Graph of amperage draw over time for Model S and Tesla wall charger?

[ajdelange]

I’m trying to find a graph of amperage draw over time for a 60 amp circuit breaker to the Tesla wall charger for the current Model S.

The current ramps up in less than a minute as the EVSE does some tests on the connection and then holds steady at whatever current level you asked for until the charge is complete at which time it ramps down to 0 over a few seconds.

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[JBee]

The current ramps up in less than a minute as the EVSE does some tests on the connection and then holds steady at whatever current level you asked for until the charge is complete at which time it ramps down to 0 over a few seconds.

Hey ajdelange! Long time no see. Welcome back.

 

[ajdelange]

Thanks!

 

[Bitmaster]

I recently installed a Chargpoint CPH50 at my house. I went with a 70A breaker and #6 THHN wire. The charger will not take wire larger than #6 and I wanted to be able to charge at its max rate of 50A if needed. I feel sufficiently future proofed.

 

[ajdelange]

I suspect an 8 hour overnight charge will not bring me up to 80% with either 30A or 50A plug, but 50A will bring it closer.

We can WAG it. At 240V 30A is 7.2 kW. 90% charging efficiency means that 6.48 kW is delivered to the battery and in 8 hr it would receive 51.8 kWh. Deduct 2kWh phantom drain from that to get around 50 kWh. Since this is a TriMotor its battery is going to be about 200 kWh meaning this 50 kWh represents about 25% SoC. Unless you are starting from 55% you are not going to get to 80%. And while we are at it 50 kWh is going to be about 111 mi added (at the CT's also WAGed consumption of 450 Wh/mi) so it's clear a 30A charger is limiting if you only have an 8 hour charge window.

With a 60A circuit the most you can draw is 48A which at 240 V gives 11.52 kW which degraded by 10% conversion loss is 10.368 kw to the battery. Eight hours of that adds 82.9 kWh which, minus 2 kWh phantom is 40.4% of a 200 kWh battery. So you would have to start at around 40% SoC to reach 80%. The estimated miles added by 81 kWh is 180 mi. Even the maximum sized charger may be marginal if only 8 hrs is available for charging and you must replace more than 180 miles per day. This may be well above the median requirement but certainly would not be rare and even individuals whose commutes are less than 180 RT miles may have the truck in services that either require more miles to be driven (manufacturers' rep.) or use the battery to run power tools on a job site, for example.

This discussion thus quickly brings us to the same conclusion as all the others that question the size of EVSE (and the wiring to support it) and that is that one should install the largest system he can consistent with financial (or other) constraints. The largest available EVSE (48A assuming the CT will be limited as Tesla's current portfolio is) is something that some will need all the time, most will need some of the time and everyone will need eventually.

So what do you do if you can't install 48A charging for whatever reason? Charge enough to get to an SC with what you have or charge longer hours. Eventually we'll find someone who can't do either. That person shouldn't buy a CT.

 

[ajdelange]

A 2170 stores from 3.5-5 amp-hours in a cell. Going from 0.1 to 0.3 would be changing charge time from (at best) 35 hours to 10 hours. Which is, well, a massive change, but also... Really a huge amount of time to dissipate the heat from it.

I found a spec sheet for a 2170 cell with capacity 4 Ah and average operating voltage of 3.65V. At this voltage it stores 14.6 Wh and one wold need, therefore around 13,700 of them for a 200 kWh battery with the actual number dependent of what values of P and S work out to be. The internal impedance of one of these cells is less that 25 mΩ so if charged at 0.1A (.025C = C/40) the heat dissipated would be (.025)*(.1)*(.1) = .00025 W and 13,700 of them (the pack) would dissipate 3.42 W. Not a lot of heat. Trebling the charge rate to 0.3A (0.075C = C/13.33) would increase these numbers by a factor of 9 leading to pack dissipation of 30.8W.

Charging a 200 kWh pack at 11.52 kW amounts to a charge rate of 11.52/200 = 0.057C = C/17.36 so we would have a charge current of 4/17.36 = 0.23 A (all of this approximate) if we used this particular 2170 to build a 200 kWh pack for a CT. This leads to a dissipation of (0.025)*(0.23)*(0.23) = 1.3 mW/cell and 18 W for the pack. Thus removal of heat from the pack at even the maximum Level 2 charge is not something that we need worry about.

