DIY EV Charging: a guide for an idiot (me)

I have a charge point. I have a charger. I have a battery. You would think I could hook these three things up and charge my battery from the charge port, via my charger. Not so fast.

Charging an EV requires a little more than that because there is signalling that goes alongside the AC between the charge point and the car. And my charger does not handle it all.

Now, I have had all this working on the bench. But that was about six months ago and I’ve forgotten how it all works. So in order to try and fix it in my head, I thought I would write a post while I’m trying to make it work. Most of this information is drawn from Wikipedia and elsewhere on the web so don’t expect anything insightful here. It’s just me trying to make sense of it all.

The Kit

What I am talking about here specifically is AC charging via a Type 1 SAE J1772 connector, using the charger from a Mitsubishi Outlander Charger.

Alongside the AC wiring (live and neutral) there are three lines that run from the charge point to the car:

  • Proximity Pilot (PP)
  • Control Pilot (CP)
  • Protective Earth (PE)

The PP and CP lines have different functions in combination with PE.

Proximity Pilot

You can think of this as means of telling the vehicle that it is connected to the charge point. In fact, it is also known as ‘Plug Present’.

The charge point shares information about whether a vehicle is connected and how much current the cable is capable of carrying by varying the level of resistance between the PP line and PE – the protective earth. The resistance changes when the latch on the charging cable is opened or closed, as well as when it is connected and charging. By measuring this resistance (e.g. with a voltage divider and analogue to digital converter), the car can know when to start or stop charging (and not to move when connected).

Under IEC 62196 standard, the resistance on this PP-PE loop is also coded to the current carrying capability of the charging cable itself.

Control Pilot

The CP line carries a square wave whose duty cycle is varied to communicate to the EV how much current is available. A 50% cycle means a peak current of 36A and a continuous current of 30A is available. Down to a 10% cycle which means 6A is available.

The car varies the resistance between the CP line and PE to change the voltage on the positive half of the square wave. This allows the car to move the charging point through various states from ‘standby’ to ‘vehicle detected’, to ‘charging’ etc.

A diode in the circuit protects the negative side of the square wave so it always stays at -12V. This ensures that you can’t just drop the connector in a puddle and have it dump all its current: it needs to be connected to the proper circuitry.

Making it work

My charger handles all the communication over the CP line, so I don’t need to think about that. I just connect my charge port up to there. But, it doesn’t handle the PP line, so it doesn’t know when it is connected and hence when to start charging. For this, I need my Battery Management System. This makes sense: you want the thing that keeps an eye on the batteries to be able to turn off charging when they are full.

How do we do this? By reading the resistance on the PP line and using that to send some commands to the charger. OR, if we’re being hacky, by just manually putting the car into a ‘charge’ mode so that it commands the charger to begin charging once we’ve connected the charging plug.

These commands are carried over the CAN bus. There are two that are important to make things work:

  • One on ID 0x285 needs to be sent to the charger so that it can tell the charge point it is connected (0xb6 in Byte 2 if you’re interested)
  • The next on ID 0x286 sets the current in amps to pull (Byte 2 again, scaled 10x)

So, I need my BMS to detect either the connection of a charge port, or the car being put into charge mode, begin sending out these messages, and then stop when the battery is full – ideally ramping current up and then down as it starts to get close to 100%.

Fortunately the hard yards have been done by other people here. Now that I’ve reminded myself how it all works, I just need to actually make it work…

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