Meaningful difference between BMS and VE.Bus reported DC current

Q&A and troubleshooting DESS
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1

Hi everyone,

When running DESS, with the “target SOC” scheduled to stay flat, it seems that our BMS-reported discharge current is meaningfully higher than the VE.Bus-reported discharge current.

Does anyone have ideas as to why this may be? When the system hits the ESS/DESS “minimum SOC limit”, the difference is much smaller (e.g., 0amps +/- 1 or 2).

This seems to be causing a “self discharge” overnight, while the system is scheduled to keep the SOC flat.

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Here’s our system setup:

  • 4x Quattro 10k units on firmware v556
  • Cerbo GX on firmware 3.60~53
  • 60kWh of Discover AES batteries, with a communicating Lynk II CAN-bus BMS (on latest firmwares)
  • AC coupled PV on the AC Output side of the Quattros
  • No DC loads outside of Quattros (I’ve confirmed the battery heater is not running as well).

We’ve previously posted on a related issue, where it seems the DESS scheduler does not take into account the power draw of the Quattro system itself. However the behavior above makes me wonder if there are multiple factors at play (DESS slowly discharging battery when scheduled to keep target SOC flat)

Thank you!

2

Thinking on this a bit more… is it possible the VE.Bus current reading excludes the power draw of the inverter/chargers themselves?

When the system is at the DESS/ESS “minimum SOC limit”, the system seems to take the power needed for the Quattros from the AC side of the system. So it would make sense the BMS and VE.Bus current report both zero current.

When at DESS “target SOC”, I believe this power is taken from the DC side of the system (if insufficient AC-coupled PV). So it would make sense the BMS reports a discharge current while VE.Bus reports zero.

EDIT: I enabled “Has DC System”, which does seem to indicate the VE.Bus DC Current value excludes the power consumption of the inverter/chargers. Would appreciate anyone’s confirmation.

i.e., Grid + PV Inverter = AC Loads
I’m fairly certain AC Loads excludes the inverter/charger power draw, so this delta is being included in DC Power.

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