Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register
  • Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
  • The final cut of TMC Podcast #29 is available now with topics time-stamped. We discussed the Tesla Cybertruck's expected 1 MW Ultra-Fast Charging capability, the Tesla Semi Delivery Event, the coming Model 3 refresh (project "Highland"), and more. You can watch it now on YouTube.

Why is home charging only 32 amps on SR when supercharging is so much faster?

sterod

New Member
May 18, 2022
4
10
Toronto
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps? Was this just a d*ck move or is there some sort of physical limitation? I don't understand if there is a different charging module used by the car for different charging methods or is all the hardware the same regardless of how it is charging?
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
16,831
22,260
Riverside Co. CA
Ok, so that means that there are two chargers onboard the vehicle and they cheaped out on the AC hardware.

Supercharging completely bypasses the chargers in the car, so supercharging rates are irrelevant to the home charging rates.

The difference between 32amps for a SR and 48amps for a LR / P is a trim distinction choice by tesla. You are looking at a difference in 32amps vs 48amps, not supercharging rates. In actual usage of charging at home, there is not likely to be much difference, since the Standard range has a smaller battery so the overall charging time is likely to be similar.
 

RayK

Safety Score 90 (Was 96!)
Apr 5, 2016
3,081
3,182
San Jose, CA
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps? Was this just a d*ck move or is there some sort of physical limitation? I don't understand if there is a different charging module used by the car for different charging methods or is all the hardware the same regardless of how it is charging?
Yes, there are different charging circuits involved. You are also comparing apples (32A 240VAC home charging) with oranges (DC charging @ 400VDC or thereabouts).
 
OK, it's like this.

First, there's the Actual Battery Charger. It accepts DC, does a DC->DC conversion where the output goes into the actual batteries.

Then, there's the rectifiers. These accept AC, either at 120 VAC or 240 AC (in the states: In the EU, they got three-phase), converts that to a DC voltage, runs it through another DC-DC converter, with the final DC available for the Actual Battery Charger.

Then: the Actual Battery Charger can accept a DC voltage from one of two places: Direct from the plug at a Supercharger or from the output of the rectifiers.

Interesting factoids:
  1. DC->DC converters are, relatively speaking, cheap and highly efficient. They take in a DC voltage and, with a certain number of big, fat switching transistors, run current first one way through the primary of a transformer, then the other way through the primary of a transformer. This is done at a switching rate as high as the market will bear, somewhere (typically) north of 700 kHz, and I've heard of ones running as high as 10 MHz. The switching back and forth is pulse-width modulated and is controlled to set the output (secondary) side of the transformer output voltage.
  2. Why the high frequencies? Because transformers are made out of inductors; the reactance of the windings of a transformer is what makes a transformer go; and the reactance is 2*pi*f*L, so (a) one gets to use smaller inductors as the frequency goes up and (b) small inductors tend to have less losses than big ones.
  3. Then, consider the rectifiers. Input frequency: 60 Hz (50 Hz in the EU). The lower in frequency one goes, the larger the various inductors and capacitors required to filter the stuff to a reasonable DC value get. It's almost trivial to filter a 1 MHz 10 kW power line; trying to filter a 60 Hz 10 kW line means Extra Big Capacitors that are expensive, take up a lot of room, and, technically, have loss factors and heating issues that smaller capacitors don't have to contend with.
Next: Why 32A at 240 VAC? Well, that's about cost. My understanding is that there are two or three rectifiers in a Tesla these days. Each rectifier can do 16A and costs $$. Now, say one has a M3 LR or P: Comes with a 75 kW-hr or so battery. A full charge, starting from zero, with 240 VAC @ 48A = 11.52 kW is going to take 75 kW-hr/11.52 kW = 6.5 hours, using a Wall Connector and all three rectifiers.

Now, take a SR. It has a smaller battery. Checking on-line.. 60 kW-hr. Suppose this car has two rectifiers (which they do), so one gets 240 VAC @ 32A = 7.68 kW, which, for a starting-from-empty charge time of 60 kW-hr/7.68 kW = 7.9 hours.

