Tesla engineer warns against using extension cords at Superchargers

[Tony Burgh]

Heat = I^2*R if I remember correctly (ChemE, not a sparky)
If the conductor is too small and excessive heat generated, it must be removed. Hence water cooling.
If conductor is adequately sized to carry current at lower resistance, less (or little) heat will be generated and little need for auxiliary cooling.

Maybe the extension cord has correct gauge copper. Maybe Tesla went cheap on copper. I hope not aluminum (ductility).

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[BeeKind]

Heat = I^2*R if I remember correctly (ChemE, not a sparky)
If the conductor is too small and excessive heat generated, it must be removed. Hence water cooling.
If conductor is adequately sized to carry current at lower resistance, less (or little) heat will be generated and little need for auxiliary cooling.

Maybe the extension cord has correct gauge copper. Maybe Tesla went cheap on copper. I hope not aluminum (ductility).

Water cooled aluminum alloy, ever wonder why they're so thick?

Copper has the problem of walking away in the night.

 

[Tony Burgh]

Water cooled aluminum alloy, ever wonder why they're so thick?

Copper has the problem of walking away in the night.

If I get a properly sized extension cord, you can bet it will walk away with me. Every time.

 

[Maquis]

Water cooled aluminum alloy, ever wonder why they're so thick?

Copper has the problem of walking away in the night.

Meth heads wont know it isn’t copper until it’s too late.

 

[BeeKind]

Meth heads wont know it isn’t copper until it’s too late.

Actual thefts have been low. Vandalism on the other hand has been on and off, high in various regions.

 

[RickKeen]

The correct gauge copper can handle the current. Its just expensive.
Nobody going to buy a $1000 extension cord.

 

[husky10101]

Their extension chord is 16 feet, you don't need a 16 foot chord to adapt to a Tesla supercharger. Only a few feet will do.

 

[Heliian]

Extension cords create heat, losses and multiple points of failure.

Literally any extension cord.

This is not rocket appliance.

 

[BeeKind]

Their extension chord is 16 feet, you don't need a 16 foot chord to adapt to a Tesla supercharger. Only a few feet will do.

NEC doesn't rate extension cords for 500A and isn't feasible. You're talking 500 MCM wiring or larger if aluminum.

I really don't think people are appreciating the wattages they're talking about here. There's nothing out there in consumer world built to do this, nothing certified, nothing that will be certified.

You know why your EVSE is limited to 19'? It's not a technical limitation, it's code being extra cautious. You really think they're going to let you run around plugging in 16' 750 MCM? How is that being cooled? What is it rated to run at? You're going to have 170F cables?

What is possible in theory and what is practical are often planets apart. Extensions are feasible and are in Europe for L2 AC charging. Here our standards is different, but we're not talking L2 now either. Extension cables to carry DCFC loads will probably always be a nonstarter.

 

[Deleted member 12322]

I'd be curious if state of charge would hook one of these up, I think they own their own DC chargers. Would be a very interesting test to evaluate heat over a half hour. I wouldn't want to plug it into a vehicle though, for risk reasons, but the equipment to bench test is also probably expensive. A possible solution is instead of making it thicker, use some form of air cooling heat sinks like rings along the cord with an insulated grab handle.

 

[Kev12345]

sounds like a Tesla problem. hopefully they start replacing all the V3's with longer cord V4's so this doesn't become an issue.

 

[Monkey]

I'm not sure I would want to do 350kW over an extension cable, but I think I'd be ok with 100-150kW. Also, we (and I assume Chevy/Rivian) only need about 1-2ft, so you can really over engineer it without it not costing a stupid amount.

The 350kW chargers are all 600V+, so that helps a bunch in terms of keeping things cooler at the higher charge rates. Tesla‘s 250kW v3 chargers are 425V, which means they’re driving nearly 600AMPS to deliver that 250kW rate. Same with the 150kW chargers out there, or 350kW chargers when they have to run in the 400-450A range with most older EVs using a 400V architecture.

The most you’re going to get for charging a Lighting is about 480A at 425V or about the peak 177kW -ish that we’ll see as a peak rate and only for a couple minutes. Even with that, the NACS adapter runs hot.

Well understood to figure out what size wire you need for 500 amps with air cooling. Easy enough to buy the right wire. Check the charts you will see you need like 373 kcmil copper. Bulky and costly, but not impossible.

