I'll touch on a couple of things on your "wish list"

If solar voltage above X use solar,
When solar voltage below X use engine
When hook up voltage present use hook up

For the first two - If solar voltage above X use solar, When solar voltage below X use engine - an option you could use is the Redarc BCDC1240D B2B/MPPT (Engine/Solar) Charger.
This charger is a 40A unit and has a Solar Priority setup - so it will automatically use Solar as much as possible and will use the alternator to top up the difference between what the solar is providing and what the battery is asking for, up to a total of 40A
In truth, the benefit of doing this is quite slight and using either seperate B2B and Solar Controllers or a Combo B2B/Solar that works with only one input at any one time will work just fine.

Your next point - When hook up voltage present use hook up - I would suggest is not really the best way to have the setup.
It you are plugged in to mains, then you are very unlikely to be running the engine, so there is little point of working out the best way to manage that combo. That leaves you with Solar and Mains that can be potentially active at the same time. As Solar is "free" power (at least once installed) it is better to have Solar Priority where possible. The way I set that up where possible is to have the Solar Controller configured at a higher charge voltage than the mains charger. That way any topping up and trickle/float charging will come from the solar when that is available and from the mains when solar is not active.

Basically Mains and Engine are always going to be mutually exclusive in practice. Solar is potentially available when parked AND when driving and should be given priority over Mains or Engine. If using separate controllers, you can with a good one set up solar to always be the priority.
Use of an Arduino is possible but would requires a series of relays and control IO setups and would be a pointless (IMO) complexity for very little, if any, return.

 

I think you're over thinking it a bit if you don't mind me saying. I have a CBE system with mains powered smart charger, split charge VSR and 150w solar with MPPT controller.

As said above with an MPPT solar charge controller it will keep your battery nicely charged and will maintain it properly. Mine even triggers a desulfation process once a month. If I'm on hook up then the mains charger is doing most of the work and of course you have endless power if the sun isn't shining. I leave my van plugged in all winter so the leisure and vehicle battery are kept in good nick.

The split charge will just help keep the leisure battery charged and let you run your fridge flat out when your on the road (with the solar panel probably adding power as well), but my understanding is that a VSR won't charge your leisure battery in the optimal way to maintain it and help it live a long life, so you should never just rely on the alternator even if you have no other options. ie in winter take the leisure battery out and give it a good maintenance charge.

In other words with the system above you fit and forget. The solar will let you stay off grid for several days, or indefinitely if it's sunny every day, the mains hook up is a no brainer if available, (especially in the South of France when the fridge will be struggling), the VSR will keep things charged on the road and let you turn the fridge up to max enroute to your site (I often have a plug in electric cool box on the go and unplug when we arrive).

 

I have a pretty simple set up. Leisiure battery connected to the vehicles main system by a voltage sensor system so when the engine is running it charges the second battery. There is also a bit trip switch in this system to limit charging and allow me to isolate the leisiure system if required.

Additionally I have a solar panel with MPPT controller, all loads are fed through the MPPT which means the batteries never go dangerously flat. This system works for me.

I don't have massive demands on the system though, couple of lights and a small electric cool box is all it has to power.

Rhys

 

Hi @TBRider,
I have very similar setup like RhysHughes + 220V - > 12V charger. I am using Victron products in T4 van. Just an idea about Victron install in a van, but I guess this will cost a fortune because of the expensive batteries set – The Victron Van install | Victron Energy and

Let me share some details about Victron products I have used
I am using AGM battery – 200Ah
Victron Battery coupler Cyrix 12/24 V 120 A - https://www.amazon.co.uk/Victron-Intelligent-Combiner-Cyrix-ct-24V-120A/dp/B0738JGVZ9
Victron Energy Phoenix True Sine Wave Inverter 12/375 - https://www.amazon.com/Victron-Energy-Sinewave-Inverter-VE-Direct/dp/B01N0W4H34?th=1
BlueSolar MPPT 75/15 - BlueSolar MPPT 75/10, 75/15 & 100/15 (12 / 24 Volt + load output) - Victron Energy
Blue Smart IP67 Charger Waterproof

Blue Smart IP67 Charger Waterproof - Victron Energy

The Blue Smart IP67 Waterproof Charger is the new professional battery charger with built-in bluetooth. Find a Victron Energy dealer near you.

www.victronenergy.com

Two 100W semiflex solar panels
If you are going to this by yourself I could try to write simple DIY plan.

