SOC discrepancy between the Renon BMS/Cerbo GX pair and a Multi RS

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Subject: The MULTIRS and Renon Bms report different battery charge levels

System architecture: Single-phase 220 V, 50 Hz installation. Communication between devices = VE.CAN and LAN network. MultiRS 48V_600Va MPPT 450_100 (firmware V1.31), Cerbo GX MK2 + GX Touch 50 (firmware V3.80), VM-3P75CT energy meter (firmware V1.11), Renon Ebricks batteries (2 x 5.12 kWh), Trinasolar PV panels TSM-455NEGR9R.28 (10 x 455 Wp)

Problem encountered: Case no. 1

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In VRM, the CerboGX/Renon Bms pair reports a current battery charge level of 39%, whereas the MULTIRS reports a current charge level of 100%.

The batteries’ actual current charge level is 39% (verified directly on the Renon battery monitor). So the CerboGX/Renon Bms pair is communicating correctly.

Consequences:

  1. Although the MULTIRS indicates that the batteries are at 100% charge, it allows the CerboGX to continue charging them.
  2. When there is no longer enough sunlight, the MULTIRS discharges the batteries, which it considers to be 100% charged, without taking into account that they are actually only at 39%. As a result, the SOC setting of 20% for stopping battery discharge is ignored, and the batteries are completely discharged…

Problem encountered: Case no. 2

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In the VRM app, the CerboGX/Renon Bms pair reports a current battery charge level of 100%, whereas the MULTIRS reports a current charge level of 47.3%.

The batteries’ actual current charge level is 100% (verified directly on the Renon battery monitor). So the CerboGX/Renon Bms pair is communicating correctly.

Consequences:

  1. The CerboGX/Renon Bms pair sees that the batteries are 100% charged, so it stops all charging from the PV panels and starts drawing from the batteries to meet the loads.
  2. The MULTIRS also starts discharging the batteries, but its reported charge level is only 47.3%. Note that, despite this charge level, it does not request power from the PV panels to continue charging the batteries, even though there is enough sunlight to do so.
  3. When the SOC calculated by the MULTIRS reaches 20%, it starts supplying the loads from the grid and the PV panels, even though the batteries are actually still charged to 100 - 47.3 = 52.7%…

Cause and resolution of the problem:

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The problem was caused by an incorrect configuration of the MULTIRS instance number in the CerboGX. With two different instance numbers, the three devices were not communicating correctly. The CerboGX pair was using the battery charge information from the RENON Bms, whereas the MULTIRS was making a theoretical charge calculation that did not reflect reality…

After setting the MULTIRS instance number to 0, the MULTIRS aligned with the battery data reported by the CerboGX/Renon Bms pair.

I hope this information will save other newcomers from spending hours trying to work out why there can be a discrepancy in SOC between the Renon BMS/Cerbo GX pair and the Multi RS.
Kind regards.

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