question

Johnson Lee avatar image
Johnson Lee asked

Multiplus II with flow battery

I am currently doing some R&D work for an upcoming flow battery manufacturer and their BMS will disconnect itself when the battery SOC is low or high.

In the setup we have a SmartSolar MPPT 150/35 and a Victron Multiplus II 48/3000. We ran a test today simulating 0V scenario by isolating the battery from the system and PV on the MPPT input.

We know from our previous tests that the MPPT will provide float voltage when it does not detect battery voltage.

In this test we had grid input into the multiplus and the transfer switch was providing power to the ac output of the multiplus. When we isolated the battery and the MPPT, the inverter would sometimes not drop power to the loads but other times it will trip the ac input circuit breaker. In this scenario the low battery light turns on and the overload light turns on.

Will the multiplus provide battery voltage when the grid is connected and why would it cause the circuit breaker to trip?

Multiplus-II
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1 Answer
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @JohnsonAWS

Yes the unit should provide DC voltage.

But, you write:

"In this test we had grid input into the multiplus and the transfer switch was providing power to the ac output of the multiplus. "

if you put power on the output of the multiplus, you can / will damage the unit, a 'transfer switch' should isolate the output of the unit and then connect the users to the source.

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russ avatar image russ commented ·

I read the setup as the internal multi transfer switch was providing ac 1 out from mains, but when the battery was disconnected it sometimes dropped the load.

Was the multi providing power assist to limit input current?

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ russ commented ·

Do -not- connect AC sources to the output. You -will- damage the unit.

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ejrossouw avatar image ejrossouw Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

If this is the case, how can you then connect a grid tied inverter which produces AC to AC1 without damaging it? Just for my better understanding please.

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ ejrossouw commented ·

a grid-tied inverter has LOM detection, and cuts production as soon as the inverter stops providing a stable 'grid' to it.

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Johnson Lee avatar image Johnson Lee russ commented ·

@Russ, that is exactly my question.


The battery has it's own internal BMS that will cut battery voltage on either low state of charge or when full. The battery manufacturer is relying on the fact that the inverter will passthrough power from the grid to the loads on AC Out 1 when the BMS cuts battery voltage to the inverter.

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Johnson Lee avatar image Johnson Lee Johnson Lee commented ·

@Daniël Boekel (Victron Energy Staff) - what is the best way forward with this? In a normal scenario where a lead acid battery bank is faulty (eg. drop cells) and below the minimum input dc voltage of the MultiPlus II, will the MultiPlus continue to supply power from AC In to AC Out?

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ Johnson Lee commented ·

A disconnected battery is different from a low voltage battery: the inverter can sense 'high DC-Ripple' and shutdown on that.

A BMS should only disconnect in case of emergency, normally it should send the min/max voltage / current to the inverter, or at least use 2 signal communication. Also your inverters lower DC limit should be configured so that the BMS doesn't have to disconnect.

So setting the low voltage cutoff voltages higher might solve these problems.

But whatever you do: don't connect the grid / generator to the AC-output of any inverter!

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Johnson Lee avatar image Johnson Lee Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

@Daniël Boekel (Victron Energy Staff) thanks for your explanation.


I should not have used the example of the low voltage battery. Also to note I am -not- connecting the grid/generator on the AC Output. It is always on the AC In and I am relying on the internal transfer switch of the MultiPlus to provide power to any loads connected to AC Out when the battery BMS decides to drop to 0V.


Unfortunately as a feature of this battery's BMS, the BMS will cut battery supply to the inverter and MPPT when the battery is either fully discharged or fully charged. It's dumb and I've told the manufacturer that they need to look into another BMS but it's what I've designed and sold them.


My question is, when we simulate 0V on the MultiPlus with grid connected on AC In and loads on AC Out, the MultiPlus will occasionally go into overload and switch off. What setting should I have in VE Config that I can ensure that the MultiPlus will not do this?

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