Why the arithmetic everybody does is wrong
Take the defaults: a 77 kWh pack going from 15 to 80 percent on a 150 kW post. That is 50.05 kWh into the pack, 52.68 kWh through the inlet at 95 percent, and the obvious sum says 52.68 divided by 150 is 21 minutes. The page says 27. The six minutes are the taper, and the gap widens fast the higher you take the target.
The reason is that power is not constant. Below the knee at 45 percent, the car takes the full 150 kW. Above it, the curve falls in a straight line towards 12 kW at the top, so by 80 percent the car is only accepting 62 kW — less than half the number painted on the dispenser. The average across the whole stop is 118 kW, and that average is the only power figure that has anything to do with how long you stand there.
The last ten points
With the defaults, going 15 to 25 percent takes 3 minutes. Going 70 to 80 percent takes 7. Same ten points, same 7.7 kWh, more than twice the time, and the ratio keeps climbing above 80. That is the whole argument for the 80 percent convention on a road trip: not that charging above 80 harms anything, but that the minutes stop buying much range.
The block near the bottom puts a number on the trade rather than repeating the rule. Stopping ten points earlier saves 7 minutes and gives up 7.7 kWh, which at 3.2 miles per kWh is 25 miles. Whether 7 minutes is worth 25 miles depends entirely on how far the next charger is, and that is not something a calculator knows.
Per-minute billing bills you for the taper
Where a network charges by time rather than by energy, the taper stops being an inconvenience and becomes a line on the receipt. The default session costs $25.29 at 48 cents per kWh. Spread over 27 minutes, the per-minute price that matches it is 94 cents. Charge the same session at a dollar a minute and you pay more, and you pay more specifically because the last third of the session delivers so little.
The perverse consequence is that under time billing a slower charger can be cheaper per mile than a fast one for a car that tapers early, and a fast charger is far cheaper for a car that holds power. It depends on the car, not on the post.
Where the model gives up
Two straight lines are a caricature of a real charging curve. Actual curves have plateaus, steps where the pack management changes strategy, and occasionally a bump back up as cells rebalance. A pack that arrives cold can sit at a third of its rated acceptance for the first ten minutes and then climb as it warms, which no version of a knee-and-tail model captures.
What the model does get right is the shape and the ordering: power is flat, then it falls, and time per point of charge rises as you go up. That is enough to answer the question people actually ask, which is how long the stop will be and whether the last stretch is worth waiting for. For anything tighter, log a session on your own car and move the knee until the total matches.
Questions people ask
How long does it take to charge from 20 to 80 percent?
It depends far more on the car than on the charger. With the defaults on this page — a 77 kWh pack, a 150 kW post, a knee at 45 percent — 15 to 80 takes 27 minutes at an average of 118 kW, against a naive division that says 21. Change the knee from 45 to 60 and the same session drops by several minutes without touching the charger at all.
What is the charging taper?
The point where the pack stops accepting the full output of the charger and the power begins falling as the state of charge rises. It is set by the battery, not by the dispenser, and it exists because pushing current into a nearly full cell is what damages cells. On this page you describe it with two numbers you can read off your own dash: the state of charge where the fall begins, and the power still going in at the top.
Why do I never see the full advertised power?
Three reasons stack. The vehicle has its own ceiling and it is often below the dispenser rating. The pack tapers above the knee, which is most of a session that goes past halfway. And a cold pack accepts far less than a warm one until it heats up. The average power figure on this page is the one that decides the length of the stop, and it is well below the peak in almost every real session.
Is the energy I pay for the same as the energy in the battery?
No, and the page keeps them apart deliberately. The dispenser meters energy at the inlet; some of that becomes heat in the cable and the cells rather than charge in the pack. At the 95 percent default, a session that puts 50.05 kWh into the pack bills 52.68 kWh. Every line on the output says which side of that loss it sits on, because a cost per mile worked from the wrong one comes out low.
Should I charge to 100 percent on a road trip?
The page gives you the trade rather than an answer. With the defaults, the ten points from 70 to 80 already take twice as long as the ten points from 15 to 25, and the ratio keeps climbing above that. If the next charger is comfortably in range, the minutes buy little. If it is not, they buy the whole trip. Charging habits for battery longevity are a separate question and this page says nothing about it.