Modbus with two master devices (adding a charger for Volkswagen ID.4)

I wrote earlier that I’m using the Carlo Gavazzi VMU-C as a datalogger and as a datasource for electricity metering. Now I got a car. Volkswagen ID.4 the be exact. It’s the “Pure” version, so the smaller one of the battery options and with a 2×16 A internal charger. I want to use home charging since it’s cheaper and really convenient. We also have a winter here in Finland. Being able to charge and preheat the car at home is a nice to have. My house has 3×25 A main fuses and even with not using electricity for heating, I still wanted to implement dynamic load management (DLM) for the EV charger.

Since I already had a modbus capable energy meter, I though it would be a nice source for the load management. Garo seems to be having EV chargers that are capable of connecting to modbus and even better, they seem to be using energy meters OEM by Carlo Gavazzi. The problem here is, that modbus can have multiple slave devices (in my case electricity meters), but only one master device (in my case the VMU-C). Luckily there are solutions to this and this is what I ended up adding to my earlier system:

  • Garo GLBDC-T222FC EV charger, limited to use 3×16 A at max
  • ICP DAS tSH-735 serial port sharer
  • Carlo Gavazzi EM-340 (as a dedicated energy meter for the EV charger)
Connection diagram for my modbus setup

The tSH-735 is a small din-rail mountable device for sharing the modbus. It allows multiple setups. I’m using it for sharing with cache enabled and with different COM port parameters for the EV charger compared to rest of the bus. I initially had quite a lot of errors but tweaking the timing parameters did the trick. The setup seems to be working well now.

Having all of the meters reachable for the VMU-C allows me to report the data with Grafana as described in the earlier posts. In the picture below the red line is charging utilization and green area is our electricity connection load (for phases 1 and 2 that are also used by the charger of the ID.4). You can see that the charger carefully adds more load when there is available capacity (eg. 15:39). If needed charging can be fully stopped rapidly (16:02). DLM seems to be working as expected and gives a peace of mind to start charging without worrying of overloading our electricity connection.

Garo dynamic load management (DLM) in action

As a side note the car feels so far solid. It’s a reasonable transport for our family with plenty of space and a nice ride. A step up for sure from a cargo bike, that served us for the last three years, now that the kids wouldn’t have fit in to it anymore.

My Volkswagen ID.4 at the local store fast charger
These ones will be now off daily duties

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