All register values read as zero or are unavailable. Why?

The FEMS App Modbus/TCP Read Access must be installed and active on the FEMS box before any registers are served. Open the FEMS App Center, confirm the app is listed under Installed, and check that the Modbus TCP service is running. Without the app, the TCP port is not open.

Register addresses are off by one compared to the FEMS system profile. What is wrong?

Some Modbus tools default to a base address of 1, which shifts all addresses by one. The FEMS Modbus TCP interface uses a base address of 0. Set your tool or gateway configuration to base address 0 so that address 508 maps directly to the soc register without offset.

The gateway connects on port 502 but returns errors for addresses above 10000. What causes this?

Registers at addresses 10019 and 10024 are BMS hardware identification registers that may only become available after the battery tower has fully initialised and the BMS has reported its topology to FEMS. Wait for the system to reach normal operating state, then retry polling those addresses.

Cell voltage min and max readings look implausible or frozen. How do I diagnose this?

Check id_of_cell_voltage_min and id_of_cell_voltage_max to identify which specific cell is reporting the extreme value. If the same cell ID always appears at the limit and the spread between min and max is large, this may indicate a weak or faulty cell in the Fenecon Home battery rack that requires service.

Free Modbus template: Fenecon Home

Name: Fenecon Home
Brand: Fenecon
SKU: c307d913-e850-448c-bb82-643821b02cbf

The Fenecon Home is a modular lithium iron phosphate battery energy storage system for residential and small-commercial PV installations, managed by the FEMS energy management platform built on OpenEMS. This ModbusCloud template reads BMS-level data over Modbus TCP, including battery pack voltage and current, state of charge, state of health, per-cell minimum and maximum voltage with cell IDs, per-cell minimum and maximum temperature with sensor IDs, BMS protection status flags, per-BCU current limits, rack topology parameters, and hardware type. Having this data in your portal lets you track battery health, spot cell imbalances, and verify BMS limit compliance without manual access to the FEMS interface.

Registers (23)

23 read / 0 write

Name	Address	Function	Type	Unit	Access
voltage DC battery pack voltage in volts.	506	FC03	uint16	V	R
current DC battery current, positive discharge, negative charge.	507	FC03	int16	A	R
soc Battery state of charge, 0 to 100 percent.	508	FC03	uint16	%	R
soh Battery state of health, 0 to 100 percent.	509	FC03	uint16	%	R
min_cell_voltage Lowest individual cell voltage in the rack, millivolts.	510	FC03	uint16	mV	R
id_of_cell_voltage_min Cell index reporting the minimum cell voltage.	511	FC03	uint16		R
max_cell_voltage Highest individual cell voltage in the rack, millivolts.	512	FC03	uint16	mV	R
id_of_cell_voltage_max Cell index reporting the maximum cell voltage.	513	FC03	uint16		R
min_cell_temperature Lowest cell temperature across all modules, degrees Celsius.	514	FC03	int16	°C	R
id_of_min_temperature Cell or module index reporting the minimum temperature.	515	FC03	uint16	°C	R
max_cell_temperature Highest cell temperature across all modules, degrees Celsius.	516	FC03	int16	°C	R
id_of_max_temperature Cell or module index reporting the maximum temperature.	517	FC03	uint16	°C	R
bp_charge_bms BMS charge balancing protection status bitmask.	518	FC03	uint16		R
bp_discharge_bms BMS discharge balancing protection status bitmask.	519	FC03	uint16		R
max_dc_charge_current_limit_per_bcu Maximum allowed DC charge current per battery control unit.	520	FC03	uint16	A	R
max_dc_discharge_current_limit_per_bcu Maximum allowed DC discharge current per battery control unit.	521	FC03	uint16	A	R
rack_number_of_battery_bcu Number of battery control units present in the rack.	522	FC03	uint16	count	R
rack_number_of_cells_in_series_per_module Count of cells wired in series within each battery module.	523	FC03	uint16	count	R
rack_max_cell_voltage_limit Configured upper cell voltage protection limit for the rack.	524	FC03	uint16	V	R
rack_min_cell_voltage_limit Configured lower cell voltage protection limit for the rack.	525	FC03	uint16	V	R
upper_voltage Configured upper DC voltage limit for the battery rack.	528	FC03	uint16	V	R
battery_hardware_type Hardware type identifier code for the installed battery.	10019	FC03	uint16		R
number_of_modules_per_tower Count of battery modules stacked in each tower.	10024	FC03	uint16	count	R