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The voltage is held constant. The intrinsic resistance of the battery grows as it becomes charged. I suppose the SC could increase voltage as the battery becomes full but that may break down the lithium cells faster than desired.
Current moves because there's a difference in charge between two points; it moves from the place with more charge to that with less. As the battery fills, it gains more charge and thus, has less of a potential difference between it and the voltage source. This inherently slows down the movement of charge as there's less electo-motive force.
When you do a supercharger rally (officially or unofficially... ), you definitely are more time efficient leaving before 100%. That assumes you have enough to make it to the next supercharger, of course.From the Tesla SuperCharger page under the "How it works" section:
Optimal Charging
The fastest way to replenish your Model S is to charge to 80% state of charge, which is more than enough for travel between Supercharger stations. Charging the final 20% takes approximately the same amount of time as the first 80% due to a necessary decrease in charging current to help top-off cells. It's somewhat like turning down a faucet in order to fill a glass of water to the top without spilling.
That's actually an understatement. Exceeding the maximum allowable terminal voltage on a lithium-ion cell is a very good way to set it on fire.I suppose the SC could increase voltage as the battery becomes full but that may break down the lithium cells faster than desired.
There's no requirement that the voltage start at maximum. It will do so only if the charger can supply as much power as the battery can then absorb. If a large battery is nearly depleted, and/or the charger is too small, it will begin by limiting the current to some maximum. This is the "bulk charge" phase. The battery's voltage will rise as it fills, and when it reaches the upper limit the charger will hold it there by decreasing the charging current. This is the "absorption" or "acceptance" phase.Every time I've supercharged, the voltage has increased gradually to its maximum of 400 volts and amperage has decreased (from 200 amps to below 20 amps near max range) as the charge state of the batteries reaches maximum....
Aaron
Sounds good. Thanks for the follow-up.
just curious. Why can't the SC's pump juice at max input until its 95% and then taper off, versus the slow taper that gets 50% in 20 min, 80% in 40 min and 100% in 75 min?
I'm sure others have already mentioned this, but imagine filling a glass with water from your bathtub spigot on full blast. You can only do that for a relatively short period of time before you overfill the glass. To prevent overshooting, you gradually close your spigot so that the flow rate allows you to neatly fill the glass without spilling. That's basically how the superchargers work, and why they have to throttle back the charge rate at some point.
Optimal Charging
The fastest way to replenish your Model S is to charge to 80% state of charge, which is enough for travel between Supercharger stations. Charging the final 20% takes approximately the same amount of time as the first 80% due to a necessary decrease in charging current to help top-off cells. It's somewhat like turning down a faucet in order to fill a glass of water to the top without spilling.
I'm sure others have already mentioned this, but imagine filling a glass with water from your bathtub spigot on full blast. You can only do that for a relatively short period of time before you overfill the glass. To prevent overshooting, you gradually close your spigot so that the flow rate allows you to neatly fill the glass without spilling. That's basically how the superchargers work, and why they have to throttle back the charge rate at some point.