Point is: That's also overnight. And it makes for a cheaper car, with two rectifiers rather than three, just like the smaller battery makes it cheaper. So, they size the rectifiers to match the battery size to save the customer some $$$.

Finally: As pointed out above, the charge rate at DC Superchargers isn't affected. You can still get 250 kW at a SC into a Tesla SR (at least, for a short while, while the battery's mostly discharged) just like the LR and P guys do, so it should take a little shorter time for an SR to get in and out than a P or LR.

Questions? Ask 'em, we got answers 😁
 

SageBrush

REJECT Fascism
May 7, 2015
13,770
19,013
New Mexico
Ok, so that means that there are two chargers onboard the vehicle and they cheaped out on the AC hardware.

No, it means that a Supercharger has a multi-megwatt electric feed and hundreds of thousands of dollars worth of charging equipment, compared to your home feed of ~ 30 kWatt and a $50 breaker.

You are comparing a 5 star hotel to a tent and wondering why the air conditioning is different.
 
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps?

Tesla didn't decide to do this, you did.

If you wanted a high current charger at home, conceivably you could have one. But you'd need to pay your utility for a direct connection to your distribution lines which might be at 13.6 kilovolts, a very large transformer, probably to 480 V three phase, or maybe higher, and then a DC fast charger. I'm guessing the hardware equipment is $250,000 alone, and there could be a few months to a year wait. You'll need to get permits and lay concrete pads to put the transformer and DC charger, get a licensed power engineer to make drawings and submit them to your utility for approval, and then pay a commercial electrical contractor to build and install it, and then get inspection from the utility before you can turn it on.

Now you can't take residential service any more so you need to pay for a new meter install of course and need to go onto large commercial rates with current demand charges so you will pay a charge monthly proportional to the maximum current (not energy) used in any 15 minute period during your billing month. That can be a few thousand $ per month with a nice super charger level current. Plus energy but that's probably a lower kWh rate fortunately.

Was this just a d*ck move or is there some sort of physical limitation?
It's the same dick reason I don't have a jet and three mistresses. Is that a physical limitation?
 

stopcrazypp

Well-Known Member
Dec 8, 2007
13,099
9,425
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps? Was this just a d*ck move or is there some sort of physical limitation? I don't understand if there is a different charging module used by the car for different charging methods or is all the hardware the same regardless of how it is charging?
The obvious answer is you don't have a 150+ kW (not kWh, which is a unit of energy) feed at your house, neither do most people. That 150kW is in DC as others mentioned, and it's pumping roughly 380V and 400A. Your typical residential home service usually is only 240V AC and 100A or 200A service, meaning all maxed out it can pump only 24kW or 48kW.

As for the onboard AC charger, the SR+ comes with a 240V 32A one (7.7kW), all the other models come with 48A (11.5 kW). The decision to have it slightly slower in SR+ is cost savings.
Onboard Charger

The reason the others are still only limited to 48A is limited space in a car (plus the fact few places have 20kW supply so they stopped offering that option). This is one 10 kW OBC that were under the rear seats of the older Model S/X (some had dual chargers for 20kW):
Charger-module_500.jpg

https://teslatap.com/articles/supercharger-superguide/
They stacked 12 of these in a cabinet to make the initial 120kW superchargers.
In case you weren't aware, a supercharger consists not only of the cable pedestal, but there is also a cabinet that holds the actual chargers:
images (1).jpg


For obvious reasons, there is no space to put 12 of those OBCs in the car. Here's an example of Model S with dual onboard chargers (where do you suggest they stash another 10 of them?):
Tesla_Doppellader.jpg


Model 3, did miniaturize that somewhat and put it the PCS that is on top of the battery pack, but still it would be a challenge to fit enough to handle 150+kW.


