Multiplus triggers fuse without DC

DIY
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1

Hello,

I have a problem and hope that someone can help me with a practical solution:

I have installed a Multiplus II off-grid system in a vacation home and now have a problem:

The system is often left unattended for weeks at a time, and power outages are common in this area. My set SoC limit is 15%.
However, if the power outage lasts too long, the SoC of the battery continues to drop. If AC power does not return, the battery pack with its BMS goes into protection mode at 2.5 volts cell voltage. Internally, the MOSFETs disconnect the output, i.e., the Victron also switches off. In this protection state, the battery can last a very long time.

Now comes the real problem: when (at some point) AC power is restored, the Victron Multiplus II switches on and could now recharge the batteries. However, the circuit breaker (MCB) of the AC supply regularly trips immediately during the relay test, which starts after about 1 minute in the Multiplus II. The inrush current must be extremely high; this is a 16 A fuse (B characteristic).

This happens reproducibly; I have recreated it independently in two locations. Reinserting the fuse does not help either; it blows again during the next Multiplus relay test. Only when a functioning battery is connected to the DC battery input again can it connect to the AC side without any problems.

Does anyone know of a practical workaround? A setting that precharges the coil or tells the Multiplus to be “careful”?

Thanks!

2

The DC input to the Multiplus has a large bank of capacitors to smooth the DC current during operation. If the battery is disconnected, when switching on ac the inrush current to charge these capacitors trips the breaker. You should always connect the battery first ideally by a DC precharge before starting ac. There is no setting to do this. If you have solar then start the solar first to get DC power to precharge the Multiplus. If you have no solar I suggest you get a small ac to dc charger so you can get some voltage on the DC bus before starting the Multiplus.

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3

Can’t you set the multiplus low battery shutoff at say 48V, versus soc percentage? And turn back on at 49V. That way it wont be draining your battery till low voltage shutdown and then have the problems of needing to precharge on the way back up? That works for me with a basic ve.direct phoenix 48/1200.

4

Good idea, but as written, it can happen that the AC mains is unavailable for a longer period of time. This reduces the risk but doesn’t mitigate the problem. :confused:

5

So your multiplus does turn off when soc hits 15%? But it continues to drain the battery? Or is that the 15% soc doesnt reflect actual reality which would be common especially if bank rarely charges to 100% and theres bms drift etc.

6

Some BMSs have high parasitic drain if not put into sleep mode.

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7

Yes I had assumed there was some PV coming in via scc which could also potentially help dark start the system.