Our Multiplus II GX was connected to a battery pack when the AC power failed for a long time. This led to the batteries being completely discharged, causing the BMS to cut off the DC supply for self-protection.
The Multiplus then switched off.
After the AC power was restored, the Multiplus switched back on. Then the 16 A fuse tripped within milliseconds, meaning there are currents 2-3 times the nominal rating (i.e. 30 - 40 Amps).
We were able to reproduce the situation where very high starting currents occur, but only when no battery pack is connected.
This means that once the battery pack is disconnected for self-protection, it may no longer be possible to recharge it on the AC side because the fuse blows on the AC side.
Is this a design flaw of the Multiplus? Is there a simple way to prevent this situation?
You are not suppose to supply AC input to the Multi without a DC source connected first. The large bank of capacitors on the DC bus is causing the high inrush on Ac side. Ensure the battery settings on the Multi caters for low DC cutoff 10% before it entirely drain the battery. Also before connecting a battery do n pre- charge or apply pv to pre charge the bus.
Connect the Voltage Sense wires of the MultiPlus II through separate low-amp fuses. This way the internal capacitors will safely pre-charge up to the battery voltage through the built-in resistor. It practically eliminates DC-side inrush current.
Remember: with the MultiPlus II, you always power up DC first, and only then switch on the AC circuits — both input and output.
Set your DC shutdown higher so the inverter stops invertering first. You should never be in a situation where your battery is totally dead and having to self protect.
So it is as simple as prevention of the situation. Either use voltage or state of charge to shut down if your battery is communicating its SOC to the system
A second way to recover is using solar to charge the batteries first.
