Is there any advantage of leaving the X plugged in even when charge is completed?

[Fiver]

This Youtube disproves that. When your pack completed its charge and it draws 0 amps / 40 amps or whatever your charger amp limit is, then you turn on your HVAC, the ampere drawn would move up to as much as 10 amps / 40 amps. That's 10 amps from shore power without using the 10 amps from the battery.

DM me if you want the wiring schematics and you can see for yourself.

 

[Batt Car]

It's actually fairly simple. The AC-DC converter puts current on the bus. When there's a draw (i.e. the heater running), it draws down the current from the bus. The current is coming from the battery and the AC-DC converter.

Now, if the current from the AC-DC converter is enough to run the accessory, great. If it needs more than is available on the bus, the battery will supply it.

True whether the vehicle is plugged in or not. You're right - there's no contactor that opens to isolate the battery and use shore power exclusively. But that said - if the battery's topped off and there's still enough current on the bus because the charger is plugged in, it's not going to draw down the battery any.

So yeah, the rule is as always: ABC. Always be chargin'. I set my Model 3 to 90% and forget aboout it. When it's home, it's plugged in. Going to do the same when the X arrives next week.

dmurphy has it right. The schematic supports this. This is the same principle Tesla uses for the power wall and every electric distribution company uses to load service. The bus is the key to supply from an energized supply or from a standby storage battery. If the bus maintains the load the battery is not discharged. If the load exceeds then the battery supplements. It is basic conservation of storage devices. Check any UPs backup system and it is the same, especially when they are leveraging low cycle lead acid batteries like in data center setups.

 

[hill]

i used to charge the model X at a the measly minimum on the HPWC .... just a few KW's. In effect, it does the same thing because the parasitic draw can be so huge I have everything running even though the car's ignition is off.
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[Fiver]

After reading another thread on this topic, I'm starting to realize the terms being used are causing confusion and I acknowledge that I'm not helping as I'm also phrasing things incorrectly regarding where power goes in the car. I'm going to take 2 posts by user ZOMGVTEK that sums up what I'm trying to say here:
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"If you’re talking about when the car isn’t charging, but is plugged in and on, the wording can be tricky. The high voltage stuff is all tied together, there’s no way or need for anything to be routed in any sort of way. What happens is the charger tries to match the consumption of the loads on the car so that the end result is zero power in or out of the battery. The battery is still very much critical for this, and it’s going to get some amount of power in or out to smooth out the minor fluctuations. The charger can’t respond quickly enough and likely doesn’t have or require sufficient resolution to ensure zero power flows in or out of the battery, but it’s likely nearly zero.

So yeah, kinda, power gets dumped into the battery. But effectively it will get used preferentially by the various loads before any appreciable amount hits the cells. But the battery is still very much connected, critical for this to be stable, and has some non zero power running in and out to make up the presumably very minor visitations between supply and demand. It’s like grid tied solar, or a power wall. You don’t directly connect it to loads, but your local appliances will preferentially use that power before the grid power since there’s a larger potential difference between the local generation and the grid power. This must be the case or else the power wouldn’t flow into the grid."

Electrically the power from the charger never bypasses the battery. Some power will flow both in and out of the battery to compensate for the difference between commanded supply and the actual load. In practice with high frequency SMPS like the PCS, you can all but guarantee some non zero power flowing ‘through’ the battery, effectively.

If the concern is the wear on the cells in this use case, it’s near zero. So in practical terms the battery isn’t being used. It does happen to be very much in circuit, likely critical for stable operation, and will have some non zero power transfer that presumably will be close to net zero long term.

One important distinction is the PCS must only ‘react’ to loads AFTER they occur. Just like a generator, the load is only noticed after it occurs, and the generator has to react to it without prior knowledge. This is why the battery is critical, it serves effectively as a capacitor in this case, otherwise the system would need relatively considerable additional local storage for stability which would add needless cost when you have a perfectly good battery. So there’s really just zero advantage to disconnecting the battery. The chemical reaction is always on and slowly degrading the pack, might as well use it to smooth out a few ripples here and there.

