Why do some registers return NaN instead of a value?

SMA inverters return NaN for registers that have no valid data at the current moment. The battery charge and discharge power registers (31393 and 31395) return NaN when no battery inverter is installed or when the battery is idle at night. Check that the battery subsystem is active and physically connected.

The gateway connects but no register values appear. What now?

The Modbus interface on SMA inverters is deactivated by default. Log into the inverter web interface, navigate to the communication or Modbus section, and enable the Modbus TCP server. Also confirm the unit ID in the portal matches the unit ID set on the inverter. A mismatch returns Modbus error code 11.

Write commands to the batterymode registers appear to do nothing.

All six BMS control registers (40236, 40793, 40795, 40797, 40799, and 40801) must be written together as a group. Writing only one register without updating the others within 60 seconds causes the inverter to ignore the command. Send all six values in sequence within that window.

Are there risks to writing the batterymode setpoint registers frequently?

Yes. Incorrect setpoints can push the battery outside its safe operating envelope and may affect warranty coverage. Write registers cyclically only at reasonable intervals (every 15 to 20 seconds is typical in practice) and always restore all six registers to their default values (40236 = 2424) before disconnecting your control client.

Free Modbus template: SMA Smart Energy Hybrid Inverter (battery)

Try the demo Shop

Free Modbus template: SMA Smart Energy Hybrid Inverter (battery) | ModbusCloud

Registers (10)

4 read / 6 write

Name	Address	Function	Type	Unit	Access
power_a Current battery discharge power, instantaneous, in watts.	31395	FC04	uint32	W	R
power_b Current battery charge power, instantaneous, in watts.	31393	FC04	uint32	W	R
energy Cumulative battery discharge energy, in watt-hours.	31401	FC03	uint64	Wh	R
soc Battery state of charge, 0 to 100 percent.	30845	FC03	uint32	%	R
batterymode_a BMS operating mode setpoint: controls charge, discharge, or default mode.	40236	FC16	uint32		W
batterymode_b Minimum battery charge power setpoint, in watts.	40793	FC16	uint32	W	W
batterymode_c Maximum battery charge power setpoint, in watts.	40795	FC16	uint32	W	W
batterymode_d Minimum battery discharge power setpoint, in watts.	40797	FC16	uint32	W	W
batterymode_e Maximum battery discharge power setpoint, in watts.	40799	FC16	uint32	W	W
batterymode_f Grid exchange power setpoint, positive to import, negative to export.	40801	FC16	uint32	W	W