Error 08 on the Must inverter: how I caught this error in SBU mode

Aug 14, '26 16:04

3 min. read

Post cover: Error 08 on the Must inverter: how I caught this error in SBU mode

This content has been automatically translated from Ukrainian.

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Why does Error 08 occur specifically at 100% charge?How to fix this in 2 minutes?What does this achieve?Observing

If you have set up off-grid power, you know the main rule: if the system is working and there is power - it's better not to interfere unnecessarily.

But the other day I noticed strange behavior. The system operates in SBU (Solar-Battery-Utility) mode. The scheme is standard:

The sun fully charges the battery to 100%.
The battery discharges slightly for the needs of the house.
The sun picks up the load again and "tops off" the battery.

These up/down movements occur when the battery is charged to 100%. It is during this period that I experience a disconnection/reboot of the station for about 5 seconds.

And during such another micro-cycle of "recharging," the power went out in the house. Meanwhile, my inverter is located far away (in the technical room), and unfortunately, my model does not have a log/error history. So the light flickered, after a minute everything works again, and what it was - think for yourself.

Today I was lucky to be near the inverter just at the moment of the next "cycle"... and on the screen it flashed - Fault 08 (Inverter bus voltage is too high).

Why does Error 08 occur specifically at 100% charge?

It turns out to be a classic story for LiFePO4 packs and Must inverters with factory settings.

Factory voltages are too high. By default, the menu has Bulk = 57.2V and Float = 56.2V. For a 16S LiFePO4 pack, this is almost 3.58V per cell.
Cell imbalance. At the very top threshold of charging, the capacity ramps up very quickly. If even one cell pulls ahead and crosses the threshold of ~3.65V, the BMS of the battery instantly opens the circuit for protection.
Inductance "into the void." At this moment, the inverter actively "pushes" current from the sun/grid into the battery. When the BMS suddenly "cuts off" the battery, there is nowhere for the energy to go - a powerful voltage spike occurs on the internal DC bus of the inverter. The inverter falls into fault 08 and cuts off the output.

How to fix this in 2 minutes?

If you have similar symptoms, it is treated not by repair, but by adjusting the charging parameters in the inverter menu. I do not have instructions in Ukrainian.

Bulk / Absorption (Program 17): lowered from 57.2V to 55.6V (~3.475V per cell).
Float (Program 18): lowered from 56.2V to 54.2V (~3.39V per cell — my Must simply does not allow setting lower).

What does this achieve?

The battery still charges to its 98–99% capacity, but the cells no longer enter the zone of aggressive imbalance, the BMS stops emergency "tripping" the terminal, and the high-voltage bus of the inverter no longer experiences shock spikes.

Observing

I literally just set these parameters. I will observe and write about the results in a couple of days. Before this, I googled the issue again and found successful cases on reddit and facebook. So I think it should work.