0.1 amps at wall current is the same as a 100-watt bulb. That's alot of potential heat.

Well, no. 0.1 Amp at 120 V is 12 W but the 0.1 amp against battery cell voltage of 4.2V is 0.42 Watt and, as we have just seen, only a tiny portion of that gets converted to heat. Most is stored in the cell to be withdrawn when it is discharged.

It really does matter the size of the entire battery pack and the ambient temperature and its ability to actively cool or heat the pack.

Not, evidently at L2 charging rates. At an L3 rate 10 times the L2 rate (i.e. 115 kW) we would have internal dissipation of 100 times the L2 peak i.e. 1800 W. That is something to worry about.

This isn't to say that maintaining temperature of the battery within an appropriate range is not very important in order to enhance its lifetime and performance at discharge. And L2 also does introduce a waste heat load from the 10% loss in the rectifier. Depending on ambient, energy may be required to dispose of this.

 

[ajdelange]

I’m trying to find a graph of amperage draw over time for a 60 amp circuit breaker to the Tesla wall charger for the current Model S.

The current ramps up in less than a minute as the EVSE does some tests on the connection and then holds steady at whatever current level you asked for until the charge is complete at which time it ramps down to 0 over a few seconds.

I should add that you can change the charging rate (current) during a charge session in which case the answer is broadened to say that the plot follows your rate requests throughout the charge. Desired rate can be set in either in the car or via the app. Per the discussions here you will probably, with a CT at least, set 48A and leave it at that but if you have demand charges in you area you may want to set it to less than that or if you want to charge a second BEV at the same time. Another, but probably relatively rare, example would be people with PV/battery systems. They want to "follow the sun" which means setting the charge rate such that the total premises demand is just under the total PV production so that all the power going to the vehicle comes from solar rather than the battery. This eliminates battery charging and discharging losses and prevents cycling of the battery prolonging its life.

 

[Diehard]

Deduct 2kWh phantom drain from that to get around 50 kWh

Would you say 2KWh is normal for 3, X, Y and it is reasonable to expect it from CT without a way to significantly reduce it?

 

[SwampNut]

It can be reduced, and the daily maintenance usage on my 3 LR is a measure 4k. That's with the Sentry on (car never sleeps) and the temperature maintenance on fan only, during warm-ish days that make the fan run.

 

[HaulingAss]

All of that is overkill unless you're running a taxi service. There is almost never a reason to home charge above 25 amps or so (about 6kW or enough to fill the car easily overnight). Your batteries will last longer being at a lower rate. If you live in a place with a demand charge, like we do, then you'll pay a "fine" for fast charging every month. All of this stuff about installing the fastest charger is a huge waste. Every charger I've helped install for my friends and their friends is a UMC connected to a 14-30 and #10 wire. The effective charge rate for a M3 is about 26 MPH. Who needs more than that at home? Even a person using one for a full time Uber service says she has at last 12 hours between being "on."

Completely and utterly wrong that your batteries will last longer charging at home at a lower amperage. The more power available, up to the maximum allowed of 48 amps for Long Range (and somewhat less for some smaller batteries), the faster your Battery's thermal management system and Battery Monitoring System (BMS) can condition the battery in cold weather. Furthermore, charging at very low currents, at low temperatures is not good for battery life because it's not enough current to create internal warmth in each cell. Charging a 75 kWh battery at 240V 48amps is basically trickle charging so it will not shorten the life over even slower charging. It's actually more efficient to charge it quicker because then the thermal management system doesn't need to add heat to the battery over as long a period of time. It can just charge it and let the battery cool down when done.

If you live in a place with a demand charge (which is not the norm for residential power) you can easily moderate the charge level as needed to avoid extra demand charges. More common than demand charges is Time Of Use (TOU) billing. In this case, you want the maximum power the car can accept to ensure you can charge to the desired level in the most economical time window. Having a powerful circuit for charging is not overkill, it's a nice feature.

I've been out and about taking care of business all morning, come home with a 65% charge, only to learn we had to make an unplanned round trip to Seattle Area and back home that night and we had to leave in under 3 hours. Having the ability to charge at 48 amps allowed me to eliminate a Supercharger stop which was more convenient and it will also cost about 60-70% less to charge at home than a paid Supercharger. Granted, this kind of spontaneous trip is not common but the ability to quickly add miles when something unexpected comes up allows me to feel more comfortable not having my car fully charged (or charged to 80-90%) all the time "just in case".

