Tesla supercharger debits me in multiples of kWh, what’s the rounding logic?

[fnegroni]

Whenever I visit a Tesla supercharger, it seems to debit my account in multiples of the price per kWh at that time.
How does it perform the rounding of the last few Wh delivered between me stopping the charge and the charger disconnecting?
At speeds above 70kW I can understand it well takes less than a minute, maybe a few seconds to deliver a kWh; but let’s say it has delivered part of a kWh in the final stage after pressing the stop button. How does it decide to round the number?
Other chargers normally charge in multiples of 10Wh or 100Wh

What do Tesla superchargers do?

 

[siteguru]

I've never used one so I can't answer that specific rounding question. (Does it round [up/down]? Or truncate to whole kWh?)

However ... and for ease of example ... at 60kW charge rate the charger will deliver 60kWh in 60 minutes; therefore it will deliver 1kWh in one minute, so a lot more time than "a few seconds" - 6 seconds would be 0.1kWh (100Wh).

 

[slmorgan42]

I've never used one so I can't answer that specific rounding question. (Does it round [up/down]? Or truncate to whole kWh?)

However ... and for ease of example ... at 60kW charge rate the charger will deliver 60kWh in 60 minutes; therefore it will deliver 1kWh in one minute, so a lot more time than "a few seconds" - 6 seconds would be 0.1kWh (100Wh).

I love a good number game lol

Ah but would you have to take loss in to consideration as kw travel to the car. Even though it is giving out 60kw there is I believe some loss in the cable and car before it reaches the battery.

E.g. when I use a 22kw (I know I can only get 11kw) I actually get about 9.7kw

Or is the charger showing the rate after loss? I have never compared what the car is showing as a charge rate against the charger itself. Mostly because my local 22kh charger doesn’t have a display.

 

[siteguru]

Even accounting for efficiency losses your "few seconds" statement is still not true - and the charger would deliver the power it says it is delivering (and charging you money for), so efficiency is actually irrelevant; it doesn't care how much the car actually stores, just how much it has sent.

 

[fnegroni]

Tesla chargers can deliver up to 250 kW and some cars charge at 170kW, 1 kWh every 21 seconds.

I did my own experiment. Stopped the charger when the app indicated 17.8 kWh delivered.
Near instantaneous stop via the app.
Got charged for 17kWh
Charging Losses around 4% (app shows 91kW, car onboard display 88kW)
Seems fair enough to me