 

Thank you all for the detailed responses. I think there's a strong possibility i am over thinking it.

The dual input MPPTs look like exactly what I want + hookup.

The only reason I wanted the mains hookup to have priority over solar, is that, the only reason I want to use mains for charging / conditioning the batteries. The method that @Hoovie suggested sounds like it will do the job perfectly.

@dobris I have been looking at Victron components over the coming days.

My intention now, is to complete a wiring diagram, which I'll post here for your comment and then spec all the components too fit. I'll likely build this out in a modular steps due to the cost. I.e leisure batteries + alternator charge first, then solar, then hookup. I'm a little bit of a control freak / perfectionist on these things.

 

doing stuff in stages due to budget makes sense but .... Do make sure you get all your cabling in place ready e.g. all the "first fix" stuff in house building terms.
For example, a common place to have a hookup plug in a T5 in under the bonnet. If you do this, then you will have a mains lead from Engine bay to cab. This will follow the exact same run as the Split-charge cable from Engine bay to cab, so it makes sense to run them at the same time.
You can add in the actual plugs and consumer units later on.

If you are a bit of a control freak you want to be able to see what is happening with the electrics probably (hence why you were thinking of the Aduino approach?). To scratch that particular itch you could (and are anyway) look at Victron kit which has probably the most advanced connectivity of the mainstream products?
As an example, I just logged onto my Campervan whilst sat at my desk in the house and took these two screenshots ...

A view of some of the devices (so Fridge is healthy at 3C, the Heaters fuel tank has used 1.436L since I last filled it up, the Charger is in Storage mode, etc)

And a summary of the charge - and energy production and use - of the system

I may be a bit of a control freak as well

 

doing stuff in stages due to budget makes sense but .... Do make sure you get all your cabling in place ready e.g. all the "first fix" stuff in house building terms.
For example, a common place to have a hookup plug in a T5 in under the bonnet. If you do this, then you will have a mains lead from Engine bay to cab. This will follow the exact same run as the Split-charge cable from Engine bay to cab, so it makes sense to run them at the same time.
You can add in the actual plugs and consumer units later on.

If you are a bit of a control freak you want to be able to see what is happening with the electrics probably (hence why you were thinking of the Aduino approach?). To scratch that particular itch you could (and are anyway) look at Victron kit which has probably the most advanced connectivity of the mainstream products?
As an example, I just logged onto my Campervan whilst sat at my desk in the house and took these two screenshots ...

A view of some of the devices (so Fridge is healthy at 3C, the Heaters fuel tank has used 1.436L since I last filled it up, the Charger is in Storage mode, etc)
View attachment 175904


And a summary of the charge - and energy production and use - of the system
View attachment 175905

I may be a bit of a control freak as well

That was exactly my plan.

I really like the look of the Victron stuff. The connectivity looks awesome too. I'll work on planning out the system.

 

So if you used this wiring set up...

How to Wire up your Camper

Wiring up your camper van can be very daunting at first, however if you follow some simple rules, it can be just as easy as wiring up a normal household plug. Below is a diagram that shows a typical...

www.switchpanel.co.uk

Could you just add an extra input to your leisure battery from an mppt controller for solar? If that makes sense?

 

If I have understood @Hoovie correctly, the MPPT will have two inputs, one from VSR and one from Solar. But otherwise i believe that circuit matches my intentions.

 

This looks easy enough for even me to understand .....but the rest all sounds waaay over my head ?