You can technically run an ICE car without a battery, usually. But the system is usually wildly unstable and the only thing helping stability in that case is the number of devices with a linear response to voltage. In a vehicle with lots of SMPS driven accessories, this falls apart in a jiffy. The system in M3 most likely wouldn’t work without the battery present.
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/edit. Just to build on this. My car right now is plugged in, but not charging. It has its charge limit set to 50%, but currently it's SoC is 61% (I charged while at the gym earlier). If I remotely turn on the heater, even though it's plugged in, it will power the heater from the battery and the SoC will drop until it gets below 50%. It will not charge the battery and run the auxiliary systems off that same HV bus (what you are calling "shore power") until it gets below whatever charge limit I've set.

Shore power indicated that everything is powered by the wall bypassing the battery, which is not the case.​

 

[Martin VanB]

The 12 volt battery won't discharge as often when you keep the charger plugged into the car. The result is your 12 volt battery will last longer.

When connected things run off the charger.

 

[Bigriver]

This thread has turned primarily into a discussion of whether all wall power goes through the battery or sometimes goes directly to powering the car. I have no knowledge of this, but will briefly return to the original poster:

does the X hold full charge if/when always plugged in?

or do the batteries have its typical idle drain and the charger will always "top off" when it sees/sense a drop? i noticed i plugged in my charger last night and got the full notification of 251 miles in typical fashion.

the charger is still plugged in, but this morning the X is now at 246 miles and the Tesla app says "charging complete". more or less curious to know the logic when the plugged-in charger continues to charge the batteries (ie, is there a threshold). hope that made some sense.

When just idle/sleeping and plugged in, the phantom drain for the car occurs the same as if it weren’t plugged in. I know this because my wall connector has a meter on it, and I once noted the readings during a week of non-use when I kept it plugged in. No wall power was used when the battery remained charged above the charge setpoint. The model X also remained unused while I was out of town for a month last year. I left it with 60% charge, plugged in, and the charge setpoint at 50%. Here is the progression:
Day 0 - 60%
Day 7 - 52% (software update)
Day 14 - 47% and it charged to 50%
Day 20 - 48% and it charged to 50% (also software update)
Day 25 - 47% when I arrived home

@animorph has it right in post #6 that it does draw wall power for heating or cooling the car. Easily checked with my setup, as all I have to do is turn on a/c or heat with my car plugged in and watch the meter on my connector tick up the numbers.

So your original question was should you leave it plugged in? And everyone responds yes. And the owners manual says yes. And I agree there is no harm in leaving it plugged in. But I also think there is no magic imparted to the car by having it plugged in when you are not charging or preconditioning. I believe the Tesla guidance was originally formulated to keep it simple and to help instill good habits in EV owners, many of us who drove our ICE cars to 0 miles range, let our cell phone batteries die before we go find the charging cable, and some who may be absent minded and forgetful. So teachTesla owners to plug in and keep us from stupidly bricking our batteries. But that being said, I don’t keep either of my Tesla’s plugged in when I’m not charging them. I don’t precondition them before driving, and if no wall power is being drawn, having it plugged in can’t have any effect, positive or negative, on the health of the battery.

 

[Fiver]

The 12 volt battery won't discharge as often when you keep the charger plugged into the car. The result is your 12 volt battery will last longer.

When connected things run off the charger.

The 12v battery will discharge just as much regardless of if the car is plugged in or not. The 12v battery is charged once its voltage drops below a certain threshold. There (to my latest knowledge) is no constant float charge applied to the 12v battery like a ICE car's alternator. The battery runs, loses voltage, and is recharged by the DC/DC system back up to it's spec. This constant charge/discharge arrangement is part of why people have 12v batteries die so often in Teslas. Being plugged into the wall or not doesn't change the things that are powered through the 12v battery. If they did a constant float charge on them (the 12v battery) they would last longer. This is part (just one part) of why Elon mentioned that he wanted to do away with the lead acid 12v battery and just power the 12v (or 48v if he get's his way) systems directly from the DC/DC converter box.

This is why if the main pack is dead, you can attach jumpers to the 12v and power up the MCU etc, but the same isn't true if you plug the car into the charge port.

