Supercharging Efficiency

[ThosEM]

We occasionally see claims made by EV and Tesla critics that superchargers are woefully inefficient, as in maybe 25-33% efficient in putting energy in the battery. I strongly suspect this is apocryphal, since I can demonstrate that the 10 kW rectifiers on board a Tesla are about 92% efficient (except in the top 10% of SOC on a range charge). But I'm unable to find a source of information about the efficiency of ether CHAdeMO or Tesla quick chargers and their rectifiers external to the car. Does anyone know of reliable data on this that could be used to refute and shut down such claims?

 

[Saghost]

We occasionally see claims made by EV and Tesla critics that superchargers are woefully inefficient, as in maybe 25-33% efficient in putting energy in the battery. I strongly suspect this is apocryphal, since I can demonstrate that the 10 kW rectifiers on board a Tesla are about 92% efficient (except in the top 10% of SOC on a range charge). But I'm unable to find a source of information about the efficiency of ether CHAdeMO or Tesla quick chargers and their rectifiers external to the car. Does anyone know of reliable data on this that could be used to refute and shut down such claims?

We've been told on several occasions that the Supercharger is made up of a stack of standard Model S charger modules (originally 9, currently 12 I believe,) so I would assume that the same efficiency data would apply.

I haven't seen the claim you're referring to about 25% efficiency, but I would assume that they are probably referring to overall efficiency starting with a combustible fuel at a power plant and going through the plant's conversion, then the grid losses, then the SpC itself. This of course assume that the electricity comes from a combustible fuel, but that's still true for most US power, and makes it kinda-sorta comparable to the efficiency of burning gasoline in a car (in which case the best hybrids can compete, but the S still dominates anything in its size/weight/power/cost class.)
Walter

 

[mikeash]

I have no special knowledge, but this doesn't pass a basic smell test.

A supercharger can put about 120kW into a car. At 33% efficiency, that would require 360kW of electricity.

Some brief searching on the internet turns up this plan for a hopefully representative supercharger installation in Madison, WI: http://www.cityofmadison.com/planning/projects/reports/89etm_site2.pdf

That shows a 500kVA transformer for three superchargers (serving six stalls). kVA converts to watts with the power factor of the equipment, which I'm guessing (and I could be wrong) is around 1 for this application. So that's about 500kW of available power for three superchargers, each of which can put 120kW into a car, so 360kW total. That means that efficiency must be 72% at a minimum, and that's assuming there's zero headroom on the transformer, which I'm sure is not the case.

33% efficiency on a 360kW charge rate would also mean that there's 720kW of heat being dissipated somewhere. Since none of the equipment glows red hot or cooks people who stand too close, it should be fairly clear that this isn't happening.

The supercharger chargers are identical to the ones in the cars, there's just more of them, so efficiency should be identical.

 

[David99]

It is as efficient as normal charging as it's made using the exact same chargers which have more than 90% efficiency. The reason the car has to cool the battery is because at 10 times the power rate you have 10 times the heat. It is not less efficient.

 

[ThosEM]

Thanks! Very interesting info, and arguments.

In the cases I'm thinking of, one finds a real hard core Tesla bear fuming in the comments section of some site that Tesla owners are burning electricity up like there's no tomorrow at superchargers with unconscionable waste of energy that never gets into their batteries. They aren't talking about electric generation plant inefficiencies. So I'd like to identify a link where I can point them the next time I encounter this.

 

[mikeash]

If you want some links to back it up, this article mentions that the superchargers use the same actual charging hardware as the car:

Inside Scoop on Tesla’s Super-Smart Supercharger Network | PluginCars.com

(It's in a photo caption under Good Economics.)

And this one mentions that the car charger is 92% efficiency:

Model S Specs | Tesla Motors

That first link also describes the 120kW capacity of the supercharger, so would go well with the transformer size indicated in the plan I linked above.

Not that this will probably work for someone who thinks this way. Worst case, ask him how he came to that conclusion, then you can try to correct wherever he went wrong.

 

[randompersonx]

Supercharging must be somewhat less efficient than charging at home, as the fans never seem to come on when charging at home, and when supercharging at a low SOC (115KW+ charge rate), both the fans on my car and the supercharger cabinet seem to be on overdrive.