The wire isn’t the only problem, it’s the connections and the heat transfer that stems from those connections, as well as resistance differentials across the system. Simply placing a beefy enough wire in the chain solves nothing, in fact can cause more problems. Electrons don’t just happily travel down a wire, they vibrate and shuffle about, which creates heat along a wire or cable. They have to bridge gaps and connections and the additional energy expended to make the jump becomes heat. This is why Tesla chose the liquid cooling on their V3 and V4 chargers cables. It helps distribute the heat load across the system. And this is not just about keeping everything from melting or burning up, we have to keep Joe User from burning his hand when he unplugs. And we have to keep the cabinets running at optimal rates, along with the charge cords and the cars, even charging at optimal rates under full afternoon sun when it’s 124°F in Phoenix. And that 2kV DLC you linked is absolutely not the right cable for this application. I will assume you picked that one just to show we can easily look up a big beefy wiring option…. Being a customer myself of that site, I don’t think they sell what we’re looking for here. To do this right is going to need an OEM wiring solution.

And as others have pointed out, there is no way any regulating body is going to certify an extension cord rated for 180,000 or more Watts, juggling upwards of 3 to 5 X or more power as the electrical service entering most homes.

 

[BeeKind]

The 350kW chargers are all 600V+, so that helps a bunch in terms of keeping things cooler at the higher charge rates. Tesla‘s 250kW v3 chargers are 425V, which means they’re driving nearly 600AMPS to deliver that 250kW rate. Same with the 150kW chargers out there, or 350kW chargers when they have to run in the 400-450A range with most older EVs using a 400V architecture.

The most you’re going to get for charging a Lighting is about 480A at 425V or about the peak 177kW -ish that we’ll see as a peak rate and only for a couple minutes. Even with that, the NACS adapter runs hot.



The wire isn’t the only problem, it’s the connections and the heat transfer that stems from those connections, as well as resistance differentials across the system. Simply placing a beefy enough wire in the chain solves nothing, in fact can cause more problems. Electrons don’t just happily travel down a wire, they vibrate and shuffle about, which creates heat along a wire or cable. They have to bridge caps and connections and the additional energy expended to make the jump becomes heat. This is why Tesla chose the liquid cooling on their V3 and V4 chargers cables. It helps distribute the heat load across the system. And this is not just about keeping everything from melting or burning up, we have to keep Joe User from burning his hand when he unplugs. And we have to keep the cabinets running at optimal rates, along with the charge cords and the cars, even charging at optimal rates under full afternoon sun when it’s 124°F in Phoenix. And that 2kV DLC you linked is absolutely not the right cable for this application. I will assume you picked that one just to show we can easily look up a big beefy wiring option…. Being a customer myself of that site, I don’t think they sell what we’re looking for here. To do this right is going to need an OEM wiring solution.

It's literally impossible with current implementations, and I don't understand why people think that it is. The issue of connection points can't be overstated. The higher the current, the bigger the problem the connection becomes. You might engineer something to work the first 100 times with perfect contact, but factors like minor arcing, corrosion, and dendrite formation will make this solution wildly unsafe over time.

You're also not controlling where heat builds up at the connection points. By deploying extension cords system-wide, you're likely to start fires in some cabinets where everything was within tolerance before. Now, because you've significantly increased the resistance, stress points in the cabinets and connections are pushed beyond their safe limits, leading to more failures.

 

[RickKeen]

It's literally impossible with current implementations, and I don't understand why people think that it is. The issue of connection points can't be overstated.

400 amps is a common sized feed for a house. U.L. certifies those components no problem. And lots of commercial and industrial systems way larger than that.

Also, the connection from the connector on the fender of the car to the battery in the car is just some wire. Same exact problem as the extension.

By far the weakest point is the connection. Tesla's standard has very small pins for the amount of current. That is where all the heat is going to be. Certainly possible conduct the heat away from the pins with a thick enough piece of copper on either side. And nothing except cost and awkward bulkiness stops the extension from being liquid cooled, either. Could have a small fan and radiator box...

It all just boils down to cost. As I mentioned, its difficult to make it cheap enough for everybody to carry one around.

The best solution would be for Tesla to have made their cables 6 feet longer to start with.

 

[potato]

400 amps is a common sized feed for a house. U.L. certifies those components no problem. And lots of commercial and industrial systems way larger than that.

With permanent connections torqued to a specification, yes. But not with connectors that the end user can plug and unplug at will. We have hundreds-of-amps capable connectors for 12 volts, for example winches, but 400+ volts is a whole other ball game.

I kind of agree this whole idea probably can't be made fully safe in the long run. Maybe for the first 100 insertions but if anything starts to wear, the slightest bit of arcing... it's game over.

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