HMJ-2: Safe MQTT commands for toggling feed-in timer without crashing the device?

[ituri]

Hey everyone,

I'm running a small home automation setup with a Marstek B2500-D (HMJ-2, FW V113) and a Hoymiles HM-1500 microinverter monitored via OpenDTU. Since I don't have a Shelly on my smart meter, I built a Home Assistant automation that pauses B2500 feed-in whenever two specific HM-1500 inputs (my street-facing panels) together exceed 250W — basically preventing the B2500 from pushing power when the panels on that side are already delivering enough.

The automation is working, but I had to figure out the raw MQTT payload format by sniffing what the Marstek app sends. I ended up using cd=20 with the full timer config string (e.g. a1=0 to disable, a1=1 to enable), published to hame_energy/HMJ-2/App/<mac>/ctrl via hame-relay with inverse_forwarding_device_ids set.

It works, but only kind of — after sending a few commands the B2500 became completely unresponsive via MQTT and the app and I had to physically restart it. So I'm not confident I'm using the right commands here, and I'd love some clarity before I keep hammering the device.

My questions:

1. Is cd=20 the correct/recommended command for temporarily toggling feed-in (timer on/off) without writing to EEPROM?
2. Is there a cleaner way to just enable/disable a specific timer period without sending the full config string every time?
3. Are there any hm2mqtt control topics for HMJ-2 that handle this more elegantly? I only see status topics in my broker, no control topics.
4. Could sending repeated cd=20 commands cause the device to become unresponsive? Is there a safe rate limit or a better approach?

Would love to know if there's a more robust approach. Happy to share my HA automation YAML if useful for docs or examples 🙂

Thanks for the amazing work on this project!