In short
- Without a data link to the battery, an Axpert estimates the level from the battery voltage.
- Lithium voltage barely moves between 30 % and 90 %, then sags under load: the estimate jumps.
- Voltage at rest, the time of day and the evening trend tell you more than the percentage.
Most Axpert-family inverters are paired with batteries that do not talk to them. The inverter only sees one number from the battery: its voltage. From that, it works out a percentage, using a table made for lead-acid.
Why lithium confuses the estimate
A lead-acid battery loses voltage steadily as it empties, so a voltage-based gauge works reasonably well. A LiFePO4 battery does not. Its voltage stays almost flat across most of the range, then falls quickly near empty and rises quickly near full. Two things follow.
First, under a heavy load the voltage sags for a moment, and the gauge reads that sag as an empty battery. That is the kettle effect. Second, in the long flat middle, the gauge can sit on the same value for hours while you are using real energy.
What to look at instead
The voltage itself, read at rest, is more honest than the percentage. On a 48 V LiFePO4 bank, a voltage that holds above 52 V in the evening means there is plenty left; one that slips under 50 V with a small load means you are near the bottom. The exact values depend on your battery, so note what yours shows on a full evening and on a hard one.
The shape of the evening tells you even more. Axpert Monitor keeps the battery voltage minute by minute and draws it next to the load. After a few evenings you know where your battery usually lands at bedtime, and a night that ends lower than usual stands out at once.
If your battery has a BMS port
Some lithium batteries can report their real state of charge to the inverter over a communication cable, if both sides support the same protocol. When that works, the inverter shows the BMS figure instead of guessing. It is worth asking your battery seller whether your pair supports it.
Setting thresholds on voltage
Since the inverter switches modes on voltage anyway, set your SBU return-to-grid threshold from what you observed, not from the percentage. Leave a margin: a short load spike should not send the house to the grid.
Common questions
Is my battery faulty if the percentage jumps?
Usually not. The percentage is estimated from voltage, which sags under load. Look at the voltage at rest instead.
Does Axpert Monitor show the real state of charge?
It shows what the inverter reports: battery voltage, charge and discharge current, and the inverter’s own percentage, with their history.
Which voltage means my 48 V lithium bank is low?
It depends on the battery. Note the voltage at rest on a normal evening and on a hard one, and set your thresholds from those.
