The Tesla High Power Wall Connector (HPWC) has its current limit set upon install. This is done via the rotary switch inside.
The screen inside the car shows this value as the maximum current I can draw from this charger. You can see the /32 here which is that indication.
The 15/32 that I have set here, is my manual setting to make the car only draw 15 amps from the 32 amps available.
The connector type is IEC 62196 Type 2. One of the pins is the Control Pilot which carries signalling information, including that information on how much current the car is able to draw (32 amps in my case).
In the unit I have here, the Control Pilot wire is the purple one. The orange wire is the Proximity Pilot.
Other electric car charging units are able to vary this signal of how much current is available to the car. This allows them to change the power that the car is consuming on the fly, and therefore can do things like match it to the amount of solar power which is available. Since the Tesla HPWC is hard set with a switch, this capability is not there.
The only way a Tesla can change its charge current (within the maximum available) is internally. This means that the car owner would have to be the one who does this change, and would have to be physically in the car to do it (no remote access for this setting).
A different way to go about charging only when there is enough solar available is by using the information from a Powerwall if installed. The Powerwall has CT clamps on the load and the solar, and an API that we can use to get that information in real time on the local network.
Although the current cannot be changed, the charging can be stopped and started again to regulate it in a duty cycle kind of way.
Given that clouds can happen at any time which would dramatically change the solar power output, it would be unwise to frequenty turn the charger on and off all the time. Instead, I’m going to measure the solar output every 30 seconds, but use an average of the latest 5 or 10 minutes to determine if the charger should be stopped/started.
Another factor that comes into play is the amount of charge that the Powerwall has in it. The Powerwall can buffer some of that power for when the clouds pass over, so it still wouldn’t be using from the grid. I want to ensure that the Powerwall doesn’t get drained, but that I can use some of the Powerwall charge in the morning to bridge the gap between the solar power available and the load power required.
Other functions could be added too, such as simply allowing the car to charge from wherever at maximum power if you’re just in a rush to charge it no matter what. Another case could be to allow the car to charge for a specific time. Basically, any kind of logic could be used to turn the car charger on and off.
The interface is currently a CLI as I haven’t finished polishing this yet, but I hope to one day just have a Raspberry Pi running it via a web server. This means there would be no need for any specific app, as you would just control it from a web browser.