Using Spare 4th MultiPlus II as Backup Charger with existing ESS and Pylontech BMS Integration

DIY
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Hi all,

I have a 3-phase, 50 Hz power grid setup with 3 x MultiPlus II 5000/48 units configured in an ESS system, working together with a Fronius Gen 24 inverter, which is AC-coupled on the AC-out side. The battery bank consists of 8 x Pylontech US5000 (4.9 kWh each), set up according to Victron’s integration guidelines. The Pylontech BMS communicates via CAN bus with the Victron system for SoC, voltage, and current data.

Issue: Occasionally — especially during autumn and winter nights — I experience loss of one or two grid phases for several hours or even up to a full day. When this happens, the ESS system goes off-grid and powers the house from batteries, since it requires all three phases to be available to operate in grid-connected (on-grid) mode and to allow grid charging of the batteries.

If the batteries reach 0% during such outages, the system shuts down completely, resulting in a blackout.

Proposed solution: To address this, I use a third-party electronic phase selector that constantly monitors all three grid phases and outputs the best available single phase — automatically switching to a working phase if another fails. This ensures at least one usable phase is always available, even during partial grid outages.

I also have a spare MultiPlus II 5000VA unit and would like to use it in this scenario. The idea is to connect its AC input to the output of the phase selector, and use it only as a charger for the Pylontech batteries during times when the main ESS system is off-grid due to missing phases.

When the grid returns to normal and all three phases are available again — allowing the ESS to reconnect to the grid — I would like this extra MultiPlus to automatically go into standby and stop charging.

My questions:

  1. Can this spare MultiPlus II be configured to operate independently as a standalone charger, and still communicate with the Cerbo GX for DVCC-based monitoring and control?

  2. Will it be able to dynamically adjust its charging current based on the battery SoC and charge limits reported by the Pylontech BMS via CAN?

  3. Are there any known issues, limitations, or best practices when using a MultiPlus like this — outside the main ESS VE.Bus cluster, but still integrated via Cerbo GX with DVCC enabled?

The goal is to maintain safe and intelligent charging of the battery bank during partial grid outages, and to make effective use of a spare MultiPlus II unit that would otherwise be idle.

Thanks in advance for any advice or feedback!

2

Hi
question 1: yes can be configured, no can not communicate

2: no not directly, but maybe you could use node red or relay outputs on cerbo and inputs on 4th multiplus

3: if your battery gets low, your 4th multi would power the whole system, if the load is too high it wil stop also

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Hi,

I have the same setup 3 x MP II 5000 in ESS configuration + 1 spare from the same batch in case one fails. At the moment I utilize the spare one as an additional charger. At the time of installing the system I was not aware that MP II charging capability is considerably throttled down from 70A per unit, even in case of normal room temperature. In my case I get around 6-6,5 kW charging from my system (ie ca 40A per one MP II). Since I don’t utilize my grid connection’s full potential I set up spare MP II as additional charger (I know it is kinda expensive charger!) as follows:

I know that one phase that is usually underloaded during charging (which usually takes place nighttime). So I connected the spare MP2 AC input into that particular phase - just as any other connection in breaker box. AC out is not connected. DC cables are connected to common bus bar. In case scheduled charging starts spare MP2 automatically turns on and starts drawing current from one phase and acting as an additional charger. The caveat is that in main VRM this charging method is considered as ordinary AC load, but I can live with that.

I accomplished the automatic start of spare MP2 via following method:
I had Rasperry PI 3 laying in my junk box, so I installed Venus OS on it and connected it to spare MP2 via MK3 USB programming cable. Of course it can be done with additional Gerbo GX directly with ethernet cable. So I had another VRM instance just for the charger (and I can see what it is doing in VRM). It is not connected at all to my existing VRM instance - totally different thing. Last step is achieved with Home Assistant. Since I already had the Victron connection in HA I just added one Automation criteria - if Scheduled charging has been turned on (in my “main” 3xESS unit) then switch on spare MP2 (it also need additional connection to HA). And also in case ESS charging is stopped then switch off spare MP2. I set spare MP2 Grid current limit so that I don’t overload my Grid input.

It is not directly the thing you want to replicate, but will give you the ideas what can be achieved with your equipment.

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