The model 3 is going to be our secondary car and will only get driven a couple of times a week for just a few miles. So charging speed is not an issue at all. I'm wondering if charging at 120V is less efficient than charging at 240V. Any idea?
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Charging 120v vs 240v efficiency
- Thread starter wesley888
- Start date
Kermee
It's Not Easy Being Green
Short answer to your question is yes, 120V is less efficient than charging at 240V. However since you've stated that you're only driving a couple of times a week for just a few miles, the difference in costs due to the delta in efficiency may be negligible.
My opinion is that unless your electrical panel is really far from the garage and costs would be really high, the convenience of being able to charge fast outweighs the small one time cost to put in a 240v outlet. My parents have 3 cars and two drivers. They thought they could get away with just the 120v as well. A couple of unexpected events happened (one of which was me visiting with my Model 3) and the hassle of only having a 120v outlet convinced them to just add in a 240v even if it would be rarely used. Now, they use it a lot more than they thought just for convenience sake of having the car always topped up to 80%.
Efficiency isn't a great word to explain the difference here. They are both just as efficient.
But, they don't charge at the same rate for two reasons, When the cars is being charged, a percentage is used to maintain the car and the remainder used to charge the battery. Since the charge rate is lower for a 120V the percent of power going is lower.
120V @ 12A is 1440 watts
If it was 240V @ 12A it would be 2880 watts.
If 120V charging is good enough, then it is indeed good enough. If it is a double 120V 15A plug, then standard type, that can probably be replaced with a single 120V 20A plug and you can get the correct Tesla adapter and charge at a little faster rate for very little investment.
But, they don't charge at the same rate for two reasons, When the cars is being charged, a percentage is used to maintain the car and the remainder used to charge the battery. Since the charge rate is lower for a 120V the percent of power going is lower.
120V @ 12A is 1440 watts
If it was 240V @ 12A it would be 2880 watts.
If 120V charging is good enough, then it is indeed good enough. If it is a double 120V 15A plug, then standard type, that can probably be replaced with a single 120V 20A plug and you can get the correct Tesla adapter and charge at a little faster rate for very little investment.
eprosenx
Active Member
I think there are a couple of factors here: Speed and efficiency. They are linked to a large degree.
When charging the car, it wants the battery to be at a specified temperature range and I believe it runs the cooling pump continuously during the charging as well. So when you are charging there are certain fixed charging losses. Let's just say they are 400 watts (totally made up #). So charging off 120v @12 amps means you are only getting 1440 watts minus the fixed 400 watt overhead is 1040 watts. But if you are charging at 240v @ 12 amps you are at 2480 watts. That is nearly 2.5 times as fast.
Also, on the pure efficiency level- A 240v rectifier is going to be slightly more efficient than a 120v rectifier. This is one of the reasons we run all of our datacenters at 208v vs 120v.
From my standpoint, the less time the cooling pump and such is running perhaps the longer life you will get out of it (though this has to be balanced against other factors like if you run things at some fraction of their rated capacity it is often times better for them).
I charge my model 3 at 48 amps 240v every day as soon as I get home just because I can. There are no time of use provisions that make it cheaper to charge later, and I have a very robust electrical service that can trivially handle it. In the event I needed to go out in an emergency for any reason it is nice just knowing the car will be at 80% within about an hour of me getting home (that is what I set my limit to).
Now if someone can provide evidence that charging slower than 48a is better for my battery then I will modify my behavior, however, 11.5kW is so vastly slower than a supercharger that I suspect it can handle it trivially.
I don't ever want to be limited by my car not having enough charge, and I don't want to have to juggle car charging every day.
To the comment about being able to upgrade from a 120v 15a receptacle to a 120v 20a receptacle: Yes, this is very often doable in garages but not always!!! I did this in my garage, but you need to check that you have 12 gauge copper wiring and a 20a breaker. If not, this could be dangerous. But yes, that extra 33% (going from 12a to 16a charge rate) is huge. Due to those fixed losses during charging it makes a huge difference. I actually have a "cheater" adapter that lets me plug my 5-20 Tesla UMC adapter into a 15a receptacle. Note that this could be very dangerous, but I have only used it once when I 100% confirmed that the receptacle I was using was on a 20a circuit and it was the only plug on it and it was only a few feet from the subpanel. This still does not mean the 15a receptacle was rated for 20a, though it worked completely fine and did not get hot at all. I would never use this adapter at home, I would always wire in the right receptacle instead.
