Since almost all public chargers are unattended and isolated, there is too much risk for the company to install a credit card reader, because it becomes a target for a thief to install a skimmer.
Also, since the average sale is small (generally less than $10), the credit card transaction fees take a big cut of profits. Adding funds to the company's phone app and then using the banked funds let's them reduce the number of credit card transactions and cut down on the fees.
Skimmers are a good point.
The normal solution for too-small-payment would be to take coins and/or bills. No need to worry about a skimmer then, and I'd be happy with that as well. Or just charge me the extra $0.50 to cover processing fees on the card to discourage that method,
but allow it.
Or have a sufficiently modern credit card system that doesn't allow the most trivial skimmers to work, like chip&pin. Unfortunately even here in Canada where we use that, we still need to allow the simpler methods to work as well (especially in the offline access case I mentioned).
That kind of helped understand what is going on.
My previous car was a 2013 BMW M3, which I think has a 20 gallon tank, but when I filled up when the orange light came on, it was around 15-18 gallons costing $60-70 at around $4 / gallon in CA (2017-18).
So long range Model 3 (315 miles) has a 75 kwh battery? Is 75 kwh battery equivalent to gallon capacity for gas cars?
So if a Tesla supercharging station says $0.30 per kwh on the GPS, does that mean a full charge of that 75 kwh battery come out to $22.50 maximum for a full charge? $0.30 × 75 = $22.50
And since Tesla's "fuel gauge" is in percent, does that mean every 10% is 7.5 kwh? So it should be $2.25 per 10% charge at that supercharger rate?
Plus I believe the speed of charging is faster when battery is low (near empty) and becomes slower when you get close to full. But the Tesla supercharger only shows kwh rate, not a time use rate. So do you only pay for the resource (Kwh) you get, even if the charging gets slower when you're nearly full? So would that mean you're may get less value if you're using a public charger that charges by the time if you try to get 90-100%?
I'm looking at my supercharging bill. I had to pay $8.70 to do a 40 to 70% before leaving Las Vegas to make it to Yermo CA to supercharge. Then in Yermo CA it cost $13.16 from a 14% to 75% charge. When I arrived home, I had 20% left in my battery. I could have saved time / money if I just did the minimum charge required. The Tesla said it only needed 20 minutes to charge so I set my timer for 20 minutes. But when my 20 minute timer was done, the car still said "needs 5 minutes", but time passed and that "5 minutes needed never went down" even though the charge was going up. I kept waiting, but then I saw the percent estimate left to my destination go to like 26% and the timer still didn't go down so I drove off. I usually like arriving home "to my charger" with 10-15% left feeling I got the most value (since home charging is cheaper). I dunno what's the extreme minimum to barely make it "home" to your wall connector is. Thing is, that percent estimate thing isn't accurate (on the dot). The estimate drops about 5% once you get close to home.
Prepare for an essay. Succinct conversation is not a skill I posses, sorry. But I will get to your questions and points!
kWh is indeed the "amount" of "fuel", like gallons for gas cars. Current Model 3 LR packs have a design capacity of roughly 77.5kWh, about 74.0kWh of which is
usable when brand new (at 0% or 0 miles on the screen's gauge, there is still another 4.5% of the remaining pack energy, but this isn't to be counted on for various reasons so it's not really considered).
So
when new, every 10% on the gauge is about 7.4kWh. As the battery degrades, the energy in each percent decreases. This is why it might be preferable to display rated range and not percentage, since there is a certain number of Wh per rated mile. See here for more info:
Should I display range, or percent?.
At per-kWh chargers like the Superchargers you mentioned, you do indeed pay for the same energy delivered even when the charge slows down (but note the gross vs. net discussion I'll have below). At the per-minute ones (e.g. some public stations, or also Superchargers in other states/countries), you
can be "ripped off" in comparison when the charge starts to slow down a lot. They sometimes have different rates depending on the power (kW) being delivered, but don't go so far to charge by total energy (kWh). It sort of approximates energy billing this way, but the very tail end of charge speeds is expensive in comparison. That said, at a public Level 2 station, you have to be at something like 97% before the car's charge rate dips below what the station can give out!
The reason the car says "20 minutes" but may actually take more is again partly due to the gross vs. net discussion below. Tesla, at Level 3 DC fast charge stations, uses
gross input power to estimate the time remaining. Of course, since net power into the battery is always lower than gross, they end up underestimating the charge time. A bug in my opinion, but inconsequential to most people and probably not a priority for them to fix.
(Sometimes I wish I was a Tesla employee that could point out and fix things like this... hire me please?). This problem does not appear to exist on AC charging, but then it has other quirks.
Tesla, like all charging stations, charges for
gross energy delivered (it used to be net at Superchargers, not anymore). The screen in the car while Supercharging shows
net power though!. This is important because you consume additional power for climate control, the car's systems, and for battery heating for a portion of the charge (which is a rather high amount of energy). The calculation ends up being, for example:
- "Full charge" of 74.0kWh, net
- $0.30/kWh, gross
- Assume 40 minutes "heating the battery", but 1 hour for the full charge (somewhat accurate on average)
- Battery heating power ~= 7kW for the AWD
- Estimated auxiliary power ~= 1.5kW (car's systems, minor climate control)
- 40 minutes @ 8.5kW = 5.67kWh "lost", but delivered
- 20 minutes @ 1.5kW = 0.50kWh "lost", but delivered
- TOTAL GROSS ENERGY = 74.0kWh net + 5.67kWh loss + 0.50kWh loss = 80.17kWh
So a "full charge" is actually $24.05, and effectively $0.325/kWh
net. I know it's like a buck fifty difference total, but what I'm showing is it's usually more than expected. For shorter charges at lower states of charge, the "lost" energy will be higher because it was likely heating the battery for the whole duration, not just 67% of the charge duration.
For example, a shorter charge might be like so:
- Precondition battery en route for faster charging, ~3kWh (very rough estimate)
- Deliver 30% to new battery in 13 minutes, about 22.2kWh net (realistic enough number at lower state of charge)
- Heat battery for the duration, AWD, so 7kW for 20 minutes ~= 2.33kWh
- Estimated aux power 1.5kW for 20 minutes ~= 0.50kWh
- TOTAL GROSS ENERGY = 22.2kWh net + 3kWh loss + 2.33 loss + 0.50 loss = 28.0kWh
Here, that works out to $8.41, effectively $.379/kWh
net. Note that this scenario was more expensive per
net kWh delivered due to the battery preconditioning losses for fast charging
and the battery being heated for the duration of the charge to maintain high charging speeds. That's an over 26% increase in cost, if still ultimately a small total cost.
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Regarding estimates and what's OK to arrive with, this varies. The Tesla trip planner is good at summer estimates with reasonably efficient driving practices, and mostly accounts for elevation changes. Anything that can significantly throw this off (bike racks, cold weather due to cabin heating, heavy rain, speeding, headwinds) will result in not being able to make the estimate. While it requires a lot more configuration (there's one for every variable I mentioned and more!), abetterrouteplanner.com is highly recommend for more "complex" trips. Finicky to configure, but way more accurate as a result.
I don't like to arrive with anything less than 10% (same as a gas car, really - that's about where the light turns on). In winter I give myself 15% or 20% because any expected issues on such drives can really suck a lot of power, and I've learned that the hard way.
