question

Thanks Kevin,

I’m on VenusOS 2.91 and GuiMods 6.15. Over the Modbus, the Venus is sending “DC System Power” with the alternator/wind/fuel cell subtracted out, as those power measures come through in their own distinct variable.

Beyond that, I can only say that Venus 2.91 without GuiMods does subtract the alternator power, although it doesn’t show anything about the alternator on the screen. And your wind/fuel cell tiles do allow DC System to display the accurate values, while Alternator miscalculates SC System. Near as I can tell, with Alternator running, DC System = (roughly)

0 - 2*alternator_power + dc_system_power_besides_alternator

So if Alternator puts out 360w and the rest of the system is pulling 20w, alternator has +360w, the battery has +340w, and DC System has -700w.

As “Fuel Cell”, FC has +360w, the battery has +340w, and DC System has -20w. Which is what I’d expect of both settings, no?

It’s very strange…

Interesting. That makes sense, as it’s always seemed like DC System was just a catch all of any power in the system that doesn’t have its own measurement.

The shunt is wired per Victron with the ‘output negative’ of the Orion going to the system side of the shunt, and the battery side goes to my system minus (busbar, on the system side of my main battery shunt).

Yes, it seems that without GuiMods, the DC system effectively removes any power generated by the FuelCell or Alternator.

Perhaps I’m expecting the wrong effect? But I was thinking, same as a fuel cell, PV, or any other generator/charger in the DC system, the alternator would be excluded from the DC System number because it has its own discrete measurement now. And GuiMods handles all of those as I would expect, with the exception of Alternator.

Per that commented code above, I would expect alternator_power to be added, along with the rest of the power generators. But perhaps the designed intent is for the alternator to be taking from the DC System instead of charging it?

I find it odd that without GuiMods, you are seeing alternator removed from DC System numbers. Since the stock code doesn't include it in the DC System calculations then it should show up under DC System. The stock code DOES include fuel cell so I would expect the behavior to be different (alternator vs fuel cell).

I too would think alternator would be the same as fuel cell, PV charger and AC charger. Since the code is commented out, it could be wrong. I simply used the commented code so alternator is subtracted.

In my initial testing of these numbers used my main battery shunt reconfigured for other things and I got the signs backwards in some cases. I got this sorted out with help from someone with an extensive DC system on his boat. But your experience seems to contradict his. I think his system may have used a smart alternator controller or DC-DC charger (Scotty) which provides alternator data without using a shunt. So maybe there's an issue with the shunt configuration relative to data from Scotty. Yikes, I'm not sure I can sort this out.

One of the issues is the use of shunts for other purposes is not covered in Victron documentation. There is a one-liner for using a shunt for a DC Meter and very little detail other than a picture of the wiring of a shunt for "other circuit". Could be some positives and negatives reversed somewhere in the code.

It would be interesting to see what your alternator shunt is showing in dbus-spy. Can you take a look at that and let me know if that number is positive or negative?

The modified code SUBTRACTS the alternator power rather than adding it as is done for fuel cell which accounts for the 2x error. If you completely uninstall GuiMods, the original systemcalc is restored and I suspect you'll see 1x the alternator power shown in DC System power.

I can change the alternator to be added to the DC System power like fuel cell but as I mentioned before, I had someone else test this with a smart alternator interface that doesn't use a shunt for providing data an they though it was correct as is. I'd need to dig onto the source of the power to decide whether to add or subtract the alternator power. Getting messy.

@Eric M.

You are correct it is strange. The system at present does subtract some currents and not others. Victron is well aware of the issue and I am sure will be fixed at some stage, but no idea when. The total today is not true DC Power. It is Power derived from adding and subtracting various measurements, so reflects what is left over and not accounted for in for instance the battery, inverter and solar. The calculation is

dc_pv_power + charger_power + fuelcell_power + vebuspower - inverter_power - battery_power

So am surprised that water generator and dc charger did not give strange results as they are not listed in the calculation and therefore would end up in the bucket once the values are subtracted from the battery power.

If you want to fix it and have shell access to your device then you can edit the relevant python file to remove the alternator current/wats.

The other alternative is to install a shunt in the negative of the true loads. Then configure the shunt as a dc system shunt. That stops the calculation above from being used and the system displays the sum of all dc system shunts.

Cheers Jon