What invertor?

[wildebus]

Reason I asked about cable sizing is, I have actually seen 'experts' on boat forums telling people how long and extra length of cable they need to fit to stop problems.

Personally I went for a B2B for two reasons. 1 because I did not know how much spare capacity my alternator had but was fairly certain it could provide me with 30A constantly. 2 it gives a proper charge cycle.

You could get relays but I don't think that Victron Li was about when I go mine, if it was nobody was talking about it but that wouldn't limit the draw

The long thin cable seems to be promoted on Canal Boats when adding Lithium to create a Lead/Lithium Hybrid battery setup. Why people don't want to do things right is beyond me

There are actually three different Victron Cyrix Battery Combiners for Lithium, the Cyrix -li being the most common and simple to use. It is in essence the same as the Cyrix -ct. You are correct, they don't limit the draw so you have to ensure the draw is under the rated current (120A).
The key difference between the -ct and -li is the disconnection voltage. the ct is mean for Lead Acid and will disconnect at a voltage that is below the resting voltage for Lithium (so not suitable, which is why the -li is the right option). No VSRs on the market are suitable for Lithium as they once connected they will never disconnect.
Two reasons why a B2B is preferable ... It provides a current limiting function so acts as a safety net against overloading; and it provides a regulated voltage and ensures the battery will charge at the right voltage rather than the random voltage an alternator may put out (some don't above 14V, some output 15V).

 

[nabsim]

Those are the two reasons I went the B2B route but I was more concerned about overloading what could be an OEM alternator, would have been around 12 years old when I was looking. Plus I new the B2B was safe for me where I didn't know about other ways of doing it. Was a simple install initially breaking into the EBL wiring as well. Have since improved by wiring direct from starter battery to leisure battery instead of using original EBL cables. Went from 13.9V to 14.4V using decent cable direct

 

[nabsim]

The way the yanks do it with a second 180A alternator and I forget what they call the relay they use is impressive though. Some of them are throwing over 200A into their hab banks. If I was starting with bare van I would probably look into this method a lot closer

 

[wildebus]

From what I recall seeing on Sprinter-Source, they seem to use chunky solenoids rather than Relays. The multiple alternators are common on boats as far as I can tell based on conversations I have had with some people I've sold kit for (both Canal Boats in the UK and Yachts in the Adriatic and Med).

 

[merl]

So you are saying that if you swap a lead battery for a lithium one, the charge current will actually go down?

Actual current will depend on the SOC of the batteries, take out a fully charged LA and fit a relatively depleted LFP and yes the current will go up comparatively but not higher than that of a flat LA, likewise a flat LA will obviously take more than a full LFP because a full LFP will take bugger all when the battery voltage reaches 14.4V because the battery and alternator will be at the same potential and NO current will flow. As you know Lifepo4 has a very flat charge profile so that wont change as dramatically as LA while charging. There seems to be a tiny area (below 5% SOC or less) where LFP's voltage is slightly lower than LA, otherwise charge current will be lower than LA at similar SOCs.

 

[merl]

Reason I asked about cable sizing is, I have actually seen 'experts' on boat forums telling people how long and extra length of cable they need to fit to stop problems.

Personally I went for a B2B for two reasons. 1 because I did not know how much spare capacity my alternator had but was fairly certain it could provide me with 30A constantly. 2 it gives a proper charge cycle.

You could get relays but I don't think that Victron Li was about when I go mine, if it was nobody was talking about it but that wouldn't limit the draw

Precisely, you'll need a certain value of resistance to limit the current to the desired level but you cant just add a short length of very thin wire, you need to spread the wattage of the resistance over a large enough area to stop excessive heat build up in the resistor(cable) and this will involve using longer lengths of thicker cable. The famous 'smoking alternator' video has a particularly little alternator spinning as slowly as they could to reduce the cooling (there's some evidence to suggest that they'd doctored it by removing the fan too) and particularly short fat cable, not a typical install by a long chalk.

 

[nabsim]

From what I recall seeing on Sprinter-Source, they seem to use chunky solenoids rather than Relays. The multiple alternators are common on boats as far as I can tell based on conversations I have had with some people I've sold kit for (both Canal Boats in the UK and Yachts in the Adriatic and Med).

Yes sorry your are right, its solenoid not relay

 

[trevskoda]

very different animals and rarely chosen based on price

Best value by far is 240V Compressor Fridge. Works well in a MH with a bit of a mod to secure the door for travelling.
The 12V Compressor versions are significantly more expensive and are are comparable in price to 3-way fridges. If you do a price-check for a given size of a Thetford or Dometic, you will see only around 10% difference between compressor and 3-way.
You need the battery setup to run a compressor, but installation of a gas fridge is much more involved and more expensive than getting a bigger battery to run the electric one

Here you go for door mod/lock.