I am reading threads similar but finding nothing useful…

I have 200 watts of solar on the camper, which charges the 2 Golf batteries, but is also connected through the trailer wiring to the tow vehicle's battery (though much loss in the wiring).

1. What goes on between the panels, charge controller and alternator? Does the alternator sense the charge/voltage from the solar and assume the battery is full?

2. What I am thinking is to put more solar on the vehicles roof (lots of room on the truck's camper shell) to supplement the alternator and perhaps the alternator does not need to cycle on while I am driving on a sunny day. I am told my alternator takes about 6 HP to run at full.
As a bonus, the solar will keep the two vehicle batteries topped off when parked too.

I also planned to put a relay switch to disconnect the alternator charging connection to the trailer, as the solar is plenty to take care of the camper batteries, thus less drain on the alternator, which means less drain on the engine. I'll do this anyway unless there is some reason not to.

thanks, and always, looking for knowledge over opinions, I have plenty of opinions and knee-jerk thoughts to deal with on my own….lol

 

1. The following assumes that the 3 batteries are effectively connected in parallel (that's how it sounds)...

The alternator puts out a specific voltage (13.8-14.8V, about 1-2v higher than typical battery voltage) and if any batteries connected to it happen to have less voltage than that, they will be charged. The lower the battery voltage, the higher the alternator current. If your trailer batteries happen to be very low, you might blow a fuse in your truck trying to charge them. I do not think the wiring (and hence fuse values) are really designed to handle much more than some lights.

The charge controller is probably more sophisticated than the alternator and likely runs in constant current mode to charge the batteries. Depending what that current is, could probably also blow fuses in the truck If the truck's battery is low.

2. You will almost definitely not see any MPG gain by solar charging while driving. The alternator does not "cycle on/off" while driving; it's always on, and it's probably always going to be outputting more voltage than your charge controller.

...but even if not... 50A is a measurement I have taken from my truck's alternator while idling with a good battery. I think that is typical. 50a x 14v =700W. Alternators are not any more than 50% efficient so it might be 1.5kw mechanical load on the engine. That is negligible for a truck outputting 100-200kw to pull a trailer down the highway.

Installing a relay is a waste of time IMO. If any action at all needs to be taken, maybe it is better to permanently disconnect the trailer power (positive) from the truck power to prevent the truck fuse blowing in the case of dead trailer batteries or dead truck battery. But then the solar panels won't keep the truck battery topped off. Your choice.

 

The truck fuse is not of concern so much, I have a relay to the trailer 12+ so all the trailer charging, lights, reverse lights are on dedicated trailer circuit.
I'm not an alternator guy, but a re-builder explained that the alternator would cycle on and off (though spins all the time) depending on demand. Even though it is spinning, if the windings are not required to charge then they are just spinning. Its the electrical load that is cause of resistance to turn the alternator. Maybe some can turn on and off, while other alternators just stay on. This way the alternator can either float or charge. But Im not an alternator guy…
I once tried running the RV fridge off DC and burned out the alternator (melted the insulation on the copper windings), so there is more to it than just alternator outputing what it outputs. This was in an RV, not truck n trailer.
So, based on my loose understanding of what he said, I was thinking to use the solar to maintain the charge in order that the alternator not have to put out much voltage, which would in turn require more power to spin it.
Alternator will work harder, and use more HP, when connected to a dead battery as opposed to a fully charged one.
Here is my battery configuration: Vehicle has two 12V connected parallel. Trailer has two 6V in series.
Trailer power is pulled off of vehicle battery node (connected at starter motor), which goes through a battery isolator/ solenoid then to the trailer 7 pin connector. The relays are for disconnecting 12+ to the trailer while driving is I so desire as I don't need 12+ to charge trailer battery, and reverse lights are on manual control (which has no bearing on the topic).
Cost is not an issue as solar power in the truck for other reasons is justified.
thanks

 

On a Round 7 Blade Type RV plug there should be a ground of 12 gauge, 12v of 12 gauge with Fuse at battery and a 12 gauge for the brakes for the RV The others are connected for lights and turn and back-up at 16 gauge wiring.