As others mentioned, if you are rich enough you can technically install your own DC charger at home, but few of us have the money for that. For example, Larry Ellison had a supercharger installed on his private island.
It appears Tesla's first Hawaiian Supercharger will be on Larry Ellison's 3,400-person island
 
Last edited:

Rocky_H

Well-Known Member
Feb 19, 2015
8,630
11,220
Boise, ID
Huh. I've seen people ask this question plenty of times on the forum, but I've never seen it where it started off right away with anger and false accusations like this. You see something new every day. I was also going to simply talk about that example of the 10 kW AC to DC converter and how there was a stack of 12 of them outside in the Supercharger cabinet. I think that illustrates it well.
 
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps? Was this just a d*ck move or is there some sort of physical limitation? I don't understand if there is a different charging module used by the car for different charging methods or is all the hardware the same regardless of how it is charging?
I know there are several replies out there that talk about AC vs. DC and supercharger vs. home, but what this all boils down to is the circuit that you have your car plugged into. Each circuit (plug) in your house has an amperage rating. Most of the plugs are either 15 amp or 20 amp. In some cases, electric stove or dryer, you may have a 30-amp circuit. A Tesla (all of them) will limit the current to 80% of the circuit rating. That's why when a Tesla is plugged into an ordinary 3-prong plug, it will only charge 12 amps (15 amp circuit) or 16 amps (20 amp circuit). My Model S will charge to 40 amps because I have a 240 volt/50 amp circuit to plug it into. 40 amps is the 80% of the 50. That's all there is to it - plain and simple. (I hope.)
 
  • Like
Reactions: DGSteig

drtimhill

Active Member
Apr 25, 2019
3,406
4,410
Seattle
I have a 2020 Tesla Model 3 standard range that can only charge a 32 amps. What I fail to understand is, if the car can supercharge at crazy 150+ kWh rates, why did Tesla decide to limit the home charging rate to 32 amps? Was this just a d*ck move or is there some sort of physical limitation? I don't understand if there is a different charging module used by the car for different charging methods or is all the hardware the same regardless of how it is charging?
Different circuity for AC (home) and DC (supercharging). And the home charging "smarts" are built into the car so it can plug into anything, while the supercharger is actively involved in power control, so much of the (expensive) power control for supercharging is built into the charger, not the car (which saves you money on the car).
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
16,831
22,260
Riverside Co. CA
Huh. I've seen people ask this question plenty of times on the forum, but I've never seen it where it started off right away with anger and false accusations like this. You see something new every day. I was also going to simply talk about that example of the 10 kW AC to DC converter and how there was a stack of 12 of them outside in the Supercharger cabinet. I think that illustrates it well.

This isnt ment to be a moderator note, just an observation from me as a regular member (not a mod).


The tone from some posters here was likely due to a couple of specific things the OP in the thread said in their first two posts. My experience here on TMC is that, while many times thread tones shift all over the place, the initial tone of a thread tends to evolve from not only what is asked / said, but how its asked / said, just like in real life.

The difference (between real life and online) is that because we cant see all the other non verbal queues that go along with communication, we insert that in our minds based on either past experience we have had with similar discussions, or how we feel or dont feel about a topic.

This is probably "front and center" in my mind right now, because (for example), recently, I have been accused here of being insensitive in mentioning to people who are railing about something or other and threatening to "sell the car" that either "other cars do that" or "they should sell the car if they are as unhappy as they say they are.

From my point of view, i am being compassionate, because I personally feel no one should be as unhappy as some people sound, about a vehicle. Life is too short for that, and its too easy to fix it (by selling the car and voting with your wallet). From some other peoples point of view, im being insensitive, or displaying a "take it or leave it" attitude.

Anyway, back to this thread... the couple statements made in the first couple of posts likely directed some (but not all) of the responses. We did also get some really good, technical responses too though.
 
OK, it's like this.

First, there's the Actual Battery Charger. It accepts DC, does a DC->DC conversion where the output goes into the actual batteries.

