Registers 842 and 843 return zero or no response. Why?

Modbus TCP is disabled by default on all Victron GX devices. Go to Settings, then Services on the GX device and enable the Modbus-TCP service. Also confirm the GX device IP address is correct and that port 502 is not blocked by a firewall or VLAN policy on your network.

The state of charge value looks correct but battery power reads as negative when the battery appears to be charging.

Register 842 is a signed int16. Positive values indicate the battery is charging; negative values indicate it is discharging. This is the documented behavior of the com.victronenergy.system service. Check that your Modbus client is interpreting the value as a signed 16-bit integer, not unsigned.

Writing to register 2901 seems to have no effect on the ESS behavior.

Register 2901 must be written to Unit ID 100 and the value is scaled by a factor of 10, so a minimum SoC of 20% requires writing the value 200. Also confirm that ESS mode is active on the GX device, as the register only takes effect when the ESS assistant is enabled in VEConfigure.

Is it safe to write arbitrary values to the ESS minimum SoC setpoint register?

Use caution when writing to register 2901. Setting the minimum SoC too low can cause repeated deep discharge cycles that accelerate battery degradation and may void the warranty of certain battery systems. Always stay within the range recommended by your battery manufacturer and test changes during supervised commissioning, not during live operation.

Free Modbus template: Victron Energy (battery)

Victron Energy is a leading manufacturer of battery storage and energy management systems built around a GX device such as the Cerbo GX or Color Control GX. This template connects to the Victron Energy (battery) integration via Modbus TCP and reads system-level battery power and state of charge from registers 842 and 843, served by the com.victronenergy.system service at Unit ID 100. It also exposes the writable ESS minimum SoC setpoint at register 2901, allowing automated depth-of-discharge control. Installers can use this template to monitor battery status in real time and adjust energy storage limits remotely through the ModbusCloud Gateway.

Name	Address	Function	Type	Access
power System battery power in watts, positive when charging, negative when discharging.	842	FC04	int16	R
soc System battery state of charge, 0 to 100 percent.	843	FC04	uint16	R
limitsoc ESS minimum state of charge setpoint; battery will not discharge below this level.	2901	FC16	uint16	W

Victron Energy (battery)

Overview

Use cases

Installation

Things to watch for

Registers (3)

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