If I were really, really short of money, the last thing I’d buy would be an air-conditioning unit.
What good is it if you don’t have the means later on to supply it with the energy it needs…
First of all, the air conditioning is essential for my dog, and secondly, I never said I was going to buy everything at once, but rather piece by piece. If you’ve got money to spare, I’ll gladly take it; my post isn’t about my finances, but about technical feasibility.
So, I wasn’t the one who said that a sensible installation was too expensive for you…
Well, anyone can make something work sensibly and correctly, but you were also the one who said that the last thing you would buy if money were tight would be an air-conditioning unit. It’s also the case that people who don’t currently have much money to spare want to convert vehicles and make it possible for themselves and their partners to take at least a little holiday. And of course, people buy things on the second-hand market, where you have to take whatever is available cheaply. Nevertheless, you don’t want to buy rubbish—especially when it comes to things that could potentially kill you. I should make it clear that I’m an electrical engineering technician specialising in event technology myself, and not a complete idiot when it comes to wiring, loads and inrush currents. As you can see above, I did give the setup some thought, and every decision has a reason that can be explained. What the people on this forum know better than I do, though, is whether there is a MultiPlus-specific characteristic that I’m overlooking here, and, if so, what it is.
Everyone who has tried to outsmart physics has failed without exception to this day, but maybe things work differently for you. Good luck…
OK, now I’m curious what physical wall I’m supposedly going to run into. I used the maximum loads in my calculations; it checks out on paper and physically too.
You seem pretty convinced that you’re right, don’t you? Otherwise, surely you would have taken one or two of the other users’ comments seriously by now? For example, the one about the planned battery setup.
How long do you think you’ll be able to run your air conditioning with your 140 Ah battery?
140 Ah at 12 V gives you 1.68 kWh. That will run your essential air conditioning for around an hour at full load—ignoring losses, of course. Really TOP!
I know you want to run two batteries, but since you’re an “electrical specialist”, you’re also aware that lead-acid batteries should only be discharged to a maximum of 50%, which gives you 140 Ah of usable energy…
I think we need to distinguish between normal operation and my worst-case assessment here.
The calculation is deliberately an extreme case: high outside temperature, a prolonged full road closure, the roof air conditioner running continuously at maximum output and not modulating down. It is precisely for this rare scenario that I want to know how long the system will hold up and where its limits are.
Of course, I would be in a much better position with LiFePO₄. I am not disputing that at all. The fact is, though, that I already have 2 × 140 Ah lead-acid batteries here, and those will be used initially. The system will be built up piece by piece, and components can of course be replaced or expanded later. The air conditioning will not be installed until spring anyway; the camper will already be used several times this year, just initially without roof air conditioning.
There is also a crucial part of my concept missing from your calculation: in the assumed worst case, the engine is running. So the air conditioner is not powered exclusively by the leisure battery. The 12/800 supplies a substantial part of the AC power directly from the vehicle side, while the 12/1200 supplements only the difference from the leisure battery via PowerAssist.
The 280 Ah correspond to 3.36 kWh nominally. Of course, under a real load, not all of that energy is usefully available from lead-acid batteries. For me, 50% DoD is a sensible limit in normal operation; in a genuine emergency, however, I would rather accept a shortened battery lifespan than switch off the cooling. I am also aware of the Peukert effect and inverter losses, and these will be taken into account in the real-world tests.
That is precisely why I do not arrive at “one hour and that’s it” in my worst-case scenario. If roughly half of the air conditioner’s power comes from the front 800 while the engine is running, the leisure battery bank only has to supply the remaining share. I will measure how long this works under real temperatures and actual power consumption, rather than trying to make the figures look good based on a datasheet.
I am fully aware that the system could be rebuilt more elegantly with a larger MultiPlus, an Orion XS and LiFePO₄. But that still does not answer my question. The components currently available are the 12/800, the 12/1200 and the lead-acid battery bank.
What I am trying to establish here, therefore, is very specifically whether there is a MultiPlus-specific technical characteristic or control edge case that I am overlooking in the 800 → AC-In 1200 → PowerAssist cascade. If there is such a point, I would be expressly grateful for the information.
And just as a side note regarding the budget: Of course, a completely new system with a larger MultiPlus, an Orion XS and LiFePO₄ would be nicer. However, if you insist that I should replace the existing, functioning components for that reason, you are welcome to cover the difference — I would be happy to install the equipment then as well. 
Until then, I am planning around the equipment that is actually available and affordable. So my question remains a technical one: Is there any device-specific characteristic or edge case with MultiPlus → MultiPlus with PowerAssist that I am overlooking?
Once again: sell the MP and get the basic setup from a Smart Battery Sense/SmartShunt, Orion XS and an MP of the appropriate rating.
Sorry for being so direct:
The current approach is typical camper behaviour: tinkering with electrical systems based on a great deal of half-knowledge—or no knowledge at all—after watching a selection of YouTube clips (the good ones are made by experts for other experts and, as a layperson, you often can’t recognise them as such, let alone use them 1:1!).
