I’ve long understood that charging a car from 80 to 90 percent range takes longer than charging it from 20 to 30 percent, although you are adding the same 30 miles to the range either way. I assumed (without much thought) that even though it was “slower,” it was somehow using the same amount of electricity. In other words, a constant XX Kw per mile.
Now I’m thinking that this doesn’t make sense. If I’m plugged in longer to get the same range added, I’m probably using more electricity per mile of range added, and it’s not constant. Thus, I might be saving a significant amount of electricity if I usually charge in the faster charging zone. If I can add 30 miles twice as fast, am I using half the electricity?
Because the charger is a computer. It is adjusting the flow rate of electricity into the battery to best optimize the battery life and separately the amount of time spent charging. While avoiding overtemping the battery.
So for each of those e.g. 30 minutes a different amount of electricity is being fed into the battery. You can’t think of it like an e.g. 150W lightbulb where half an hour of power consumption is half an hour of power consumption.
Overall the conversion rate from electricity through your meter to mileage from the car ought to be close to constant. Aggressive driving can waste charge, super-efficient driving can save charge. Quick charging in the steep part of the refill envelope is perhaps slightly more efficient than is other sorts of charging. Etc.
So there is room for a bit of finagling. But not much. Plug it in and let the computers do the worrying.
You can see it when a Tesla is plugged into a supercharger. At low charge, it starts off charging at a crazy rate, adding over 1000mi/hr and tapers downward as the charge increases.
Here’s some sample screenshot data for supercharging a Model Y from Hertz on a recent trip to NJ, battery charge, power, how fast the battery is charging, time.
36% 163kW +604mi/hr 10:55PM
58% 94kW +387mi/hr 111:05PM
66% 79kW +327mi/hr 11:09PM
As you can see, the kW adjusts downward. Unless you really need it, there’s no point in charging beyond enough to reach the next charger on your trip and a comfortable safety margin. (Tesla tells you in real time how busy the next charger(s) are.) I think the superchargers are 350V so 163kW is if math serves me, 466Amps. Those are thick cables.
Incidentally, this was a long range model Y so 100% is I think, 324mi. I added about 100 miles in 14 minutes Some of the newer chargers are much faster. Also, battery temperature speeds charging. You can tell the Tesla to “precondition” the battery so it’s already warm when you arrive, charges faster. I probably should have done that in this case.
Others have pretty much got it already: the rate of change just changes how fast you add miles, not the efficiency per mile.
The one place where there is a little bit of variability (though still not a lot) is at very low charge rates–in particular, when using a standard 120v outlet. This tends to be noticeably less efficient than a faster rate. The reason is that the car has to be “on” for it to charge, and this may require fairly significant power. Possibly in the ballpark of 200 watts. A standard plug only supports 1440 W charging, though, so you’re losing on the order of 15% just to keep the car on.
The power waste is the same at higher rates, but because the charge session takes less time, there is less actual energy wasted.
I should also mention that most utility laws forbid reselling power by the kilowatt, so most chargers (Tesla and other) charge by the minute instead. That’s another reason not to fill too much at superchargers. Don’t fill that last little bit if it’s going to cost substantially more.
In terms of charger cost, my experience with Tesla is the cost is about half what the equivalent gas would be for the same trip. Whereas, charging at home it is about 1/10.
Charging at home, the Tesla battery has no problem continuously absorbing the steady level of power I throw at it ( 26A, 240V). It will do 40A but one night I popped the main breaker for my house, so I’ve told the car to limit charge to 26A. This still adds a steady 38km/hr (23mph). I haven’t tried pushing it to over 95% where I presume I would see the same slowdown as with superchargers. Per Tesla recommendations, I charge to 80% and rarely drain below 20% for battery health. (Maybe 90% or 95% for a road trip) Allegedly (never tried this) standard household 120V 12A (a 15A circuit) will give 5km (3 mi) per hour.
I think the problem was the utilities did not want some enterprises creating smaller sudividers reselling power (i.e. “we’re the only power supplier for this subdivision/ condo / HOA”.) I assume utilities wanted control of the grid right to your door. So other than EV charging, there was no valid reason for someone to resell power by the kWh and plenty of reasons not to allow it. It seems like the EV lobby has been making headway. I wonder if these exceptions apply only to EV charging?
Only in America. Over here in Europe where we have more volts to start with, we pay by the KwH for our electrickery. And if you plug into a “public” charger, that can charge what they like, so it’s lucky they are in competition.
L2 AC home charging will pretty much never be slowed down due to battery chemistry. 40A at 240V is only 9.6kW. L3 DC fast charging can be 150kW or more (some EVs will take 350kW peak).
Well, there’s my answer. Unlike “supercharging,” my level 2 charging at home uses a constant amount of electricity regarrless of how full the battery is.
I’m not sure what the fix is, though. When there’s only a limited number of charging locations, paying per minute will encourage people to free locations as soon as they have “enough” energy. Paying per energy unit does not encourage people to move on quicker.
This will change as more charging locations become available. But right now? I think paying per minute puts the cost on the scarcer resource: time at a location.
Seems to work out ok in the 30 states that allow it (like where I live). And Canada is switching now:
The fact that battery charging slows down after 80% should be encouragement enough most of the time. Traveling is slower if you wait until 100%. And there’s the gentle encouragement that the default charge limit is 80%, which you have to override each time if you want to go higher.
I’m thinking of the scarce charger problem more in the context of office buildings, apartments, etc. Where the selfish user would rather not bother to move their car promptly so someone else could charge.
As you say, for the chargers at stops along highways used mostly by folks on long trips, their own natural desire to get to their destination will largely ensure they don’t waste charger occupancy time for their personal convenience.
For L2 chargers, there isn’t really the same 80% problem–they aren’t “wasting” the charger by essentially trickle-charging it. Because L2 charging is already trickle-charging.
Past 100%, stations can charge idle fees. Superchargers do this already. I don’t know about others.
In managed locations like apartments or office buildings, you can have someone record license plates and impose a penalty on the owners somehow. This also solves the problem of non-EV drivers just using the parking spaces for their own convenience.
For places like malls and grocery stores… eh, probably not much you can do. Maybe the local security can get them towed if they’re really blocking a space.
Between charging as in supplying power to a battery, charging as in pricing & paying for stuff, and charging as in indictments, whole lotta ways to write ambiguous confusing sentences
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