Reserve for car charging not being “honored”

[Steve2498]

I have a Model S and what I assume is a 1st generation Tesla wall charger (2015).

i set the Powerwalls to self generation mode with the reserve for power outages at 50% and the “share with vehicle” at 15%, I.e. “save for home use” is 85%.

last night the car was charged from the PowerWalls until they hit 50%. I was expecting the grid to take over once the car charging drained the PowerWalls to 85%. Then once the car was charged, the PowerWalls would again feed the house until they hit 50%.

Am I misunderstanding how the “share with vehicle” setting is suppose to work?

thanks.

 

[jjrandorin]

Yes, you are misunderstanding how it works because that setting is only used when off grid at this time. When on grid, it is not used.

Because there was a recent app update, I checked in my own tesla app again, and the verbiage is still there on that feature "vehicle charging when off-grid", so this is still the case.

 

[Steve2498]

Thanks for the clarification.

So that brings up another question.

Let’s say your settings are as I describe in my original post: 15% for charging and 50% reserve for outages, and you are off grid, how does the gateway know if the load is coming from the car charger or some other load in the house? For that matter, how does the car stop charging, while letting the battery drain down to the 50% level. Clearly there are still some things I don’t understand about this.

 

[jjrandorin]

When you are off grid, there is no "reserve" level. If there were, then when it hit that level, your home would be unpowered altogether.

The reserve level is for when you are on grid, how much power do you want to reserve (not use) in case there is a power outage. By definition, if you are in a power outage, then you would want to use the power you reserved for a power outage.

The question on "how does the gateway know where the power is coming from", when you plug in to charge, there is a handshake that the car does with whatever charger you have plugged in. I am fairly sure that the "stop charging" is the car stopping its charging, not the gateway stopping providing power.

I believe thats why when the feature was first released, it was only for some models of teslas (3 and Y) and not others (and is not dependent on having a wall connector).

Note that, there are some of us which wish the car would do that "balancing" that you asked about in the first post, when on grid as well. It would be nice to be able to let it automagically only drain the powerwalls down to a certain percentage, when on grid, then switch to grid charging for the rest (without that being the set reserve).

 

[Steve2498]

When you are off grid, there is no "reserve" level. If there were, then when it hit that level, your home would be unpowered altogether.

The reserve level is for when you are on grid, how much power do you want to reserve (not use) in case there is a power outage. By definition, if you are in a power outage, then you would want to use the power you reserved for a power outage.

The question on "how does the gateway know where the power is coming from", when you plug in to charge, there is a handshake that the car does with whatever charger you have plugged in. I am fairly sure that the "stop charging" is the car stopping its charging, not the gateway stopping providing power.

I believe thats why when the feature was first released, it was only for some models of teslas (3 and Y) and not others (and is not dependent on having a wall connector).

Note that, there are some of us which wish the car would do that "balancing" that you asked about in the first post, when on grid as well. It would be nice to be able to let it automagically only drain the powerwalls down to a certain percentage, when on grid, then switch to grid charging for the rest (without that being the set reserve).

Regarding your comments about a reserve when off grid…of course, I wasn’t thinking that through.

I think I understand your explanation of how it behaves, but I’m still left with, “how does it do that”? Once you hit the threshold set for use by the car, the gateway somehow tells the car to stop charging?

 

[jjrandorin]

Regarding your comments about a reserve when off grid…of course, I wasn’t thinking that through.

I think I understand your explanation of how it behaves, but I’m still left with, “how does it do that”? Once you hit the threshold set for use by the car, the gateway somehow tells the car to stop charging?

Im not sure, actually. I think (but dont know) since both devices are in the tesla app and tesla has data for both, the gateway is reporting to the tesla app battery charge level, and when it hits a certain charge level, the car likely sees that charge level and throttles itself back.

For tesla, their chargers are all "dumb" chargers. The chargers dont have scheduled charging, etc. They just announce how much power is available, and the car requests what it wants.

My guess (and want to stress guess) is, is that when the threshhold is reached, the gateway is signaling the tesla app of its status, the car sees the status (threshhold hit) and throttles itself down to a trickle charge. I dont know, how it works but thats what I think.

 

[MP3Mike]

I think I understand your explanation of how it behaves, but I’m still left with, “how does it do that”? Once you hit the threshold set for use by the car, the gateway somehow tells the car to stop charging?

The car and Gateway both talk to the Tesla servers, so they share information that way. If your Internet and/or the cellular network is down during the outage it probably can't honor that reserve setting.

 

[miimura]

The feature doesn't rely on direct communication between the Powerwall system and the car and the car charging equipment doesn't matter. It could be a cloud data implementation as stated above. However, the Powerwall system could also be doing something to the micro-grid waveform that the car is looking for to trigger charging stop. It could be as simple as changing the frequency from 60.0Hz to 60.2Hz or 59.8Hz and back in some pattern. I don't have any actual details for how it is really done.

 

[Steve2498]

This has been enlightening. While factual knowledge seems to be scarce, I appreciate all the informed speculation on how this works!

thank you

 

[jhn_]

The fact that it’s only functional while off grid lends me to believe the communication to the car from the gateway in the Powerwall installation is via frequency shifts. That and Tesla says no internet connection is needed for this to function. It’s not all speculation though. Other than the exact method of communication, Tesla outlines the feature pretty thoroughly here:
Vehicle Charging During Power Outage | Tesla Powerwall

If it was via the gateway signaling up to Tesla’s servers and then back down to the car, it would have been a no braining for them to implement this feature for when the grid was active. And a lot of us have been asking for a way to have the car use excess solar before sending to the grid for a looooong time.