Hello Sten,
Below is just new information from REC.
Some off my suggestions are available in the new firmware 2.8.
see below information from REC.
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Newest firmware version Victron 2.8 or SMA SI 2.6 uses CFVC coefficient from 0.1 to 1.0
Battery Pack’s Charging Algorithm:
The communication between the REC BMS and the Victron GX device is established through the CAN bus. All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to the GX device in each measurement cycle.
The charging current is controlled by the Maximum charging current parameter sent to the GX device. It’s calculated as
Charge Coefficient CHAC x
Battery capacity CAPA. The parameter has an upper limit which is defined as
Maximum Charging Current per Device MAXC x
Number of Inverter/Charger Devices SISN. Lowest value is selected:
Table 9: Maximum charging current calculation.
SETTING |
VALUE |
UNIT |
Battery Capacity (CAPA) |
100 |
Ah |
Charge Coefficient (CHAC) |
0.6 |
1/h |
Maximum Charging Current per Device (MAXC) |
75 |
A |
Number of Inverter/Charger Devices (SISN) |
2 |
n.a. |
Charge Coefficient CHAC x
Battery Capacity CAPA = 0.6 1/h x 100Ah = 60 A
Maximum Charging current per device MAXC x
Number of Inverter/Charger devices SISN = 75 A x 2 = 150 A
Maximum charging current is set to
60 A due to lower value of the
Charge Coefficient CHAC x
Battery Capacity CAPA.
When the highest cell reaches the
End of charge CHAR voltage setting, charging current starts to ramp down to 1.1 A x
Number of Inverter/Charger Devices SISN until the last cell rises near the
End of Charge Voltage CHAR (CC/CV). At that point the Maximum charging voltage allowed is set to Number of cells x (
End of Charge Voltage per cell CHAR–
Maximum Cell Float Voltage Coefficient CFVC x
End of charge hysteresis per cell).
End of Charge SOC hysteresis SOCH and
End of charge cell voltage hysteresis CHIS is set to prevent unwanted switching. SOC is calibrated to 100 % and Power LED lights ON 100 % Charge optocoupler is turned off. Maximum allowed charging current is set to 50% to allow supplying DC loads from charging devices like MPPTs. Charging current is limited to 30 % of the maximum charging current, but more than 5 A near both ends of temperature (
Max cell temperature TMAX and
Min temperature for charging TMIN) and when the battery is empty (Max discharging current is set to zero).
Charging is stopped in case of systems errors (See System Errors indication chapter). SOC is calibrated to 96 % when the maximum open circuit cell voltage rises above the 0.502 x (
Balance start voltage BMIN +
End of charge voltage CHAR), minimum open circuit voltage above balance start voltage and system is in charge regime.
In case BMS is not able to control the MPPT/Non-Victron charging sources directly (MPPT should be set to charge when the remote is in short), a small signal relay can be used to amplify the signal. MPPT should be programmed with its own charging curve set as End of charge voltage x number of cells. Digital output may be programmed with another task on request e.g. heater, under-voltage alarm, …
Figure 14: Charging diagram.
Maximum Cell Float Voltage Coefficient (CFVC):
Maximum Cell Float Voltage Coefficient CFVC has been introduced into the charging algorithm to enable cell float voltage change after the full charge. It may be set from 0.1 to 1.0 of the
End of Charge Hysteresis CHIS. When
End of Charge Hysteresis CHIS and
End of Charge SOC hysteresis SOCH have been met, full charge is enabled again. @ CFVC 50 % of maximum charging current is allowed to supply DC loads from MPPTs directly without discharging the battery pack below
End of Charge Hysteresis CHIS and
End of Charge SOC hysteresis SOCH.
Battery Pack’s Discharging Algorithm:
Calculated maximum discharging current is sent to the GX device by CAN communication in each measurement cycle. When the BMS starts/recovers from the error or from Discharging SOC hysteresis, maximum allowed discharging current is set. It is calculated as
Discharge Coefficient DCHC x
Battery Capacity CAPA. If this value is higher than
Maximum Discharging Current per device MAXD x
Number of Inverter/Charger Devices SISN, maximum discharging current is decreased to this value.
Table 10: Maximum discharging current calculation.
SETTING |
VALUE |
UNIT |
Battery Capacity (CAPA) |
100 |
Ah |
Discharge Coefficient (DCHC) |
1.5 |
1/h |
Maximum Discharging Current per Device (MAXC) |
100 |
A |
Number of Inverter/Charger Devices (SISN) |
2 |
n.a. |
Discharge Coefficient DCHC x
Battery Capacity CAPA = 1.5 1/h x 100Ah = 150 A
Maximum Discharging Current per device MAXC x
Number of Inverter/Charger devices SISN = 100 A x 2 = 200 A
Maximum discharging current is set to
150 A.
When the lowest cell open circuit voltage is discharged bellow the set threshold CLOW maximum discharging current starts to decrease down to 0.02 C (2 % of Capacity CAPA in A). After decreasing down, maximum allowed discharging current is set to 0 A. SOC is reset to 3 % and Discharging SOC hysteresis is set to 5 %. If the cell discharges below
Minimum Cell voltage CMIN, BMS signals Error 2 and SOC is reset to 1 % and internal relay switches off. If the Charger/inverter is connected to the grid maximum allowed discharge current is drawn from the grid. Otherwise, 100 % load current is drawn from the battery until maximum allowed discharging current is set to 0 A. Discharging current is also limited near both ends of temperature (
Max cell temperature TMAX and
Min temperature for charging TMIN) to 30%, but more than 5 A. If the minimum cell discharges under the
Cell-under voltage protection switch-off CMIN x 0.95 for more than 30 s BMS goes to deep sleep mode to protect the cells from over-discharging. OFF-ON switch sequence wakes the BMS from this state. CLOW cell voltage setting should be set to the voltage that corresponds to 3 % of the usable capacity.
Figure 15: Discharging diagram.