I think you mean a NEMA 14-50. Every plug in the US is a “ NEMA plug”. That’s like saying something weighs pounds without saying how many. Household outlets are NEMA 5-15 outlets, etc.
http://www.trci.net/media/4682/nemaplugreceptacleconfigurations.pdf
Thank you--this bugs me as well. How many metrics is it from Austin to San Antonio? No--"metrics" isn't a unit--it's the name for the whole system of units.
We have done 110 charging at our apartment for approx 1 year until the S sold. So, it works great for us. you usually can get approx 40 to 50 miles in the car a night. My wife's comute is under 25 or so a day. On hotter days, the car will consume more range miles that that however it has worked out great. I tell ya what I really like about 110 charging. i'm always hovering around the 50% area of the battery. On weekends, we are at a S/C . The Efficiency as you speak of was mention above and I agree, it's all about the time vs charge. With the 3 you may get more miles per hour than the S. Now we have ordered the X, I'm wondering if 110 will work out for us since it's a much hungry car.
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They are NOT the same efficiency, 208/240 is ~10% more efficient as I. Less power lost in transmit (heat) then 110/120
They are NOT the same efficiency, 208/240 is ~10% more efficient as I. Less power lost in transmit (heat) then 110/120
Jedi2155
Model 3 has Arrived.
You have too much confidence in me lol. I'm sure others have done a more thorough analysis than what I plan on typing here.
The efficiency between 120v/240v is likely similar but the losses with 120v is significantly greater. The charging efficiency refers to the actual conversion efficiency between AC and DC and both could be > 90% (240v is usually greater by a small amount).
The important to thing to understand is there will be a constant power draw from other systems while you are charging that is not insignificant (all those extra computers that can't go to sleep while charging for example.) which can amount to 300-600 watts. Lets call this system losses assume 540w in this example.
So if you're charging at:
120v / 1440w - 540w loss = 900w is available to go in the battery in 1 hour.
This means only 900/1440 = 62.5% of the available power goes to the battery.
When the available power goes up, those constant system loss stays the same and charging efficiency usually increases too based on a curve. So using the same example above assuming same efficiency we get this example:
So at:
240v / 2880w - 540w loss = 2340w is available to go in the battery in 1 hour.
This means only 2340/2880 = 81.25% of the available power goes to the battery.
Now this number increases up to a particular point where the charging efficiency starts dropping and being a bigger impact than the system loss. There was another thread where an individual determined that 25-30A @ 240v seems to be peak efficiency. Far below the max 48A input capacity of the Model 3 LR, but still reasonably fast. The overall efficiencies at those points is between 88 to 97% efficient.
@Jedi2155 That is a great explanation and should be pinned somewhere to explain the main reasons why it's really not charging efficiency but instead charging time really that is affected by 120v vs 240v. An interesting take on it would be to see if the math works out to see if it actually costs less going one way or the other. In theory, it seems the costs should be close unless you have some sort of demand pricing for electricity.
AlanSubie4Life
Efficiency Obsessed Member
This is the latest data I have seen gathered here. Thanks for @darth_vad3r for the time spent gathering the data from the API and plotting it vs. his model.
Plot of efficiency vs. Charge Rate
Note that this data was taken with the screen in the car on (but HVAC off), so the overall efficiency is a little lower than it would be if you left the car plugged in with the screen off (it's something like 50-150W of additional overhead...this is an approximate, not exact estimate of the additional overhead). Note that the overhead makes a BIG difference at the low end - so it might be better to look at the red line model for your projected efficiency.
I think people have also been able to fit data to a model reasonably well, with about 200-250W of overhead (with the screen off).
Obviously, this is all at a nominal 208V. Not 120V. But I think due to the very large overhead (independent of input voltage), the efficiency hit due to 120V will be dominated by the lower available POWER (Voltage * Current) - not the somewhat lower efficiency of the AC-DC converter when operating with a 120V supply rather than 208V/240V.
