DC Microgrid

My last entry on connector sparked some discussions in the altpwr telegram, and I've done a bit more research and thinking since, so I'd like to share some more connector options.

• IEC 60309
  
  Commonly known as CEE plugs, this standard has an option for low-voltage DC as well, mostly white 2/3 pin plugs. From my understanding, for 48V we would need a white casing with the keying at 10h, which are available in 16A and 32A. They seem to be a little hard to come by, but PCE has them in their product catalog, and Conrad seems to carry them. As with the other plugs from the IEC 60309, they are probably pretty study and weatherresistent.
• RJ45
  RJ45 is everywhere, and most of us probably have a few meters of networking cable lying around. Given that and the existence of Power-over-Ethernet might make this a decent candidate. There's a bunch of cheap adapters for so-called passive PoE, which is essentially just 12V or 24V across the two unused pin pairs when running 100Mbit Ethernet across a standard ethernet cable. Given the size of the connector pins, you're not going to move much current across these connectors.
• Weipu SP21
  A type of connector that came up in discussions at 36C3 and in the altpwr group. There's a 5pin version with 2 large power pins, and three smaller pins that could be used for communications. 

The microgrid is supposed to consist of many components and should have a number of devices connected to it. So one of the obvious questions is: how do we connect those components together? And one very obvious part of that questions refers to the type of plugs we use to do so.

There are a number of different connectors out there that are used for DC power, for different voltages and purposes. Let's take a look at some and their advantages and disadvantages.

• DC barrel plugs
  They have the DC right in their name, and are used for all kinds of devices. They are very common, but also come in a variety of different sizes.
• Car cigarette lighter plugs
  Used to connect to the normally 12V DC onboard systems in cars. Also very common, but there's a lot of low quality plugs out there.
  There's a smaller variant based on the ISO4165 standard as well
• XT60
  One example of many different battery connectors mostly used in the RC world. Small and sturdy, some can carry quite a bit of current (or at least that's the theory)
• XLR-4
  The 4pin variant of XLR plugs is sometimes used to carry 12V. But it also has numerous other uses.
• MC4 connector

  These connectors are mostly used for solar systems. The connector only has a single pole, but they are pretty weatherproof.

• SELV connectors
  There is an IEC directive for DC connectors, IEC 60906-3:1994. They look sturdy, and there's special keying in the connectors for different voltage levels. They are also incredibly hard to get a hold of.
• Speakon
  Not quite a standard DC power connector, the Speakon plugs can carry decent DC currents, and have a good locking mechanism. But they are normally used for connecting speakers, so there's a certain risk of connecting two very different systems to each other, and in the meantime breaking both.
• Anderson Powerpole
  Powerpole connectors are common in the amateur radio community, and come in different sizes/current ratings. The bigger connectors have a coloring scheme for different voltages.
• USB
  Nowadays, USB connectors are the standard connectors for low-current 5V DC connections.
• Screw terminals
  Too obvious to not mention, there's always the possibility to use screw terminals. Good for more permanent connections, but not exactly what we're looking for here I guess

There's also a list on wikipedia on DC connectors, which has some of the connectors that I mentioned here.

Now where does this leave us? Well nowhere really. So far, we haven't found the ideal connector, especially for higher grid voltages such as 48V. The most common ones like DC barrel plugs or the car lighter plugs are great if we want to support connecting all kinds of DC consuming devices, but they are not ideal for connecting different components of the grid together.

For the consume01 unit, we chose car lighter plugs for connecting devices, because that's what most devices on hand are using. For the grid side, we went with a Speakon socket, because of the locking mechanism (and because that's what we had on hand). Ideally, we would've preferred a SELV socket, but they are hard to come by. Another good alternative might have been Powerpole.

So we'll be using Speakon for most of our grid connections for now, because that's a sweetspot of features, availability and price, but we'll keep an eye out for better alternatives.

Updates: Picture & link for the SELV connector, and addition of the ISO4165 plug

The first unit we built for the microgrid is called Consume001, and as the name suggest, its intention is to enable convenient consumption of power from the grid. In its initial and simplest form, the unit consists of a power "inlet", in this case a 4-pole Speakon socket (plugs and sockets are probably worth a project log themselves), a DC power meter, and some 12V car power sockets (with switches).

Above is the first version we put together at CCCamp19 (turns out making a square hole without the proper tools is one hell of fun). The DC power meter is called GC90, and has a TTL port to allow reading it out with a microcontroller. So the list of possible/planned additions is the following:

• Remote readout of the power meter, e.g. via LoRaWAN
• Fuses
• Different power outlets, such as USB C, DC barrel connectors
• DCDC converter to allow different input voltages
• a few more "input" connectors to allow better spreading of the grid
• integrated battery backup
• complete replacement of the power meter with a microcontroller that then also controls the DCDC meter and can connect/disconnect loads depending on grid voltage

I started working on the remote readout as one of the first things. The power meter uses a micro USB for its TTL connection, which is a little weird but also makes it easy to interface. Here's the first shot at setting this thing up:

Notice the small USB plug on the left side fo the power meter. There's a chain of boards, first a USB A breakout (mostly hidden by the case cover), a level shifter (5V of the power meter <-> 3.3V of the Arduino), an Arduino LoRaWAN dev board, and an FTDI for programming. I'm currently building a dedicated PCB for that.

Right now, there are no fuses in the unit but most of these car plugs already have on in them. Still, this is high on the list, especially for the input.

The DCDC converter should be really easy in its simplest form, just putting a ready made unit between the input and the power meter.

The last idea is probably the most complicated but also the coolest in terms of what it would allow us to do. Let's see what happens...