Daisy-chaining two Multiplus in a special setup

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1

MultiPlus Compact 12/800 + 12/1200 cascade in a Transit Custom – build and rationale

Hello everyone,

I am currently planning/building a somewhat unusual 230 V system in a Ford Transit Custom and would like to hear about your experience with, or get a technical assessment of, combining two MultiPlus Compact units.

I am aware that connecting the AC output of one MultiPlus to the AC input of another MultiPlus is not a conventional Victron standard topology. In my case, however, the cascade has been chosen deliberately because I have two separate 12 V battery systems and, at the same time, want to limit the maximum load on the vehicle side through the hardware.

Basic layout

Vehicle side:

150 A smart alternator

→ Starter battery

→ Victron MultiPlus Compact 12/800/35-16

→ 230 V AC

→ AC input of the MultiPlus Compact 12/1200/50-16

Living-area side:

2 × 12 V / 140 Ah lead-acid batteries in parallel, i.e. 280 Ah nominal

→ MultiPlus Compact 12/1200/50-16

→ 230 V loads

Solar will later be added to the vehicle roof, connected via an MPPT directly to the living-area batteries.

One of the largest planned loads is an inverter-controlled roof air conditioner with approximately 3.6 kW of cooling capacity and approximately 1.3 kW of rated electrical consumption.

Why is the 12/800 on the vehicle side?

The 12/800 is deliberately intended to form the interface between the vehicle and the living-area system.

The main reason is the physical power limit.

I do not want the living-area system to be able to draw theoretically unlimited power from the starter battery and alternator. The 800 limits the transferable power through its hardware alone.

At around 650–700 W of AC output power, the additional load on the vehicle’s 12 V side is roughly in the region of 55–60 A, depending on battery voltage and efficiency.

This means that the 1200 on the living-area side cannot suddenly demand 80, 100 or even more amps from the vehicle side.

Naturally, at a hot idle the alternator may in some circumstances supply less than the vehicle’s own consumption plus the MultiPlus require. The starter battery would then temporarily provide part of the power. After installation, I intend to test this behaviour under real-world conditions using a DC current clamp and vehicle diagnostics.

The 800 should otherwise work as “dumbly” as possible:

- install the latest firmware

- configure it fully once

- control the remote input via a relay connected to the ignition positive

- no further operation during normal use thereafter

When the ignition is active, the 800 is enabled. Outside engine operation, the ignition positive on this vehicle is only live in a few brief comfort-related situations. Given the MultiPlus’s very low no-load consumption, this is acceptable for my application.

Why is the 12/1200 on the living-area side?

The 1200 is intended to be the actual “system manager” for the living area.

It is connected directly to the 280 Ah living-area batteries and handles:

- supplying the 230 V loads

- charging the living-area batteries

- PowerAssist

- limiting the AC power drawn from the upstream 800

- later, control and monitoring via a VE.Bus Smart Dongle

While driving, the 1200’s AC input current will initially be limited to around 3.4 A.

At 230 V, this corresponds to a maximum of approximately 780 VA and is therefore deliberately close to the power rating of the upstream 800.

If a load requires more than the 800 can provide, the 1200 is intended to supply the difference from the living-area battery via PowerAssist.

Example:

The roof air conditioner requires approximately 1.3 kW electrically.

The 800 supplies a limited portion from the vehicle side.

The 1200 supplements the missing portion from the living-area battery.

This prevents the starter battery/alternator side from being loaded with the air conditioner’s full power requirement.

The 3.4 A is initially a test value. If it turns out that the 800 permanently reduces its output thermally or becomes unstable at high ambient temperatures, the limit could, for example, be reduced to 3.0–3.2 A.

Solar

Solar will later be connected directly to the living-area batteries.

While driving, this would theoretically provide three energy paths simultaneously:

Alternator → 800 → AC → 1200

Solar → MPPT → living-area battery

Living-area battery → 1200 PowerAssist

This means that, particularly with the air conditioner, no single source has to handle the entire load.

Shore power

Shore power is to be fed into the AC input of the 800.

This gives:

Shore power

→ 800

→ AC output

→ 1200

→ loads

The 800 can charge its own starter battery while the 1200 charges the living-area batteries.

Both MultiPlus Compact units have a 16 A transfer path, so when shore power is available, the devices do not have to invert the entire load.

With actual shore power, the 1200’s AC input limit can of course be set correspondingly higher. The approximately 3.4 A is primarily intended for operation downstream of the 800 while driving.

That is precisely why I would rather use the VE.Bus Smart Dongle on the 1200: the AC input limit, PowerAssist and living-area battery are the relevant parameters there.

Why not simply use a larger DC/DC charger?

A conventional DC/DC charger would of course be the more standard solution.