 

[mxnym]

OK, there's never been any Tesla that can do the charging you describe in kW, I believe you meant to say amps. The original S/X had either a 40amp single or 80amp dual charging setup, which (in the US) provided at max, 22.16kW. (80amps@277v). Most charged with 40amps @ 240v for 9.6kW. Skip ahead a few generations, and currently all shipping Tesla's come with either a 32 or 48amp onboard charger.

This is correct, I confused my terms, so that diagram does match up with the first gen S where you could have 40 amp or 80 amp charging depending on whether or not you got the optional dual charger.

Everything inside the car is DC powered. That DC current is pulled from the battery (350v to 400v depending on car model), and is stepped down through the DC/DC converter en-route to the various sub-systems inside the car. The majority of which are 12v, but some systems (cabin heat, air conditioning, battery coolant, drivetrain) run at different voltages, but are still DC.

I relate exactly what the repair manual says: "The vehicle recognizes when the AC charge source is connected using CAN communication with the BMS. When charging from an alternating current (AC) source, current flows from the source through the connector to the charge port. From the charge port, it passes through the HVJB (High voltage junction box) and then is routed by bus bars to the on board charger(s). The charger(s) convert AC to DC and supply current to the battery.
The purpose of the HV battery is to provide power to drive the car and run all the accessory systems. It is the primary energy source for the vehicle. The DC-DC converter also functions as a high voltage junction block, distributing current from the HV battery to the A/C compressor, coolant heater, and cabin heater.

I'm not arguing these points, my contention is the same as others, the power doesn't go THROUGH the battery. The equipment effectively runs off of the charger, and as a side effect, minimal amounts of power effectively go into and/or come out of the battery because the bus does not control power flow directionally and there is no isolation performed.

/edit. Just to build on this. My car right now is plugged in, but not charging. It has its charge limit set to 50%, but currently it's SoC is 61% (I charged while at the gym earlier). If I remotely turn on the heater, even though it's plugged in, it will power the heater from the battery and the SoC will drop until it gets below 50%. It will not charge the battery and run the auxiliary systems off that same HV bus (what you are calling "shore power") until it gets below whatever charge limit I've set.

Shore power indicated that everything is powered by the wall bypassing the battery, which is not the case.​

Sure, the BMS will prioritize reaching the desired charge, but you've effectively told it to do that. In the opposite direction, when you have scheduled charging set, this isn't what happens. For instance, I have my battery set to charge to 80%, yet when I get home for lunch and the battery is at 68%, if I plug in, it doesn't start charging. If I turn on the A/C, the HPWC relay flips, but it still doesn't start charging, the battery remains at 68% even when I use keep climate on for 30 minutes in spite of the fact that my 100A HPWC circuit wired to 72A X charger could easily charge the battery up during that time. Your two piece argument that it isn't "shore power" has a problem in each piece. First, the suggestion that it's DC means it's not shore power is effectively an argument that a charger isn't a transformer. Second, the suggestion that the battery has to power the components before switching to the power means it isn't shore power is effectively a suggestion that they must turn everything off before flipping the power on even the largest of military watercraft. Do you really support that argument once it's rephrased that way? It is more reasonable to suggest that it isn't necessarily shore power because the BMS only uses the shore power when there is enough demand to justify it, and perhaps that's just what you mean.

The 12v battery will discharge just as much regardless of if the car is plugged in or not. The 12v battery is charged once its voltage drops below a certain threshold. There (to my latest knowledge) is no constant float charge applied to the 12v battery like a ICE car's alternator. The battery runs, loses voltage, and is recharged by the DC/DC system back up to it's spec. This constant charge/discharge arrangement is part of why people have 12v batteries die so often in Teslas. Being plugged into the wall or not doesn't change the things that are powered through the 12v battery. If they did a constant float charge on them (the 12v battery) they would last longer.

I believe your knowledge is correct here, based on the same observation I mentioned earlier. The HPWC has a relay, so there is presumably no AC power drawn unless that relay is flipped, and that relay isn't flipped unless there is sufficient demand to justify it as in my example above. As such, the 12V equipment must run off of the 12V battery even though plugged in (IOW, plugged in is not NECESSARILY / not ALAWYS shore power).