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Power use while plugged in but not charging

Swordtail

Member
Oct 23, 2020
26
8
UK
I'm planning on getting a PodPoint 7kW charger in the UK for my Model 3 I get next week. Tesla recommend you leave the car plugged in all the time, even when not actively charging.

I assume the charger will let the car draw even minimal amounts of power as it likes, even when not actively charging? But will the charger report this minor power usage along with the main power usage during charging? Or is it so minor it's not worth worrying about?

Sorry, I'm one of these people who like to know what's going on to the n'th degree, especially for exciting new things!

Cheers.
 
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Zoomit

Active Member
Sep 1, 2015
2,266
4,400
SoCal
Generally, the car won’t draw power from the EVSE unless it’s charging, preconditioning, or you’re in it and the HVAC is on. This is based on a few years of data from my OpenEVSE.

I believe the recommendation to always leave it plugged in is a vestigial directive that was important when phantom drain was significant and owners risked harming their battery when it drained quickly and stayed at low SOC.
 

brkaus

Well-Known Member
Jul 8, 2014
8,172
6,725
Austin, TX
@Zoomit has the more complete answer. Thanks.

The car will only pull power from the wall in specific conditions. Mostly charging but the others mentioned are also true.

the wall connector has contactors (relays) that enable power flow when the car wants it. I can listen to the WC and know when it’s connecting and disconnecting.
 

brkaus

Well-Known Member
Jul 8, 2014
8,172
6,725
Austin, TX
Yes, the BMS will use shore power to keep the battery happy under extreme temperature conditions.
Does it? I thought it used the battery power and if it drops below a threshold it will trigger shore power as part of a recharge cycle (which would then use shore power for heating and bringing the battery charge back up).
 
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camalaio

Active Member
May 28, 2019
1,483
2,188
Vernon, BC, Canada
It will pretty much only use the charge station for larger loads (charging, heating). The car still draws small amounts of power (plugged in or not), but these will be directly served by the battery rather than the wall power. This usage indirectly shows up as charging more than otherwise expected based on trip meter calculations ("phantom drain").

The recommendation to always plug in is a fairly non-detrimental suggestion that simply lets you avoid most issues you'd experience otherwise. There's a lot of cases where you can use more power than expected while parked, and being plugged in means you won't be surprised with not being able to make it to work in the morning. The car maintains itself perfectly fine when not plugged in, it just might do something you're not expecting. Rather than explain all these behaviours, simply "keep it plugged in" is short and sweet and does the job.

Does it? I thought it used the battery power and if it drops below a threshold it will trigger shore power as part of a recharge cycle (which would then use shore power for heating and bringing the battery charge back up).

Both things are true.

The basic summary of the behaviour is any large draw will (eventually) trigger the use of wall power. Small draws (e.g. just the car being awake for any reason including Sentry) will use battery power alone until the percentage drops a couple points, then it will recharge to the target percentage.

Any form of heating, cabin or battery, won't be a "small" draw, so these typically engage the wall power fairly quickly.
 

Swordtail

Member
Oct 23, 2020
26
8
UK
The tesla manual says "Whenever Model 3 is plugged in but not actively charging, it draws energy from the wall outlet instead of using energy stored in the Battery. For example, if you are sitting in Model 3 and using the touchscreen while parked and plugged in, Model 3 draws energy from the wall outlet instead of the Battery."

This was what made me wonder if the wall charger will recognise such a small power drain and switch itself on so the car can take it, and if it does, if the wall charger would log the usage.

Cheers.
 
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Zoomit

Active Member
Sep 1, 2015
2,266
4,400
SoCal
This was what made me wonder if the wall charger will recognise such a small power drain and switch itself on so the car can take it, and if it does, if the wall charger would log the usage.
The EVSE communicates with the car via the SAE J1772 protocol. It tells the car the maximum amperage available and the car tells it how much of that it wants. Any shore power transfer, for whatever usage by the car, will only happen if the two devices agree on that amperage and the EVSE closes it’s relay to allow current to flow at the amperage that the car decides to pull.

If the EVSE logs any power output, it will log even the “small” amount of just sitting in the car fiddling with settings *if* the car decides to pull it from the wall.

Here’s output from my OpenEVSE. The car was sleeping and unplugged. I plugged it in at 2:39 and it sat for 5 min not using any shore power while I swept my garage. I then got in the car and had the HVAC on, as indicated by the ~2 kW draw. I turned the HVAC off and the power draw dropped to 500 W (2A) for less than a minute then stopped. I’ve been in the car now for 5 minutes with the radio on but it is not pulling anything from the wall.

07A8C12D-6FE3-4E47-AE6B-E6347A97B693.jpeg
 
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Zoomit

Active Member
Sep 1, 2015
2,266
4,400
SoCal
For example, if you are sitting in Model 3 and using the touchscreen while parked and plugged in, Model 3 draws energy from the wall outlet instead of the Battery."
As I sit in my car typing this, the car is on but it is NOT drawing energy from the wall.

So I guess don’t believe everything you read in the manual.
 

camalaio

Active Member
May 28, 2019
1,483
2,188
Vernon, BC, Canada
The tesla manual says "Whenever Model 3 is plugged in but not actively charging, it draws energy from the wall outlet instead of using energy stored in the Battery. For example, if you are sitting in Model 3 and using the touchscreen while parked and plugged in, Model 3 draws energy from the wall outlet instead of the Battery."

This was what made me wonder if the wall charger will recognise such a small power drain and switch itself on so the car can take it, and if it does, if the wall charger would log the usage.

Cheers.

The manual has a lot of good info, but some parts are a bit of a stretch (like recommending to precondition the cabin of the car for over 30 minutes... what?). This part you found is just plainly wrong. It would be true if climate control is on, or in some fairly contrived cases, but normally not.
 
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Swordtail

Member
Oct 23, 2020
26
8
UK
Interesting stuff, thanks. Perhaps it depends whether the "feature" in use is powered by the 12v battery or the main battery. AC and heaters (I guess) would use the main battery (and so use wall power if plugged in) and the radio/display maybe the 12v battery. Perhaps when the car wants to charge the 12v battery it would draw energy from the wall rather than the main battery.
 

TheRFMan

Member
Dec 15, 2019
554
428
Ottawa, Canada
The EVSE communicates with the car via the SAE J1772 protocol. It tells the car the maximum amperage available and the car tells it how much of that it wants.

That is not quite how it works. Aside from the EVSE reading some voltages to ensure that the car is connected and then that it wants to request power, data communication is unidirectional from the EVSE to the car. The EVSE tells the car its maximum current capacity (via a pulse width modulated signal). When the car requests the power connection, the EVSE closes the contactor and pretty much connects the car's onboard charger directly to the wires going to your breaker panel. The car can then draw whatever current it wants up to the maximum advertised by the EVSE. The EVSE has no means of controlling the current going to the car, aside from blowing a fuse if the current exceeds the EVSE's capacity.

At a high level, EVSE's a just high-current switches with some safeties built-in.
 

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