Also, remember that the maximum current of your charging circuit is not only for charging the battery but is also used to pre-condition the cabin before you unplug. This increases battery life by reducing battery cycling since your cabin is already up to (or down to) temperature when you unplug the car. A battery's life is directly proportional to the charge/discharge cycles. I charged an entire winter on a 120V 15-amp circuit and when the battery was cold it took forever to get the battery warm enough to start charging - all the current was going to heat the coolant solution so it could be circulated around the battery before charging it. Worse, when I defrosted the car and pre-heated the cabin, I was actually drawing down the battery to do so! The car was still charging from the night before and when I pre-conditioned the cabin the charging stopped and the miles of range remaining started decreasing before I even unplugged the cable.

While not everyone will absolutely need the maximum possible circuit size, I get tired of dumb advice to skimp on power "because it's not absolute necessary at this point in time". The fact is, the world is going electric, and we are not a third world country. That means that same circuit might be eventually used to charge 2 EV's (or even 3 or 4 EV's). I'm not saying everyone should go all out and put in a 100-amp circuit and be done with it because every situation is different. It might make sense to just put in a 30-amp circuit if that's all that your service can accomodate at the moment and you are not ready to upgrade. But if it's just a matter of running large enough wire for a 60-amp circuit, then there is no reason to try to save a few bucks by installing a smaller wire and breaker. A 60-amp circuit could charge multiple EV's with the power-sharing built into many charging solutions.

I advise people to not be penny-wise and pound foolish. It's almost always cheaper to do it right the first time than constantly needing to incrementally upgrade the service as needs change (500-mile Cybertruck will be a natural for a circuit of 60 amps or more) or as more EV's are added to the family. Spending on upgrading your homes electrical infrastructure is actually a good investment because electricians will not be any cheaper in the long run and there will only be fewer gas cars and more EV's.

 

[HaulingAss]

How is it saving money to run the 4AWG? Surely not the power lost in resistance.

I just ran the math based on the following assumptions:
- on an average day, you charge at 40amps for a half hour. (obviously there would be variation on this in real life, but average it over 365 days and maybe this is close enough)
- power costs $0.24/kWh
- your length of cable is 10ft from the breaker box to the wall charger (obviously you want this to be as short as possible)

P = I^2 * R, so at 40amps your 4AWG cable is losing 3.98W of power per 10ft length
while the 6AWG cable is losing 6.32W due to resistance. That's a difference of 2.34 W per 10ft of cable.

At $0.24/kWh and charging 30mins/day, you only save 10 cents per year by using the 4AWG cable.

You can modify the assumptions, but unless you're running a very long cable it's not going to change the result by much....and then you're paying way more for the cable anyway!

Most resistance in a circuit sized to code is in the terminations. At a minimum, there are two in the breaker panel, and two in the Wall Connector. If you ever modify the circuit (for example by adding a junction box to add another Wall Connector sharing the same circuit, you add two more). Every one of those terminations has more resistance than your 10 feet of wire.

A larger gauge wire spreads over a larger surface area when tightening the termination screws and thus will have less resistance and less heat buildup. Also, a higher amp breaker will have considerable less resistance on the contacts than a lower amp breaker that must be run according to the chosen wire size. The long and short of it, is that the simple calculation you used doesn't do justice to the actual electrical losses over time of running the minimum wire/breaker sizes.

While there is nothing wrong with running a circuit at its maximum legal load for long periods of time as long as the install is done using best practices and without sloppy workmanship (because the continous use is already built into the calculations with a 20% reduction in rated capacity), it's not always optimum for a variety of reasons. One of those is that the line voltage to your house can sag below the commonly accepted 5% and any resistance in your circuit will simply make that worse by the time it gets to your car. Having solid circuits in your end of things provides more margin of error during heavy brownouts. Because at some point the car will decide the voltage is too low and shut down.

