I assume using the ground within the trailer/camper plug?
IF the solenoid you added breaks the hot charge line (didnt say if you added new charge wire or are interupting existing line) your good.
If existing or prior line still connected and not controlled by 'solenoid' then, depends on whether or not your truck kills power when off? Assuming your truck doesnt or added solenoid to get more charge?.