I say this because this plot shows that the efficiencies for low currents at 208V/6A are pretty close to what I've seen reported for 120V/12A charging (~70%).
Summary: Yes, charging at lower power makes a big difference.
Worth noting:
The 120V charging will hurt you, as described above, but actually for just a few miles a week of driving, the vampire losses may end up significantly affecting your overall efficiency as well. They have been reduced lately (they are down to 1-1.5miles per day for a lot of people). But for a rarely driven vehicle those losses do really add up. For 10k miles a year, with the new lower vampire drain (we'll see if it stays that way in winter!) it's about 4-5% overhead. But if you're only driving 5k miles per year, that's nearly 10% overhead.
If the vampire losses go back to where they were before a couple months ago (hopefully they don't!), you could be looking at 20% overhead.
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darth_vad3r
Well-Known Sith
I think you misunderstood the explanation a bit. The overhead still costs you power and thus money. Whether you include the overhead in the “charging efficiency” or the “overall efficiency” is just semantics. You can’t exclude the overhead from your power bill!
You could theoretically charge X amount in one hour at 120V and if the charging efficiency was the same, charge the same X amount in 30 minutes at 240V. The extra half hour spent at 120V you have to still “pay to keep the lights on” (car fixed overhead).
Excluding installation costs, charging at 240V will cost less than charging at 120V.
When I get a chance I may repeat the test that produced the 208V chart @AlanSubie4Life linked to above at home on 120V. Should be a lot quicker with only 8 data points from 12A down to 5A vs the 26 data points I used to make the chart from 30A down to 5A at a ChargePoint station
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I don't really disagree, but just how much $$ does it matter. 2kWh @ $0.10/kWh is only 20 cents.
My biggest issue with these "efficiency" discussions is that it really doesn't matter that much. It's somewhat like the "charge to 80% or 90%" discussions. Most people didn't give a crap about the efficiencies with gas guzzlers, why worry about things that make even less difference now? It's a combination an range anxiety and analysis paralysis. Did everyone fill up in the mornings when it was cooler? How far out of the way did you drive to save $0.10/g? I feel that we are worrying over things a lot smaller.
When new drivers start reading the threads, they get totally freaked out.
That's not what should be happening. We should be talking about how you don't have to worry about the battery. Maybe just a few simple things that can make your battery a little happier. But not "If you ever charge to 100% your battery is going to die a premature death" And that's what a LOT of people get when they read the forums.
AlanSubie4Life
Efficiency Obsessed Member
In short, some people in California (on the wrong electric plan, or because they may have no choice) may pay as much as $0.47 per kWh (it can be as low as $0.10 per kWh, but one may have to factor in increased AC costs as a result of that plan choice)! In a RWD that's $0.12/mile (best case, using EPA 26kWh/100mi). (Compare to a less than 8 cents/mile in a Prius in California - obviously not an equivalent vehicle, but as a means of transport it is comparable).
I think people should be informed about the exact numbers and factors, to the best of our knowledge, and then they are free to make their own economic decisions. We do not need to proselytize.
The question was about efficiency, and the answers have been provided. The OP can parse the data, using the 26kWh/100 mile number (which does not include vampire losses) for an RWD vehicle, which is provided by the EPA/Tesla assuming the optimal charging efficiency (not 120V charging). For example, using 120V charging at 5k miles per year, it is more like 37kWh/100mi (26kWh/100mi*0.9/0.7*1.1).
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I don't know that is what happens, but it's a possibility.
Have you been here, though?! (sarcasm)
This is an enthusiast forum. People aren't having any of that if you just tell them, "Don't worry about it." I've seen that, and they will not accept that answer, because they say they know it's not completely vital, but they want to find out what is best, what is ideal, even if it makes very little difference. And I would agree for myself, that I would want to know the ideal, but it's up to me how close I want to try to get to that and if it's worth the effort.So I try to stick with what is informative, but not scary. I tell people that best is staying nearer to the middle state of charge, but each person has to balance that with having enough range for their own comfort level to have the car be practical and usable. That way, each person gets to know it's a balanced preference decision and doesn't feel like they are breaking a rule if someone says some exact number but their circumstances mean they need to go a little outside that.
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