I have a situation with an offgrid system that I would like to share with you to try to resolve.
This is the list of equipment used:
3x Quattro 48/15000/200-100/100 230V
1x Cerbo GX
1x GX Touch 70
1x Orion-Tr 48/24-5A (120W) Isolated DC-DC converter
6x SmartSolar MPPT RS 450/200-Tr
2x Inversor Fronius Symo 15.0.3
15x CEGASA eBick 280 PRO
The battery technology is LFP, with 15 280 Ah prismatic cells in series.
The layout is shown in figure 1. The Fronius is connected to the AC outlet. There is no dedicated energy meter for the load.
When the SOC reaches 100%, the voltage requested by the BMS (CVL) drops and enters a current fluctuation between charging and discharging. The battery charges and discharges at low current and quickly (figure 2). The attached video help understands the situation.
The system has been running for more than 3 months with the same behavior.
I have 2 hypotheses for the origin of this problem:
- The CVL drops from 51.9V to 50.9V. In this condition there is a voltage drop of ~0.2V due to loss in the wire. It may be that the voltage reaching the battery is very close to the battery's open circuit voltage. A possible solution would be to adjust the value requested by the BMS by increasing the voltage a little.
- At the same time that SOC = 100%, AC consumption also starts to fluctuate a lot, which to me doesn't make much sense. However, as the system has AC photovoltaic coupling and does not have an energy meter dedicated to the load, the AC Consumption value is an estimated value and not measured. The system then tries to reach the charge value and in this adjustment the battery compensates for the estimation error. There is also the fact that the CCL only goes to 41A and may be too low a value for the system to control. In this case, the solution would be to install a dedicated energy meter for the load.