Every electrical system has to be treated as a one-off. Your approach, with a tight budget, lots of improvised tinkering and inefficient use of every available resource, is not consistent! In the camper scene, this approach is often implemented using cheap Far Eastern products, which constantly have to be upgraded and reconfigured because nothing fits together properly!
Victron is the opposite: careful planning and proper workmanship result in long-lasting, trouble-free operation of a system that can also be expanded without difficulty if requirements increase! That’s also why some components may initially appear a little more expensive, but they provide additional value too!
Example:
Someone builds or buys a motorhome and decides against an MP. They first install a larger charger so that the lead-acid batteries can be kept properly charged. So far, so good.
A budget price is required, so cheap equipment costing no more than €300–400 is installed, along with cheap pressed-sheet-metal cable lugs and, in the worst case, CCA cables, since pure-copper cables (fully tinned stranded conductors!) are more expensive.
Some minor changes to the 12 V and 230 V systems are also necessary.
A short time later, an e-bike falls into their hands. But how are they supposed to charge it? There is no charger powered by the onboard electrical system, so an inverter is needed! But it mustn’t cost anything, so in comes a €300-class unit. It is installed with cheap materials again. Ouch—the motorhome’s electrical system now really needs to be dealt with properly, since all the sockets are also supposed to be usable and some circuits must not be supplied via the inverter (circuit charging). Let’s say that costs another €200. That brings the total to €800–900 already.
After the inverter has been operating for a short while, it becomes apparent that it fails under higher loads or during longer periods of operation.
Since the lead-acid batteries are still in good condition, the connections are checked, revealing that the sheet-metal cable lugs are overheating and that the cable is also corroding rather quickly (CCA cables deteriorate very easily in high humidity). So this has to be corrected too, this time with proper cables and cable lugs. Another €100–150.
Now that everything is working well, a coffee machine also comes on board. The cheap inverter can’t power it, so another inverter is installed, again costing €300–400.
Then, one or two years later, the owner is parked at a remote campsite and wants to switch on the coffee machine in the morning when the inverter shuts down due to low voltage. Coffee is only possible once the engine is running. Since they are going to pass a camping dealer with a workshop on their route anyway, they stop there.
Oops, the leisure battery only has a short remaining service life. Lithium batteries are currently on offer, or there is an opportunity to acquire suitable lithium batteries cheaply because of a previous conversion. The camper owner takes the plunge, gets 400 Ah for €699 and has it installed immediately for a further €200 labour charge, with several unsuitable cables having to be replaced again.
Operations continue, but at some point it becomes apparent that the charging warning light occasionally comes on. Once back in Germany, they go straight to a workshop just across the border, where an incipient alternator problem is found: severe carbon-brush wear and a partially scorched winding. Right, the alternator is replaced—€300. They are also advised to install a charging-current limiter or to configure an existing one properly. That cannot be done with the EBL, so a battery-to-battery charger has to be retrofitted.
The question is: how much money would an initially implemented “expensive” Victron solution have saved, and how much unnecessary stress and trouble would it have avoided?
Several user examples at Overview - Victron Energy illustrate this too. In my experience, I see it happening again and again.
A sound foundation is important for your new installation!
For the optimal interaction of your components in the event of a possible future changeover to a lithium battery, or to make the best possible use of the existing batteries for cost reasons, it is crucial not to make any mistakes now (a lithium battery will not tolerate errors such as overcharging or undervoltage!).
That’s why you should connect the Smart Battery Sense or SmartShunt to the battery, the Orion XS between the starter and leisure batteries, and the appropriate MPPT (plus BT dongle).
This gives you the basic system. You don’t need the Cerbo yet; the components’ smart networking is sufficient to distribute the battery data to all participants. A PV charger added later can also be integrated, and when you inevitably switch to lithium later on, there won’t be any major costs as long as the foundation is right and only the programming needs to be changed.
I believe there is a fundamental misunderstanding here about what I am trying to achieve with the build.
I am not trying to achieve complete energy independence, nor am I planning a camper that can power all 230 V appliances, including the air conditioning, exclusively from the battery and solar for several days. I am fully aware that this would require a significantly larger battery bank, LiFePO₄ and probably a larger MultiPlus as well.
My use case is different:
Both the MultiPlus 12/800 and the 12/1200 are already available. The 2 × 140 Ah lead-acid batteries are also already available. Solar will be added, and individual components can of course be replaced or expanded later.
The 800 is deliberately installed on the vehicle side. This gives me, firstly, a hardware-based limit on how much can be drawn from the starter battery/alternator and, secondly, additional AC power while driving.
The 1200 is deliberately connected to the habitation battery and is intended to provide the difference via PowerAssist when a load requires more than the 800 can currently supply.
