SuperCharging starts to feel slow

[David99]

Tesla said being able to charge a Lithium battery within minutes is possible. The technology is there. It would take serious cooling, though. And of course it would take serious power (around 1 megawatt). Both these things are hard to realize. Building a battery pack with a cooling system that dissipates 100-200 kW of heat is quite a challenge. Getting a megawatt of power is another challenge. Last but not least, the cables would be too heavy and thick to handle. So there are practical limits.

I always hoped that Tesla would come out with an updated Firmware that would run the battery cooling system at max capacity and thus enable faster Supercharging (staying at 120 kW for a longer time). I noticed that even when Supercharging in 100+ degree weather and full desert sun, the battery is cooled just using the radiators and fans. The AC unit does not kick in. If I turn the AC on manually, I hear the cooling fans run at a slower speed which indicates the AC unit provides better cooling. At one public meeting either Elon or JB said Supercharging is actually pretty conservative. They could charge faster but they were unsure about the long term effect on the battery and wanted to stay on the safe side. Tests with Leafs showed that fast charging vs normal charging makes only a small difference in battery aging. And the Leaf doesn't even have any battery cooling at all. I still have hopes Tesla will eventually allow a less conservative Supercharging profile resulting in an overall faster charge speed.

Here is the video (from 2 years ago) where Elon says that Supercharging is very conservative: Elon Musk Townhall meeting Amsterdam (HD): Firmware Version 6.0, SuperCharger Locations Europe, etc. - YouTube

 

[apacheguy]

How much heat is actually being removed by the cooling system? If I'm charging at 120 kW that does not mean 120 kW of heat is dissipated. Otherwise charging would be 0% efficient.

 

[David99]

How much heat is actually being removed by the cooling system? If I'm charging at 120 kW that does not mean 120 kW of heat is dissipated. Otherwise charging would be 0% efficient.

A Supercharger is about 90% efficient, but we don't have to worry about it much as it's outside of the car. The battery itself is also around 90% efficient. So roughly 10% is lost in heat. If you charge at 120 kW, you have to deal with 12 kW of heat generated inside the battery that needs to be removed by the cooling system. But only for the first 10 min or so. Then the taper kicks in and less and less heat generated. That's all pretty reasonable in normal weather conditions. But if you're in the desert even that can push the cooling system to the limit. Getting significantly higher rates would require some major upgrades both in the batter/car and the Superchargers.

 

[Cottonwood]

A Supercharger is about 90% efficient, but we don't have to worry about it much as it's outside of the car. The battery itself is also around 90% efficient. So roughly 10% is lost in heat. If you charge at 120 kW, you have to deal with 12 kW of heat generated inside the battery that needs to be removed by the cooling system. But only for the first 10 min or so. Then the taper kicks in and less and less heat generated. That's all pretty reasonable in normal weather conditions. But if you're in the desert even that can push the cooling system to the limit. Getting significantly higher rates would require some major upgrades both in the batter/car and the Superchargers.

Your estimate of battery loss is too high. My experience has been that the total round-trip, in-and-out efficiency of the Tesla battery is about 97%, or a 3% loss. On top of that, only 1/2 to 2/3 of that happens on the charge cycle. Therefore 2% is a better, upper estimate of battery losses during charging. 2% of 120kW is 2.4kW. 2.4kW needs to be dealt with, but is very manageable.

As you said, there is about a 10% loss in the Supercharger cabinet. If you plug in to a Supercharger and then go over to the equipment enclosure, the fan sound getting rid of the 12 kW (10% of 120kW) is pretty obvious.

 

[AWDtsla]

Your estimate of battery loss is too high. My experience has been that the total round-trip, in-and-out efficiency of the Tesla battery is about 97%, or a 3% loss. On top of that, only 1/2 to 2/3 of that happens on the charge cycle. Therefore 2% is a better, upper estimate of battery losses during charging. 2% of 120kW is 2.4kW. 2.4kW needs to be dealt with, but is very manageable.

As you said, there is about a 10% loss in the Supercharger cabinet. If you plug in to a Supercharger and then go over to the equipment enclosure, the fan sound getting rid of the 12 kW (10% of 120kW) is pretty obvious.

I would assume that efficiency is inversely proportional to charging current, if nothing else but I2R losses even inside the electrodes themselves.

 

[Cottonwood]

I would assume that efficiency is inversely proportional to charging current, if nothing else but I2R losses even inside the electrodes themselves.

Even 120 kW charging is less than 1.5C on an 85. The 2-3% battery efficiency is consistent with Supercharging.

 

[AWDtsla]

Even 120 kW charging is less than 1.5C on an 85. The 2-3% battery efficiency is consistent with Supercharging.

Eh... gut feeling is much more. Knowing that the 5.5kW pack heater raises the pack temp like 2 degrees per minute? Seems like for active pack cooling to turn on, especially given that 120kW is only maintained for minutes, would mean losses are far far higher than 5.5kW when at 120kW.