[EN] Monitoring a Victron Multiplus inverter

In previous posts, I've described how I monitored the BMV battery monitor (reading what goes in and out of the batteries), and the MPPT solar controller (reading what comes from the solar panels). Most of the electricity, coming from the shore or from the fuel generator, is inverted back from 24VDC to 230VAC, to feed the boat's equipment, by the charger-inverter.

The protocol used by the Victron Multiplus isn't as straightforward as for the MPPT or the BMV. Indeed, inverters need more advanced communication systems: they can be hooked up together, either in parallel to addition their maximal output power, or with a pi/3 phase-shift to produce a 3-phase 400VAC power supply. The protocol needs to be not only write-friendly for embedded microcontrollers, but also read-friendly. Although it's not published, Victron essentially states in its documentation "it's tricky to implement, but if you're sure you want to do it, write us and we'll send the spec". Sure enough, that's what I did and how they responded :)

I'm not publishing the spec, because it's theirs and they must have their reasons not to open it up more than they do, but it's clear, no-nonsense and not ambiguous. I wish all IETF or OMA specs high-profile public specs were as well written as that.

What to tell to an inverter?

I'm not going to implement, let along test, any code that might modify the inverter's settings in non-trivial ways, save for what can be done by a remote panel (limit current input, switch the charger and inverter functions on and off). Bricking the family's 230VAC supply would get me in too much trouble, and ban me from pursuing any monitoring expriment :)

So the functions I'll use are:

version, returning the device's firmware version, because it's a safe and easy first test:
```
> :assets.multiplus:version()
= { mode = "VE.Bus",
    version = 1130126 }
```

led, returning the status to be displayed on remote panels, mostly LED indicators in a state among on, off, blink and inverted_blink:

> :assets.multiplus:led()
= {
   inverter = "on",
   temperature = "off",
   overload = "off",
   mains = "off",
   float = "off",
   low_battery = "off",
   absorption = "off",
   bulk = "off" }

state, which conversely to led, sends to the inverter what can be set from a remote panel, i.e. the switch's state (on, off, charger or inverter) and the 230VAC input current limit (up to 16A on my system):
```
> :assets.multiplus:state('on', 16)
```

frame, returning instant statistics about an electric line, either the DC one connected to the batteries, the AC one in a signle-phase install, or each of the phases on a 3-phase system:

> :assets.multiplus:frame 'AC'
= {
   inverter_current = 9.1,
   inverter_factor = 1,
   phase_name = "L1",
   n_phases = 1,
   bf_factor = 1,
   mains_current = 0,
   mains_period = "n/a",
   voltage = 229.84 }
> :assets.multiplus:frame 'DC'
= {
   used_current = 6.27,
   provided_current = 0,
   inverter_period = 13.6,
   phase_name = "DC",
   voltage = 27.37 }

Implementation details

I won't describe the implementation in detail: there's no point if I'm not going to publish the spec it's based on. The subset of commands I'm using here is fairly standard: frames with a length prefix, a command byte to make sense of the arguments in the remaining of the frame, and a checksum byte. Relevant data are encoded as bytes, of (decimal) fixed-point multi-byte numbers.

The implementation is cut in two main modules, victron.mk2.encode and victron.mk2.decode. They convert between binary representation and Lua tables. Apart from translation, the code does typical management of serial data over Linux (through USB): read a frame (frame size then frame content), retry in case we stumbled in the middle of a frame or if checksum indicates a problem, close/reopen the port in case the USB/serial adapter gets lost, etc.

What do do with this?

For energy monitoring, the values in frame 'AC' .inverter_current and frame 'AC' .voltage allow to compute the amount of 230VAC current consumed in the boat.
frame 'DC' .provided_current indicates what's pushed form the shore or the fuel generator in the batteries.
frame 'DC' .used_current, combined with the MPPT figures, allows to compute the consumption of 24VDC devices.
led and state(switch, limit) allow to reimplement a software remote panel. Victron provides high-resolution pictures of their products, including the remote control panel, so with a quick CSS hack, a web app should be quickly pieced together. An Android app might follow, but I feel in no hurry to go down that rabbit hole :)

Bateau Elorn