That is precisely why an Orion XS does not simply replace the 800 in this particular setup:
With an Orion XS, the path would be
Alternator → Orion → habitation battery → 1200 → load.
That would still mean that the entire AC load would have to pass through the inverter section of the existing 1200.
With my arrangement, on the other hand, the path is
Alternator → 800 → AC → 1200/transfer path → load
and the 1200 merely supplies the missing power via PowerAssist from the habitation battery.
That is the actual reason for the cascade. Charging the habitation battery while driving is a welcome side benefit for me, but it is not the main reason for this architecture.
I do not dispute at all that an Orion would be more efficient for charging alone, 12 V → 12 V. I am also aware that I need to check the alternator’s actual load, particularly when idling in hot conditions. I will do that under real-world conditions using current measurements and vehicle diagnostics.
The lead-acid batteries are not an indication of what I intend the system to look like in five years either. They are simply what is available now. I am expanding the vehicle in stages. It will already be in use this year, while the roof-mounted air conditioning is not planned until next spring. If LiFePO₄ becomes sensible and financially viable later on, the battery side can be redesigned accordingly.
Your example involving CCA cables, cheap inverters, crimped sheet-metal cable lugs and components repeatedly retrofitted at random therefore simply does not describe what I am planning here. “Using existing second-hand Victron hardware” does not automatically mean “throwing together an electrical system on the cheap”.
On the contrary, I am specifically trying to find out before installation whether there is a device-specific peculiarity with this unusual topology that I am overlooking.
So my question remains very specific:
Is there a technical reason, or a MultiPlus-specific characteristic, why MultiPlus 12/800 → AC-In MultiPlus 12/1200 → PowerAssist should not operate reliably?
In particular, I am interested in synchronisation, rapid load changes, behaviour at the AC input current limit, possible control-loop hunting, and what happens when the upstream 800 reaches its power or temperature limit.
If there is a deal-breaker there, I explicitly want to know about it. A completely different system with an Orion, a larger MultiPlus, a Cerbo and LiFePO₄ would, on the other hand, be a different task from the one I actually asked about here.
And if it absolutely has to be this complete replacement system: I would of course be happy to take the components if you would like to pay for them.
No one can say how your system will behave in the worst case (control oscillations, surge events caused by safety shutdowns), because no one would build a system like this (if only because of the conversion from 12 to 230 V and back again), at least not a true professional!
As I already said, you’d be better off using the standard basic configuration. It won’t be long before you want to expand it or have to modify it on the battery side. Based on the basic configuration, you can then expand the existing system without any problems.
Your current setup will damage both batteries in the long run and will also cause damage in the event of effects propagating back from the secondary-side network. No one can predict where these will occur (climate control or MP); that is then a matter of component tolerances and design/manufacturing variations. Possibly both together! If something goes bang, the system will shut down immediately! With all the consequences that entails for you!
A trained Victron installer or dealer will advise you against it!
So, once again and for the last time from my side: save your money and don’t make any more expensive mistakes with the basic installation!
P.S. Have you noticed that much more experienced installer colleagues have already withdrawn from this discussion?
Furthermore, an event technician is not trained in the details to the same level as a Victron installer. If you have more background knowledge, you understand some of the interrelationships better—especially with a full electrician’s training, rather than as an EuP with limited basic knowledge. (This is not meant negatively here; I know a trained baker who produces sought-after model railway specialty parts worldwide on request. Never judge a book by its cover!)
Integrating the alternator is the next hurdle. What year was your vehicle manufactured?
Have a look at this https://www.victronenergy.de/upload/documents/The_Wiring_Unlimited_book/43562-Wiring_Unlimited-pdf-de.pdf
Also this Lithium battery systems | Nordkyn Design in preparation for a lithium conversion.
But that also means your engine has to keep running the whole time. The alternator will get pretty hot, though, as it’s not getting any airflow from driving to cool it down. That won’t work for long. And if the engine has to be running anyway, wouldn’t the engine-driven air conditioning be sufficient?
I’m afraid that whichever option you choose, the whole thing ultimately depends on the battery capacity.
The car doesn’t have air conditioning. And the alternator is under a load of approximately 80 A and can deliver 150 A according to the datasheet.
If you think you know better about everything anyway, what’s the point of this thread???
Why don’t you just put the whole thing together? This is exactly the right weather to test it under real-world conditions.
Even without air conditioning, you should be able to find some 1,000 W device to test it with.
(Who buys a vehicle without air conditioning these days?)
I read through it, but unfortunately couldn’t find anything for my use case. The bit about lithium batteries is a good guide, thanks.
I want to know whether any strange phenomena can occur when two MultiPlus units are connected together like this, as this isn’t mentioned in any manual I’ve found.
Sure, I’ve got enough burdens as it is, and that’s what I intend to do. It’s just that there’s a real possibility that someone with experience in this area might know something I don’t, and that a danger could arise that I’ve overlooked.