Interaction of Solar and Alternator

"Even two straws cannot feed Sumo Wrestler"

You need to know facts

How many amps are flowing when driving on the highway in bright sunshine. "Amps?" means responding with a number.

Rustofer wrote:
I have measured approx 14V at the camper connection, so there is alternator voltage there.

Despite what you have measured, as a retired automotive engineer I can promise you that voltage will drop probably down to about 13V which is NOT sufficient for charging the battery in your RV.

SoundGuy wrote:

If you're attempting to use your vehicle's charging system for recharging a trailer's deep cycle battery you'd be FAR better off using a DC > DC charger, one example being the CTEK 250S which also allows for charging from your solar panels.

Redarc also makes a similar charger.

Concur !

On a sunny day in diffused trees shade, solar still works a little. Add another 100W panel. Your existing solar wire should be sufficient for 200W, solar controller may or may not need to be replaced with slightly bigger one.

If 2*100W won't fit, try single 250W - it will need MPPT controller type, cost from $100.

I would think that alternator could add about 35 AH in 5 hours, even with a thin wire. It is possible that battery was already near full, say, 80-85%, and didn't accept much current - this COULD be the fault of solar :), but at this point it's the battery that decides how much current to accept, no matter what charging source you use.

The long wire and low alternator voltage is the issue not the solar.
Really need to get some amperage measurements along with voltage into a low battery to diagnose the issue.

You need to measure the amps at the camper battery itself with and without. You could risk your 10a multimeter if it has a fuse for blowing the 10a, but with no fuse it could fry the ammeter part.

You can park in the sun and try it different ways engine on and off, solar on and off. The battery will accept charging amps from both solar and alternator until the battery voltage rises to the same as one of them. After that the higher voltage source will do it all.

Alternator voltage usually starts in the 14s and drops to high 13s so after that, the solar set at 14.x will do it all--which could be what you are seeing.

The positive wire is so thin on the truck/camper connection you won't get many amps. It is not like "alternator charging" with a motor home, but is more like a trailer 7-pin.

If you are in a provincial park in the woods for several days you really need a small generator to use in allowed gen hours, but there is not much room to carry one in a TC set-up.

Maybe swap the small 100w panel for a 250w (and a controller for that--needs MPPT) or get more 100w panels up there and a higher amp PWM controller, and spend more of the camping day out of the woods someplace. Even the campground parking lot, then return to your spot in time for supper.

Generally all battery chargers play nice with each other. One hundred solar watts for two 6 volt batteries (200 AHs?) is not enough. In a sunny area (AZ, NM, SoCal) the general standard is one solar watt for every battery amp hour. In BC I’d go 1.5 or 2. I have 300 solar watts for 240 battery AHs.

Solar controller is usually smarter than alternator. If anything, solar does not impede the alternator work.

Rustofer wrote:
Thoughts?

If you're attempting to use your vehicle's charging system for recharging a trailer's deep cycle battery you'd be FAR better off using a DC > DC charger, one example being the CTEK 250S which also allows for charging from your solar panels. Redarc also makes a similar charger.