Comment by cromka
3 days ago
“and claims a charging rate of up to 11 kW and with an efficiency of about 90% for easy overnight charging.”
That’s a some 9kW lost on a 80kW battery. I get the convenience, but I think the best use for those would be when standardized and installed in public parking spots, for a brief charge bump, not to charge fully at home, e.g overnight, which is what Porsche aims for.
> That’s a some 9kW lost on a 80kW battery
It's about the same as wired AC charging. There are always charging losses:
https://insideevs.com/features/711659/ev-charger-efficiency-...
The 90% efficiency of the wireless transmission would be on top of the efficiency of typical charging (which is mostly losses after the power reaches the vehicle).
It's 90% from grid to battery.
4 replies →
> That’s a some 9kW lost on a 80kW battery.
*kWh
Using the right units is important.
"This may sound like one of those overly pedantic fixations... but it's not. Power and energy are very different concepts." -Technology Connections
https://www.youtube.com/watch?v=OOK5xkFijPc
I keep wondering why a robot-vacuum-charging-dock style solution hasn't materialized. We've seen to robotic arm demos but those aren't around because they are overkill for the task. But something that the car autonomously and accurately docks into and attaches from either the front fascia or underneath. Same charging standard but with a more forgiving connector design that guides and locks in.
Charging rate of 11 kW, with efficiency of about 90%, means that the charger sucks around 12.2 kW of power from the grid, of which 1.2 kW is continually lost*, and only 11 kW is effectively being stored in the battery at any point in time.
I don't understand the mention about battery capacity (80 kWh, which would translate to around 7½ hours of charging at 11 kW, if a battery could charge at a uniform rate) or where the "9 kW lost" figure comes from.
*Which is probably useful to melt snow above the device, when installed outdoors on cold climates.
The average person is spending about $1.50 to $2.00 in electricity per day to drive a car.
This would cost $0.15 to $0.20 per average trip to NOT plug in.
Average person spending $100k on a car would likely pay that without a second thought.
The Cayenne seems to need about 315Wh / mile. I assume a substantial number of them are sold in California, where rates are often above $0.50/kw. I think $1.50 would get you about 9 miles.
I drive a more efficient PHEV and spend much more than $1.50/day.
unit confusion
It will still be 90% efficient regardless of who uses it. In fact if it is used more, the total losses will be higher.
(Some of this becomes clearer when you have a feeling for the units: kW is a measurement of power, not energy)
> not to charge fully at home, e.g overnight, which is what Porsche aims for.
Wouldn't an overnight (usually cheaper) 10+ hour charge be a great use for (slow) 11 kW charging?
you think Porsche owners give a shit about efficiency? Laughable at best.
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