Then, there's the rectifiers. These accept AC, either at 120 VAC or 240 AC (in the states: In the EU, they got three-phase), converts that to a DC voltage, runs it through another DC-DC converter, with the final DC available for the Actual Battery Charger.

Then: the Actual Battery Charger can accept a DC voltage from one of two places: Direct from the plug at a Supercharger or from the output of the rectifiers.

Interesting factoids:
  1. DC->DC converters are, relatively speaking, cheap and highly efficient. They take in a DC voltage and, with a certain number of big, fat switching transistors, run current first one way through the primary of a transformer, then the other way through the primary of a transformer. This is done at a switching rate as high as the market will bear, somewhere (typically) north of 700 kHz, and I've heard of ones running as high as 10 MHz. The switching back and forth is pulse-width modulated and is controlled to set the output (secondary) side of the transformer output voltage.
  2. Why the high frequencies? Because transformers are made out of inductors; the reactance of the windings of a transformer is what makes a transformer go; and the reactance is 2*pi*f*L, so (a) one gets to use smaller inductors as the frequency goes up and (b) small inductors tend to have less losses than big ones.
  3. Then, consider the rectifiers. Input frequency: 60 Hz (50 Hz in the EU). The lower in frequency one goes, the larger the various inductors and capacitors required to filter the stuff to a reasonable DC value get. It's almost trivial to filter a 1 MHz 10 kW power line; trying to filter a 60 Hz 10 kW line means Extra Big Capacitors that are expensive, take up a lot of room, and, technically, have loss factors and heating issues that smaller capacitors don't have to contend with.
Next: Why 32A at 240 VAC? Well, that's about cost. My understanding is that there are two or three rectifiers in a Tesla these days. Each rectifier can do 16A and costs $$. Now, say one has a M3 LR or P: Comes with a 75 kW-hr or so battery. A full charge, starting from zero, with 240 VAC @ 48A = 11.52 kW is going to take 75 kW-hr/11.52 kW = 6.5 hours, using a Wall Connector and all three rectifiers.

Now, take a SR. It has a smaller battery. Checking on-line.. 60 kW-hr. Suppose this car has two rectifiers (which they do), so one gets 240 VAC @ 32A = 7.68 kW, which, for a starting-from-empty charge time of 60 kW-hr/7.68 kW = 7.9 hours.

Point is: That's also overnight. And it makes for a cheaper car, with two rectifiers rather than three, just like the smaller battery makes it cheaper. So, they size the rectifiers to match the battery size to save the customer some $$$.

Finally: As pointed out above, the charge rate at DC Superchargers isn't affected. You can still get 250 kW at a SC into a Tesla SR (at least, for a short while, while the battery's mostly discharged) just like the LR and P guys do, so it should take a little shorter time for an SR to get in and out than a P or LR.

Questions? Ask 'em, we got answers 😁
Wonderful discourse indeed! I am solidly educated on these things but I still learned so much from you. What a well thought system this is. I’m further realising how much real value I’m getting for the money. Mr. Musk has given us so much top quality engineering. These days top quality technical design like this is dead rare. Refreshing.
 
Some of the responses in here are just downright rude. This person obviously just doesn't understand that "home charging" is level 2 (and always will be) and that "Fast Charging" is "Level 3." This post is a prime example of the learning curve that many people who aren't geeks like us are going to experience when making the jump to EVs--maybe we can try and be nice and not jerks?

OP - TDLR: Level 2 Charging is limited in speed for technical reasons, not "dick move reasons". "Home Charging" is level 2. Supercharging is "Level 3," and of course will be much faster. If you want to know more, keep reading.

When you plug in at home (using the mobile charger that came with your car or a dedicated box on your wall in your garage)--your car has to "convert" that electricity (AC) into a form your car can use (DC) You are limited on how fast that can be by whatever charging cable/box you are using, your car and by the size of the circuit breaker you use in your electric panel.

