Adding a second battery + Multiplus to an existing one, a mobile battery + multiplus at that

Q&A and troubleshooting
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1

I have 3 victron multiplus 2 (5000V) in 3 phase mode. Grid connected with 48V main-battery. All load is on the critical load output of the multipluses (AC1, microgrid), so is a SMA solar inverter which feeds in power which is first used to charge the battery, then fed back into the main grid. There is a Cerbo which also knows the power used in the house and the power used from the grid. (2 meters), it also knows solar generation power. Everything can go to island mode and also via frequency shift throttle the SMA.

Now I want to add another single multiplus 2 (3000VA) with its own battery, which is however mobile in an RV and quite substiantial with 16kwh. To not leave the battery unused if the RV is unused I want to connect the RV-multiplus 2 to the microgrid created by the main 3-phase multiplus, I guess via its AC1?. This RV-multiplus is supposed to provide power to the microgrid, when the main-battery is empty and supposed to charge when the main-battery is full. To do this I want the 3 multiplus to raise the frequency when the slave rv-multiplus is supposed to charge, say up to 50.6hz and configure the SMA that it throttles for frequencies above that.

So far so good. Seems all within specs. I believe…

Now the questions.

1. How does the slave multiplus know when to discharge? My main multiplus are discharging so that they keep grid power at zero, how does the slave know when and how much to discharge?

2. How exactly would the RVmultiplus be connected? does it need both ACin and AC1 connected “together” to the house microgrid?

3. When I am on the road with the RV and am on a camping site with electricity I want to plug in the RV and CHARGE the batteries no matter what. So no frequency shift to do so. How would I solve that?

  1. Am I right that this frequency shift control only works when the main grid is down? so what are the alternatives?
2

Yes, frequency shift only works when off grid.

To solve this, you will need considerable custom code controlling the slave system in the RV, which will have to be set up as an externally controlled ESS.

The RV would connect to the house system using AC in ONLY, but you may also need a LOM disconnector. Note that local regulations may prohibit this type of connection over a ‘plug in’ type connection (as opposed to hard wired) due to exposure of live contacts due to an accidental disconnect.

Generally, I would only try this at the DC level, using dc/dc converters if the battery voltages are dissimilar.