In most cases the 12v from the Battery goes to a Auto Reset Fuse for this that would mount to the truck. Not all setups are setup right and most times the 12v is omitted. Then once the thing is setup right and your RV wiring is setup right to take the 12v from the truck there is a part in the RV that takes the input to the RV battery, From the battery there is a connection that goes to 12v Panel Charger when your on shore power...

To add a Solar system you would need to get a Converter to tie into your system all inputs.

 

On a Round 7 Blade Type RV plug there should be a ground of 12 gauge, 12v of 12 gauge with Fuse at battery and a 12 gauge for the brakes for the RV The others are connected for lights and turn and back-up at 16 gauge wiring.

In most cases the 12v from the Battery goes to a Auto Reset Fuse for this that would mount to the truck. Not all setups are setup right and most times the 12v is omitted. Then once the thing is setup right and your RV wiring is setup right to take the 12v from the truck there is a part in the RV that takes the input to the RV battery, From the battery there is a connection that goes to 12v Panel Charger when your on shore power...

To add a Solar system you would need to get a Converter to tie into your system all inputs.

I think you replied to the wrong thread. In this thread the vehicle has a dedicated trailer circuit wiring, not from the factory, and the trailer has solar already.

 

The truck fuse is not of concern so much, I have a relay to the trailer 12+ so all the trailer charging, lights, reverse lights are on dedicated trailer circuit.
I'm not an alternator guy, but a re-builder explained that the alternator would cycle on and off (though spins all the time) depending on demand. Even though it is spinning, if the windings are not required to charge then they are just spinning. Its the electrical load that is cause of resistance to turn the alternator. Maybe some can turn on and off, while other alternators just stay on. This way the alternator can either float or charge. But Im not an alternator guy…
I once tried running the RV fridge off DC and burned out the alternator (melted the insulation on the copper windings), so there is more to it than just alternator outputing what it outputs. This was in an RV, not truck n trailer.
So, based on my loose understanding of what he said, I was thinking to use the solar to maintain the charge in order that the alternator not have to put out much voltage, which would in turn require more power to spin it.
Alternator will work harder, and use more HP, when connected to a dead battery as opposed to a fully charged one.

Well sort of.

All alternators stay on all the time trying to put out around 14 - 14.5 volts at up to whatever current capacity that can sustain given either their design limits or input RPM.

They don't cycle themselves on and off but use a PWM (Pulse Width Modulation) control of their internal rotor current as part of the voltage regulator feedback loop that tries to keep the system voltage stable. My guess is that where you get the on/off functioning idea from.
IF they did that your lights would be constantly getting bright then dim then bright as you drive at night and they don't unless the alternator is trying to run a load beyond its working capacity and the load is pulling the voltage down. High powered audio systems exhibit that effect very well.

As for burning one out it's like Strantor said. Automotive application alternators are not that efficient. most are in the 50 - 60% range at best and are not designed to handle running at their rated current limits for more than a minute or two.
I consider them by design to be over priced under built junk compared to the standard commercial application ones that a big truck or piece of heavy construction or farm machinery would use.
Most to those run at 75 - 90+ efficiency and by design can handle 100% output continuous duty operation that would burn down a automotive unit of like ratings in minutes.
The sad part is a good quality commercial application alternator can oftentimes be had for a fraction of the price of a automotive grade alternator as well.
I now put 150- 160 amp commercial units on our farm tractors as their old stock units go bad and they only cost around $140 from Napa. Whereas the OEM units are the same ones used in automotive applications, but put out 1/2 - 2/3 the power and cost 1.5 - 3 times as much, cant handle long term high load operation which is normal on farm equipment that has been updated to have as many as 8 - 10 55 - 65 watt halogen floodlights.

Now for fuel savings you won't see a thing in reality given how small of parasitic load charging even a huge set of batteries is for a pickup engine. Especially so given that with the inherent voltage drops that come from the long supply wire going to the trailer even at a flat dead state of charge those extra batteries likely never pull over 30 - 40 amps peak for very long to begin with.
The total energy required to charge three big group 31 100+ AH deep cycle batteries after engine efficiency losses is the equivalent to burning a quart or so of gas of which if your towing rig burns fuel like mine that's about 2 minutes worth of highway driving on a good day.
If you can't afford the price of that you probably can't afford your pickup and camper let alone the solar panels either.