I am new to this forum so forgive me if proccedures are not correctly followed.

I have an Axpert VM3 5kW installed and is working as a UPS (No Solar). The battery is a BSL 51.2v 100Ah LiFePO4. Where I live we experience many 2 to 4 hour power outages (Loadshedding). The normal load on the inverter is between 250Watts and 1500Watts.

The Axpert VM3 5kW performs very well with no problems with the exception that when we have a power outage the inverter fans work normally when the load is <1000Watts. The fans cycle on and off depending on the load.... but.... when the invert load gets to just above 1000Watts the fans then ramp up to absolute maximum speed and remain at maximum speed even when the load reduces to below 300W for the rest of the loadshedding cycle.

I cannot believe that this is a normal situation and have read on this forum previously that this problem can be resolved by upgrading the firmware. The unit was installed in August 2020. I have checked that the intake air filters are clean. I would appreciate some comment  and if possible input and assistance. Many thanks

Could it possibly be temps causing the high fan speed? What are your temps doing before, during and after the load? Perhaps it is struggling to cool itself.

If you could give info on your firmware version (U1), maybe someone can help as to whether it can be updated.

Sadly, high fan speeds are one of the compromises with the VM (Value Model) series. You might be able to replace the factory fans with quieter ones and/or silicone mounting kits.