LifePO4 25,6V Leistungsanzeige (Watt) inkorrekt

[Cartman586]

What happened?

Hallo, da ich hier Jungs vom Fach und Entwicklung habe folgendes Problem, ließt man oft:

Ich besitze einen Hoymiles HM-800, dieser produziert insgesamt 810,9W (siehe Bild Shelly). Dieser Wert wurde mit einem Messgerät überprüft und ist korrekt, dieser wird auch eingespeist.

Tatsächlich zeigt die OpenDTUonBattery aber bei 100% Regulierung lediglich 494,6W an.

Dieses Phänomen passiert nur dann, wenn der Wechselrichter an einer LifePO4 Batterie hängt, in meinen Fall 25,6V 100Ah.

Von 810,9W realen Wert und den falschen 494,6W Wert haben wir einen Unterschied von 61%.

Wäre es bitte möglich, einen Offset in % einzubauen, um dem Problem entgegen zu wirken?

Hängen Solarmodule am Wechselrichter, so sind die Werte korrekt. Nur bei den Akkus stimmt der Wert nicht.

Bitte um Hilfe, danke

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Hello, since I have here guys from the field and development the following problem, you often read:

I have a Hoymiles HM-800, which produces a total of 810.9W (see picture Shelly). This value was checked with a measuring device and is correct, it is also fed in.

However, the OpenDTUonBattery actually only shows 494.6W at 100% regulation.

This phenomenon only occurs when the inverter is connected to a LifePO4 battery, in my case 25.6V 100Ah.

From 810.9W real value and the false 494.6W value we have a difference of 61%.

Would it please be possible to add an offset in % to counteract the problem?

If solar modules are connected to the inverter, the values are correct. Only with the batteries is the value incorrect.

Please help, thank you

To Reproduce Bug

LifePO4 25,6V Akku anstecken Connect LifePO4 25.6V battery

Expected Behavior

Korrekter Leistungswerr

Install Method

Pre-Compiled binary from GitHub

What git-hash/version of OpenDTU?

2024.01.17

Relevant log/trace output

No response

Anything else?

No response

[tehzap]

I had a similar problem initially when operating both mppts on a 24V battery. I observed different power input on both channels, although obviously the same is expected. Also the AC power output was fluctuating between the real and an odd number. The real output was measured by a Shelly in series, which showed that actually only the measurement was fluctuating but not the real power output. What solved the issue for me was connecting the negative channels using a Y cable instead of leading two separate negative cables to my busbar. Since then I got identical power output and no more power fluctuations in openDTU.

Pretty weird. How can it make a noticeable difference whether you connect the two negative input plugs via a Y cable rather than via two regular cables that are connected a little further away via a busbar? All these cables have essentially zero impedance, so in both cases you effectively short-circuit the negative input connectors.

Tbh, I have no clue. I just interpreted it such that the inverter is really sensitive to slightly varying resistance. Here you can see the difference in channel power output before and after the cable change.

[spcqike]

I just interpreted it such that the inverter is really sensitive to slightly varying resistance.

it indeed is. as the DC(-) ports are connected, it is important that all DC(-) connections have the same resistance. from the inverter to the battery. if one cable is slightly longer, if its missplaced at the busbar, if the connection is loose, ... whatsoever, the resistance is slightly higher.

the inverter measures the dc current in the negative path. so if one connection has higher resistance, more current will flow through the other connection and therefore it measures more current. even tho thats not the "real" power that channel delivers, its measured as such.

edit: that said, it should be possible, to connect DC1(+) and DC2(-) to the battery and produce power. the power must be generated by Ch1, but it would measure and think that the power comes from Ch2. if anyone wants to test this, just use low limits, as the inverters PCB probably isnt designed to handle such missmatches in power. That just can't occur, if you use solar panels. but with batteries, it certainly can. thats an exaggeration of what happens if your resistances are not equal.