Anton avatar image
Anton asked

Fuses and Earth of the DC side of 3-phase system

Hello, how to understand the "It is essential the negative battery terminal between the units is always connected. A fuse or circuit breaker is not allowed" in the Multi II Manual? The interconnection between the units is meant? The BMS in error case disconnects the minus pole of the battery by N-MOSFETs, there is no way to do it in other way. And the DC terminals of the Multis see no difference (except of the stray capacitance) which pole is disconnected. Is it correct to earth the plus pole of the battery and insert the fuse and the DC switch into the minus line which can be interrupted by the BMS anyway? Thanks.

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2 Answers
Alexandra avatar image
Alexandra answered ·


There must be an unbroken negative between the three units in a three phase system. Look up Wiring unlimited there are some diagrams in there that clarify it.

Also check out the three phase training found on victron professional.

You can break the negative on the bank after the bus bar. Which is what the bms will do.

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Anton avatar image
Anton answered ·

Hi @Alexandra

Thank you. Understood, the power ground is connected to the digital ground, it makes a ground loop, not the best solution for the EM immunity.

As the BMS MOSFETs are in the negative battery conductor, I need to ground the plus. For Multis and SmartSolar MPPT OK.

But I need to connect this (plus-earthed) system to the Raspberry Pi via MK3 interface (correct?). Has it an galvanic insulation, or an USB isolator is necessary?

Or simply earth the minus pole and not care about the BMS switch? In this case would not the battery be directly earthed (as recommended) but the MPPT and the Multis.

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Alexandra avatar image Alexandra ♦ commented ·
I have never added a usb isolator an it never crossed my mind to do so. So far have not blown any comms or had other issues.

Follow the battery manufacturers recommended way of grounding you will be fine. Usually it is just the battery case.

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Anton avatar image Anton Alexandra ♦ commented ·

I am the battery manufacturer :-). And see no necessity of earthing for the battery itself but important for providing a quite potential for the MPPT and the Multis (EMC and lightning protection). Now I found anorher problem - the RS485 of the JK BMS is grounded to the - of the battery and as known there are MOSFETs between that and the - of the busbar. If the MOSFETs switch off, the desaster is the same as in case of disconnecting of the - between the three Multis. The only what can work safely is earth to the - of busbar and an isolator for the RS485 of the BMS.

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