Marstek Venus A (battery)
The Marstek Venus A is an all-in-one AC-coupled home battery with integrated inverter, designed for balcony and residential solar installations. This Modbus TCP template connects via an RS485-to-Ethernet adapter and reads instantaneous AC power, cumulative energy, and battery state of charge. It also exposes four write registers that control RS485 enable, user work mode, force charge/discharge mode, and charge power setpoint. Using this template in your ModbusCloud portal, you can monitor and remotely control the Venus A without relying on the Marstek cloud app.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 7, read and write
- Brand
- Marstek
- Last updated
Registers (7)
3 read, 4 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| power | W | Read | |||
| energy | Wh | Read | |||
| soc | % | Read | |||
| batterymode_a | Write | ||||
| batterymode_b | Write | ||||
| batterymode_e | Write | ||||
| batterymode_h | W | Write |
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Use cases
- Monitor battery state of charge continuously to detect when the Venus A drops below your daily reserve target.
- Track cumulative energy throughput over months to verify expected cycle count against LFP degradation curves.
- Alert on unexpected charge or discharge power readings that may indicate a misconfigured work mode.
- Compare instantaneous AC power against site consumption to verify the anti feed-in mode is holding grid export near zero.
- Report daily charge and discharge energy to building owners for self-consumption performance verification.
Installation
Connect an RS485-to-Ethernet adapter to the Marstek Venus A RS485 port using pins A(+), B(-), and GND, then configure the adapter at 115200 baud, 8N1, TCP server mode on port 502.
Place the adapter on the same LAN as the ModbusCloud Gateway, or connect the Gateway directly to the adapter's Ethernet port, ensuring the device is reachable on port 502.
In the ModbusCloud portal, add the Gateway, import the Marstek Venus A template, and set the unit ID to match the slave address configured on the RS485 adapter (default is 1).
Verify the connection by checking that the soc register (32104) returns a plausible value between 0 and 100; if reads fail, confirm the adapter baud rate and that no other client holds the single allowed TCP connection.
Things to watch for
Why do the batterymode write registers have no effect?
The Venus A requires RS485 control mode to be enabled before any write register in the 42000-42999 range takes effect. Write the enable value (0x55AA, decimal 21930) to batterymode_a (register 42000) first. Only after that will batterymode_e and batterymode_h be accepted by the battery.
The gateway connects but power and soc read zero or fail entirely. What now?
The Venus A allows only one simultaneous Modbus TCP connection. If another client such as Home Assistant or evcc is already connected, the ModbusCloud Gateway will be refused or receive no data. Disconnect the competing client, or schedule polling so only one connection is active at a time.
Could writing incorrect setpoints to the force charge register damage the battery?
Yes. Writing an excessively high charge power to batterymode_h (register 42020) while RS485 control is active can cause the battery management system to trip protection limits. Always verify your target watt value is within the battery's rated charge power before writing, and set batterymode_e (register 42010) to 0 (stop) before disabling RS485 control.
The soc register value seems off by a factor of ten. Is this a scaling issue?
Some firmware versions and RS485-to-TCP adapters apply a scale factor to the SOC register, where the raw value represents tenths of a percent (e.g. 900 = 90%). Cross-check the raw value returned against the Marstek app display. If there is a factor-of-ten discrepancy, apply a scale of 0.1 in your portal register configuration.
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