Battery interconnect logic

If the battery combiner relay is engaged when travelling, the solar is already being added to the alternator's power (as the two are combined), so there's no need to do anything additional for that particular use.

There might be some circuit that senses somehow when the converter is running and/or possibly when the solar is charging and combines the two batteries at that point, too, to make sure the chassis battery stays charged. Typically that would be part of a larger charge control unit or a fancier relay generally termed a BIRD (which stands for bidirectional something something device, I think).

A picture of the relay with the wires might be helpful. I can't think of any reason to have four control wires for this one relay. Usually there would be just the heavy wires on either side to either battery, a single control wire, and possibly a ground connection if it isn't grounded through its mount.

The control wire, in one typical and relatively simple setup, goes to the emergency start button, which is a SPDT switch. The normally closed contact goes to the run circuit on the chassis, or something similar, and the normally open contact to the house 12V system somewhere (via a fuse, etc.). There are fair few possible variations; that said, what I've described is how my 1998 Coachmen is wired up.

I guess my real problem is that I don't know how the alternator and solar system cooperate. I want to make sure that I use all of the available solar before even thinking about the alternator. For instance, I am considering running the fridge through an inverter - at least to the extent possible on solar. If not enough, I would just switch to propane. I just don't want to drive down the road with full batteries and nothing to load the solar with...if I have it right!

https://www.ebay.com/itm/Sure-Power-1315-200-Bi-Directional-Battery-Separator-12V-200-Amp-/272122850467

Read the directions. Install and it solves 3 dozen questions and issues automatically. Make sure the model you arder is BI-DIRECTIONAL

Thanks for the link. I'll check it out. I think I will try removing the control wires and see how it goes. The idea is to let the alternator work on the chassis batteries and the solar on the house - if that makes sense...

Just like the relay does only autotically.
When your converter is finished charging the house batteries then it maintains the engine battery......automatically. it's a tough system to try and top. Need to jump start house to engine? Add a wire and it does that too.

I'm going to have to think about this some more. My main goal is to maximize my solar system (340w) so I really don't want to get the alternator involved at all when it comes to the house batteries. If the solar can't handle the load even with some logic I would just could not use it and would go to a backup strategy- like propane. I'm just trying to understand how things work together..

I do believe you're overthinking this. Multiple DC power sources (such as the alternator and solar controller) don't specifically have a hierarchy of which one supplies what power, but rather the system just reaches some point of equilibrium based on the attached load (including battery charging) and the current vs. voltage characteristics of the sources. For sources controlled by a computer or microcontroller, such as the solar charge controller or a modern vehicle's charge controller, those curves can change dynamically.

At any rate, the solar controller will supply what power it can to the system. If you're driving and the solar doesn't happen to provide enough to run the fridge, the alternator would make up the excess. I don't know that there's a particularly easy way to get it to switch to propane at that point, nor does it really seem sensible to me to do so; switching momentarily to propane every time you drove under an overpass seems like a questionable thing to do. On the other hand, I think it's a mighty useful thing to have the alternator charge the house batteries while driving, and it doesn't noticeably affect fuel mileage (especially for a motorhome, where the fuel consumed powering the alternator is negligible compared to that consumed pushing a big square box through the air at highway speeds).

As an aside, I gather you're running your RV fridge via an inverter. Do keep in the back of your mind that an RV absorption fridge uses a lot more electric power than a compressor style fridge; that's the price you pay for being able to run it on propane. If you're not able to keep up power with the solar installation, replacing it with a residential fridge might make good sense. I think most people run their RV fridges on propane when not hooked up to shore power or running a generator, rather than operating them through an inverter.

Thanks for the replies and input.

My real goal is to try to make maximum use of the money I spent on the solar system. The fridge is just something that seems doable (vs hot water for instance). So if I'm driving along and by - say - 11 the house batteries are fully charged the power available from the solar system has no place to go. My fridge uses 330 watts while running on AC - certainly more than I can produce on solar at any time. My idea was to use an inverter to run the fridge for a time based on keeping the batteries at a suitable SOC to get through the night if no hookup is available and the weather cooperates - no AC required. I can just turn the ac power off to the fridge to have it switch to propane as needed. Doubtless, this is not a very practical application but I enjoy such things. I would prefer to keep the alternator out of the equation as no matter how much it still requires fuel to work. (I'm into home automation and am instrumenting the unit to monitor/control electrical flows and device states where practical)

Is there something better to do with excess solar? Other ideas?

What’s your budget?

If you want to really improve your battery charging, prioritize the solar over the alternator input, and improve the charging performance over what the alternator can do on its own, instal a DC-DC charger that has both alternator and solar inputs.

A further benefit of the DC-DC charger: If you have a 3-way fridge, the DC mode will work noticeably better due to the charger maintaining the voltage at 13.5-14.6 volts vs 12 or less without it. It surprised me when I discovered my fridge DC mode actually worked after installing mine. A 25-30 amp DC-DC charger can do that all day long, drawing power from your solar panels when it’s available, and the alternator when it’s not.

:):)

Not looking for more equipment - just trying to make the best use of what I already have...and no DC fridge.