With regular charging, the heat can likely be dissipated simply by natural air flow.

That said, I would be stunned if there was more than a 10% penalty for running the fans.

 

[Johan]

Supercharging must be somewhat less efficient than charging at home, as the fans never seem to come on when charging at home, and when supercharging at a low SOC (115KW+ charge rate), both the fans on my car and the supercharger cabinet seem to be on overdrive.

With regular charging, the heat can likely be dissipated simply by natural air flow.

That said, I would be stunned if there was more than a 10% penalty for running the fans.

The fans coming on don't say a thing: with a SC you're basically concentrating the charge over a much shorter period of time, hence the need for more cooling. Doesn't really say anything about efficiency.

My guess it's close to the same as on-board AC charging, since the SC is made of a stack of the same chargers. But a tad less efficient due to more heat build up in the power electronics and wires.

 

[David99]

Supercharging must be somewhat less efficient than charging at home, as the fans never seem to come on when charging at home, and when supercharging at a low SOC (115KW+ charge rate), both the fans on my car and the supercharger cabinet seem to be on overdrive.

I think people sometimes don't understand the actual meaning of efficiency. Efficiency is the percentage of input vs output on a system. It is not an absolute number.

During normal charging you draw 10 kW from the grid for one hour which is 10 kWh. Because of the losses in the charger, that ends up adding about 9 kWh at the battery. The efficiency is 90%. In this example the loss is 1 kW over one hour time frame. On a Supercharger you are pushing in an average of 100 kW for about half an hour. The efficiency is the same (90%) but now the loss is 10 kW over 30 min. While 1 kW during normal charging is easily dissipated naturally, the 10 kW loss during Supercharging is a serious amount of power, hence the Tesla's active cooling system must help to remove the heat from the chargers (where the majority of loss happens). So the amount of energy lost is the same if you charge at 100 kW for one hour or at 10 kW for 10 hours. The only difference is that the losses are over a different time frame, hence the active cooling needed when charging faster.

Now if you want to be technical, Supercharging is a little less efficient because the active cooling does require some extra energy that isn't needed when charging slow. But that difference is small. I would estimate around 3%.

 

[randompersonx]

The fans coming on don't say a thing: with a SC you're basically concentrating the charge over a much shorter period of time, hence the need for more cooling. Doesn't really say anything about efficiency.

My guess it's close to the same as on-board AC charging, since the SC is made of a stack of the same chargers. But a tad less efficient due to more heat build up in the power electronics and wires.

If you have NO fans normally, and Maximum fans during supercharging, it does say something about efficiency, because the fans require energy, and therefore make the process less efficient.

I am saying that the fans are likely the primary source of decreased efficiency.

I am not saying that the fans are a huge decrease of the efficiency, but it is not zero.

 

[Saghost]

If you have NO fans normally, and Maximum fans during supercharging, it does say something about efficiency, because the fans require energy, and therefore make the process less efficient.

I am saying that the fans are likely the primary source of decreased efficiency.

I am not saying that the fans are a huge decrease of the efficiency, but it is not zero.

Having to run the fans adds some overhead power to the charge, certainly.

OTOH, the shorter charging time means less time the car has to keep the computers awake to monitor everything, reducing the overhead.

Without more information I'm not sure I can say which is the larger effect - I don't know how many of the computers Tesla keeps awake or how much the Tesla computers draw. The Volt seems to keep ~100W of computers awake during the charge process - going from a 10 hour charge at 120V to a four hour charge at 240 reduces the power required by about half a kWh in neutral weather.

If you assume Tesla has a similar amount, and that half? of it is not the charger module itself, then at 10X the charge rate, you'd need 500W of additional cooling to match the computer overhead that you are saving.
Walter

 

[Gabzqc]

Fantastic Supercharger construction planning document mikeash ! Do you know where we could find these for other Supercharger stations?!

 

[mikeash]

Gabzqc: I just stumbled across the document while searching the web for what kind of transformer supercharger installations have. Google thought it was a top response to my query, and they were right. I imagine finding others will depend entirely on the local jurisdiction. If the locality makes their documents available publicly so they can be indexed, Google ought to turn them up. Some may require going into their own permitting sites. A few still don't have these things publicly accessible online at all. Unfortunately, since these things are all done by local governments, there's no standardization.