Also, I didn't check your math, but you need to double the distance (to account for each conductor in the circuit). If you are able to mount the Wall Connector 10 feet away from the service panel, it would not be uncommon for the actual wire length to be 20 feet once it is properly routed in a neat manner, especially if there are any obstructions that prevent the most direct route. It's amazing how much longer the wire needs to be to account for the 2 vertical feet going up (or down) into the service panel and also a similar situation exists routing into the Wall Connector. So, it wouldn't be uncommon for that 10 feet of horizontal distance between the panel and WC to turn into 40 feet of resistance (20 feet x 2 conductors) and that's in addition to the resistance in at least 4 connections and the two contacts inside the breaker.

I've done the minimum sizes allowed by code and also over-sized circuits depending upon the particulars of each circumstance. Sometimes it makes sense to oversize, sometimes it doesn't. I've also thrown an extra conductor in a 60-amp circuit just in case someone at a later date wanted to replace a Wall Connector with a NEMA 14-50 receptacle or another EVSE with different requirements. It only added about $20 to the cost of the job and it would be impossible to add it at a later date without replacing the entire circuit and conduit.

When it comes to EV charging, I do recommend upsizing when it makes sense. You end up with a more robust system that runs cooler. More importantly, I recommend taking extra care to make sure each connection has the best possible contact by properly forming the wires during install and making sure they are laying naturally without undesirable tension in them.

None of this is all that expensive in the grand scheme of things. Do it right once and don't worry about it again.

 

[jerhenderson]

38’ cable run, 60 amp breaker, 220-240 vAC

I'm running a Tesla wall charger on a 32A breaker, running about 150' to the car. works great.

 

[SwampNut]

Nice wall of text, but the bottom line is that most everybody can do fine with two EVs and 30a charging.

 

[jerhenderson]

Nice wall of text, but the bottom line is that most everybody can do fine with two EVs and 30a charging.

yep .... you'll see people6 that cry you

Nice wall of text, but the bottom line is that most everybody can do fine with two EVs and 30a charging.

yes, tho you'll hear people falsely whining that you must have 60A, etc.

 

[HaulingAss]

Nice wall of text, but the bottom line is that most everybody can do fine with two EVs and 30a charging.

Do you mean "most people" or "most everybody"? Because I'm not sure what "most everybody" really means if it doesn't mean "most people".

And I don't think you understand what the phrase "wall of text" actually means.

I agree, more than 50% of people with two EV's "could do" with one 30 amp circuit. But it's important to point out, that doesn't mean everybody should do that. Often it doesn't cost much more to put in a solution that will not only "work for" 95% of people, but that 95% of people would find easy and convenient while also minimizing wear on the battery. Most families could get by with one bathroom. That doesn't mean you should only put in one bathroom.

The last thing you want for two EV's is one 30 amp outlet if convenience matters to you. Because EV's perform better and last longer when they are plugged in when not being used, it makes sense for every EV to have a charging cable at home and enough power available to pre-heat both cars without drawing into the main battery. That roughly describes a 60 amp circuit, depending upon the vehicle. It would be even better if one or both could add range while they were conditioning the battery in cold weather.

Those in climates where it never gets cold or in temperate climates with attached garages might be happy with less. It really depends upon the situation, but it's less than ideal to limit the life of the car's battery or the convenience of its users because someone was too cheap to spend $50-$1000 more for a system that avoided uneccessary compromises (while in some situations it makes sense to make hard decisions and compromise).

For those who still doubt this good advice, it might pay to engage your brains for a moment:

Facts:

1) It costs a lot more for EV manufacturers to include higher amperage on-board chargers than absolutely necessary in millions of cars.

2) Manufacturers include on-board chargers requiring circuits of 40 amps (for a 30 amp maximum on-board charger) to 60 amps (for a 48 amp on-board charger) to 100 amps (for an 80 amp on-board charger on versions of the F-150 Lightning). While they do facilitate the use of smaller circuits for those who can't install larger circuits by allowing charging at reduced rates, it costs them additional money to install these bigger, heavier, higher amperage circuits.

Why would they do that if "most everyone" is fine sharing a 30-amp circuit between two EV's? Because it's less than ideal, particularly in climates that have cold weather.

But the real reason I spoke up to begin with was the severely misguided notion that charging below the maximum rate of the on-board charger will extend battery life. In cold weather cases it actually reduces it. When you pre-condition your cabin or pre-heat your battery, it actually degrades your battery to not have enough power available. And it doesn't need to be arcitic cold, below zero Fahrenheit, for this to be the case.

That was false and misguided advice that shows a complete misunderstanding of how to best care for an EV. It's backwards.

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