The design intent behind EVs is that this type of charging is usually supposed to be done during a long period of inactivity (such as when you're' asleep or at work). Filling the car will take a few hours but the amount of money billed for this (usually charged per kWh consumed) is the lowest of the two charging options. This is where you'll see EV manufacturers hype up the cost difference between fuel and electricity. The hype (again, in MOST markets) is justified, the cost is much much lower than gas.

When you plug in to a commercial "fast" charger out on the road --be it Tesla's Superchargers or one of the other brands out there --the electricity is in the same form as the way it's used and stored in your battery. Because your car doesn't have to "convert" this, the speed can be much much faster. But you also pay a LOT more per kWh for this convenience (in many markets). Quick charging like this is primarily designed to be for road trips when you have to charge to make it to where you're going. There was a time when this was always cheaper than gas but recent price hikes have made this on par with gas in some situations and more expensive than gas in others. For this reason, most of us in here agree that if you primarily charge at home--you're always going to be spending less to run your car.

The last point I wanted to make is ts that the SR+ (and the newer version of this, now just called RWD) is limited to 32a when slow charging.
The LR and the P can accept up to 48a when being "slow charged" -- this is also a "trim level" difference as another person in this thread pointed out. The amount of "miles" gained back per hour when charging at 32a is around 30mph. The other more expensive Model 3's can get speeds as high as 44mph when charging from a box at home capable of outputting that much. MOST of us here don't find this to be THAT big of a difference.

Hopefully this is clear, even though it was a lot of info.
 

Rocky_H

Well-Known Member
Feb 19, 2015
8,630
11,220
Boise, ID
Some of the responses in here are just downright rude.
The original post that started the thread was downright rude, before any responses.
This person obviously just doesn't understand that ...
Then someone should start off with simply asking a question instead of pairing the question with an accusation.
maybe we can try and be nice and not jerks?
The original post started off being a jerk. Have you seen how forums (any forum?) goes? If people start off being decent and asking questions, they get answers. If they start off being rude and ranting, then they get less nice responses. This is fair.
 
  • Like
Reactions: Regaj and SageBrush

RayK

Safety Score 90 (Was 96!)
Apr 5, 2016
3,081
3,182
San Jose, CA
My take on the OP's question is (was) that it was asked from an ignorant position. Sorry, but that's my take on it. Most of us here have a fairly good knowledge about how our EVs work, that's could be based on our life or work experience (in electronics, software development, etc.) That's not always the case with a new poster. We don't know their background. Yes, the question could have been asked with much less rancor. And yes, the responses could have been more gentle. That the OP responded later on (post 12) acknowledging something new had been learned is what really matters.
 
  • Like
Reactions: ItsNotAboutTheMoney

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,344
18,538
San Diego
It accepts DC, does a DC->DC conversion where the output goes into the actual batteries.
I don’t think that this is the case. The DC fast charger negotiates with the car to provide the correct voltage/current directly to the battery.

There is a DC-DC in the vehicle, but that converts from HV to the “12V” bus which powers the computers and such (and charges the 12V battery). Not sure the specs exactly but something like several kW output.

Corrections appreciated if I am wrong, but having a DC-DC converter at 250kW input in the car seems problematic even with “low” loss. Plus there is not a need since the fast charger can make appropriate adjustments just one time, as it converts AC to DC.
 
  • Like
Reactions: Rocky_H
I don’t think that this is the case. The DC fast charger negotiates with the car to provide the correct voltage/current directly to the battery.

There is a DC-DC in the vehicle, but that converts from HV to the “12V” bus which powers the computers and such (and charges the 12V battery). Not sure the specs exactly but something like several kW output.

Corrections appreciated if I am wrong, but having a DC-DC converter at 250kW input in the car seems problematic even with “low” loss. Plus there is not a need since the fast charger can make appropriate adjustments just one time, as it converts AC to DC.
In which case, I stand corrected. This does imply that the city-power-to-DC rectifiers are also directly connected to the batteries and have variable output voltage. Which kind of sounded wrong to my ear. I need to find a technical manual for a M3 or something and dig.
 

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top