However, AC coupling offers me several interesting features:

1. The vehicle side is power-limited through the 800’s hardware.

2. The 1200 can use PowerAssist.

3. The starter and living-area batteries remain electrically separate DC systems.

4. With shore power, both battery systems can be charged through their respective MultiPlus chargers.

5. The energy source at the 1200’s AC input can change without anything having to change on the actual living-area side.

6. The living-area battery deliberately serves as a large buffer for briefly higher loads.

Critical point: MultiPlus → MultiPlus

This is the part where I am particularly interested in other users’ experience.

I understand that using one MultiPlus as the AC source for a second MultiPlus with PowerAssist is not a typical or officially standard Victron system design.

For this reason, before putting the system into productive use, I particularly want to test the following situations:

- synchronisation of the 1200 to the 800

- behaviour during rapid load changes

- PowerAssist under high load

- behaviour when the 800 undergoes thermal derating

- switching the vehicle-side MultiPlus off and back on

- loss and restoration of the AC input

- switching between inverter operation and shore power

- low state of charge of the living-area batteries

- hot engine / hot alternator at idle

- possible control oscillations in which the 1200 repeatedly accepts and rejects the source

The two devices will not be connected in parallel via VE.Bus. This is solely an AC cascade with separate battery banks.

If anyone has already operated a MultiPlus or Quattro as a limited AC source upstream of a second MultiPlus with PowerAssist, I would be particularly interested in experiences regarding stability, synchronisation and any settings that may be required.

Ultimately, my goal is:

Driving: the alternator automatically supports the living-area system.

Stationary: solar takes over as far as possible.

High load: the living-area battery provides buffering via PowerAssist.

Shore power: both battery systems are charged automatically.

So the unusual part has been chosen deliberately – mainly to maintain a hard power limit between the vehicle and living-area sides without having to forgo PowerAssist on the load side.

2

Why are you converting from 12 V to 230 V, only to step it back down to 12 V again?

In terms of efficiency, that’s abysmal.

An Orion XS DC/DC converter has adjustable power output and can also be controlled via Cerbo (overtemperature, etc.).

With the software coming soon, it will also support reverse charging (the secondary charges the primary).

P.S. Please remember to provide clear documentation for service cases when using such workaround circuits.

3

An in-depth description is all well and good, but this is a bit too detailed for me again — I didn’t read all of it.

I’d also like to ask why you aren’t using an Orion XS to recharge/support the domestic battery.

The only “advantage” I can see in your plan is that the MultiPlus for the domestic system can be smaller, since some of the power is supplied by the other MultiPlus.

However, this means you can only use the air conditioning while driving.

Why use lead-acid batteries for the domestic system nowadays?

4

The main idea was to avoid using the Orion. And neither of the two MultiPlus units would be able to handle the roof air conditioning on its own, hence the combination. Yes, fair enough, it’s certainly not the most efficient setup — I’ll give you that. But perhaps I should also mention that the two MultiPlus units are roughly 5 metres apart (one at the front under the seats and one at the rear left of the vehicle.

5

So the main reason was to combine the power.

And the lead-acid batteries, because they were cheap and money is tight for me.

You’re actually half right about using the air conditioning while stationary.

Since the shore power is connected to the first MultiPlus in this case, and both of them switch the 230 V through via the transfer relays, that has been taken into account.

6

The battery capacity seems far too small to me for loads like that.

280Ah at 50% DOD = 140Ah × 12V = around 1.7kW

In other words, after just over 1 hour 20 minutes, the leisure battery will be flat.

The starter battery is normally not suitable for cyclic operation, and the engine still needs to start as well….

I’d work backwards.

How long should the air conditioning run for?
How much power does it use while running?
What battery capacity do I need for that at x DOD?

I’d also use an Orion XS and power the air conditioning exclusively from the leisure battery.

7

A question? So you want to boost the current from the 800 MP using the 1200 MP’s Power Assistant? The Power Assistant’s minimum input current is 4 A. But the 800 MP can’t supply that much, so you’d be constantly overloading it. I don’t think that’s going to work well.

8

According to what I’ve found online, the input current limiter goes down to 1 A with firmware 536. Mine has the processor from the 2607 series.

9

I’m calculating this with the engine running and the 800 connected to the ignition-switched positive. My theoretical calculation model is based on the complete-road-closure scenario, which means that, if the alternator can still deliver anything at all, it will provide 75 A at most, which the inverter and the car will have to share.

According to the manufacturer, the air conditioning is an inverter unit with a maximum power draw of 6.3 A. So the 1,200 alone is too small.

I calculated the whole thing so that, in an emergency, the air conditioning can run for about two hours because of my dog.

10

Feel free to go for a 2000 or higher if necessary, and manage the charging via an XS (there are also 70A versions, or you can connect the relevant models in parallel), plus a Cerbo (which will also provide you with other useful services when you retrofit solar and the MPPT controllers can therefore operate and be controlled within the Victron network).

