Effects on calculated SOC for unaccounted consumption on battery

Modifications
1

Hello,
I am running the Rpi (Venus) together with a fan, directly from the battery (48V, Lifepo4, 14KWh). This results in an unaccounted consumption of ~5W continuously. Slowly and because in winter the battery never gets recharged to 100%, the calculated SOC in the inverter tends to get more and more error (~30% within a mounth).
The inverter is a MultiRS 48/6000 and I do not have a shunt to measure the current externally. The BMS is independent (not connected to the Rpi).

My question: Is there any way I could add this predictable consumption in the inverter so that the calculated SOC takes into account my external consumption of ~5W (100mA@48V) ?

Thank you,
Regards

2

I guess the easiest would be to add a SmartShunt.
The small 300A Shunt should be enough for your system.

You can somehow use NodeRed to add a permanent phantom load of 0.5A.

3

Thanks for the quick response. I guess the correct way would be to add a SmartShunt as you said.
Since this is not a really big issue and I can overcome it by manually charging from grid to 100% from time to time, I was looking for a simple tweak of some sort. I don’t have any experience with NodeRed but I’m sure something could be done.
In theory, if I was to have access to the internal current measurement in the inverter and I could add an offset of 0.1A to the measured current, it would solve the problem instantly :slight_smile:

4

Have you tried adding the load to the inverter battery terminals?

5

That’s exactly where it is. But since the current sensor is inside the inverter, it doesn’t know about other currents being drawn by other consumers that are tied directly to the battery.

6

Ah sorry, misunderstood this

Interesting how the float voltage isn’t being maintained. How is the Multi programmed.

7

In winter, it’s hard to get to bulk/float voltage on the battery. If it doesn’t reach the Bulk voltage, it doesn’t reset to 100% SOC. The BMS itself (JK) has a more precise SOC measurement because these extra consumers are actually connected “after” the BMS (not on the battery terminals). Unfortunately, I haven’t connected the JK BMS in the Rpi, although I understood it could be done using the RS485 serial port.
The Rpi uses a CAN interface to connect to the MultiRS. The MultiRS is set to ESS “Optimized without battery life”, minimum SOC set to 40%. AC input is set to connect if SOC drops below 40%.

There are two parameters available called “Peuker Exponent” and “Charge efficiency factor”. I was wondering if changing those a little bit couldn’t account for this extra consumption…

8

Have you tried connecting the JK to the Usb? There is an article somewhere on it. If i come across it again will link it.

The peukerts and efficiency factor won’t change much for a parasitic draw. It needs a shunt measurement.

9

The above is the cheapest way to send the real SOC to the Multi RS. Make an effort and connect it.
Not to mention that it’s also a professional solution for the Multi RS to know the real SOC, directly from BMS.

PS:
Nasol ca de mai mult de o saptamana nu avem soare pe meleagurile noastre.
Probabil s-a suparat pe starea noastra de spirit zilele astea… :slight_smile:

10

One other reason why I haven’t connected the JK BMS into the Rpi (yet) is the linearity error in the JK current measurement. I did calibrate it at one point and it works well around that. Unfortunately, for other currents further away the reading has some error. The MultiRS is significantly more accurate when measuring the battery current.
I’ll probably spend some time in the future and try to connect the JK and Rpi together as you guys recommended.
Thank you.

PS for Alex:
La Cluj e oricum ceata aproape toata iarna si rar vedem soarele :grinning:
Asteptam zile mai bune :slight_smile:

11

Any idea which D-Bus paths we need to write to add a DC phantom load?
I have the same issue, the JK BMS does not measure current below 0.5A.