For your minimum setup in the current configuration (which can then be expanded without any problems), I would recommend an Orion XS (50–70A), a MultiPlus 2000–3000, a Cerbo plus display (Mk. 2 with a 50 or 70 Touch display), as well as a Smart Battery Sense (for the lead-acid batteries).
Since you are still using lead-acid batteries in the living area, but these will suffer pretty quickly when operating the inverter if you draw more than 30% of their capacity (the maximum depth of discharge is 50%, but the lead-acid batteries reach the end of their service life very quickly at that level), you should already be thinking about LiFe batteries here as well!

Here, it is advantageous to install a version with CAN bus (Battery-Can), as it can be integrated neatly with your Victron system without significant additional costs if you buy the right core components now. (People often cobble things together using cheap components, which then turns into a veritable series of replacement exercises.) That is one of the advantages of the blue system: every box adds value, and the overall system can grow with your needs.

11

If you’re happy to pay for it, sure, but the fact is I barely have any budget.

12

So don’t make any mistakes with the basic equipment now! You’re also welcome to get advice from a registered dealer/installer near you.

If I were starting out now, the Orion XS and an inverter (suitable for air conditioning) would be my starting point; the lead-acid leisure batteries would be next on the list (if the inverter causes them to give up the ghost prematurely). Naturally, you’ll also want to be able to run the air conditioning while stationary, so get some PV installed (fill the roof straight away! And allow for the appropriate controllers and modules — in other words, so-called 24 V modules with an Umpp of 30 V or more. In most cases, a parallel connection will then be sufficient).
For the time being, the components’ smart network via Bluetooth is sufficient here (the MP needs the Bluetooth dongle to join in and be controlled), since you can use your lead-acid batteries optimally with the Smart Battery Sense, and subsequent upgrades (without a Cerbo) can still use battery data within the network (especially for charge control in winter).

Then, if there’s anything left over, install a Cerbo. At the latest, though, when switching to lithium, the Cerbo becomes interesting if you want to use additional functions and the lithium batteries need to be charged optimally.

13

Suppose you’re on the road and your worst-case scenario occurs: you end up stuck in a traffic jam for an extended period in summer.

An Orion XS would keep the power flowing here without significant conversion losses. If the starter battery were overloaded (voltage drop when the alternator reduces its output because of temperature), it would then reduce or stop charging (the system would cycle) until the starter battery had recovered.

The MP would power the air conditioning until it reached the configured undervoltage protection threshold, then issue a warning and eventually switch off.

However, if you have solar panels on the roof, you’re not in such a bad position in bright sunshine, with power coming from the solar charging system and the alternator.

In winter, though, the heating is running while the solar panels contribute little. The alternator then has to provide all the power for the leisure batteries on its own.

14

Be careful not to burn out the alternator if you draw high currents continuously.
I’ve already suffered quite a few setbacks with projects like this.

There are alternator regulators that have temperature monitoring.

15

I think we’re talking slightly at cross-purposes. I realise that, if the system were being planned from scratch, an Orion XS + larger MultiPlus + LiFePO₄/Cerbo at a later stage would be the more conventional solution.

That isn’t my starting point, though: both the MultiPlus 12/1200 and the MultiPlus 12/800 are already available, as is the 280 Ah lead-acid leisure battery bank. So I’m not asking which system I should buy instead from new, but how to implement the existing combination properly and identify any possible edge cases.

The main reason for the second Multi isn’t primarily to recharge the leisure battery, but to combine the AC output via PowerAssist.

An Orion XS would indeed transfer energy from the alternator to the leisure battery, but the entire load of the rooftop air conditioner would then still have to pass through the inverter section of the existing 1200. It is precisely that power limit that I want to work around with the cascade: the 800 supplies part of the power directly to the AC side, while the 1200 supplements the remainder from the leisure battery via PowerAssist.

On the other hand, I consider the point about the alternator important. Although the 800 already limits the maximum load on the vehicle side in hardware, it does not actively reduce its input current like an Orion XS if the vehicle voltage drops because the alternator is hot and reducing its output.

I will therefore test its behaviour while idling hot, using a clamp meter and the vehicle diagnostics, and, if necessary, also provide a suitable low-voltage/remote shutdown for the 800, so that the starter battery does not quietly supply the missing current over an extended period.

Solar is already planned as well, and LiFePO₄ may be added at some point later. For now, though, the lead-acid batteries will be used because they are already available.

So my actual question remains:

Has anyone had practical experience with MultiPlus → MultiPlus over the AC path, particularly with PowerAssist, rapid load changes, synchronisation and possible control hunting?

16

My answer is, unfortunately, just reality: I can’t simply get a different setup; that’s beyond my current financial means.

17

That’s why the 800 is installed in front of the starter battery, so this risk is reduced at the hardware level.

18

Am I the only one who doesn’t understand something here??? :face_with_monocle:

19

What exactly do you mean?

20

I assume you’re referring to the 3.6 kW cooling capacity. In this case, 3.6 kW refers to thermal power, not electrical power.

Electrically, we’re talking about 1,450 watts at the air conditioner’s maximum setting.