So here is an interesting question -- what is the optimal speed to drive the car at on long trips? The idea is to minimize the door-to-door time. Obviously if you drive faster, you can spend more time on the charger, so there must be some tradeoff. If the trip ends after the final recharge cycle at your destination (which isn't normally what most people would consider the end of the trip), and chargers are available anywhere you want (you don't need to worry about stretching a long leg between two chargers), then I believe the optimal speed is the one where the power use during the drive is equal to the power provided by the next charger. This is interesting, because the faster the charger, the faster you should drive! If you know the kWh of your next charger, set the cruise control on level group until the power meter is showing the power provided by the charger. An 80A charger is probably well over the posted speed limit!
Ken
You may be right in proposing that rule of thumb, Ken.
This is an almost exact analog to the question of how to optimize the achieved cross-country speed when flying a glider: in classic thermal soaring, the glider descends while making forward progress over the ground and then stops and circles to climb when it encounters a thermal (rising air). Turns out that the optimum speed to fly in the cruising portion of the flight is simply
the speed which minimizes the time required to reach the top of the next thermal. Clearly, the stronger the next thermal is the faster you should fly, in order to get to it sooner, accepting the penalty in extra altitude lost because you'll make it up by climbing faster when you get there. Trouble is, you never really know the strength of that next thermal or where you're going to find it, so it's a lot of guesswork. (Whereas in an EV you should have a very good idea where your next charge is located and what the rate of charge will be. Don't leave home without a plan!)
In an EV, the same rule, restated, is this: the optimum speed to drive is
the speed which minimizes the time required to reach the desired state of charge at the end of your next recharge stop. The higher the charge rate, the faster you can drive (and should, to minimize total time en route). It's worth noting that driving 10% slower than optimum costs you less time overall than driving 10% faster than optimum.
Perhaps counterintuitively, in a glider faced with a headwind or flying through sinking air, you need to
increase your cruise speed, because even though you descend at a steeper angle through the air you will spend less time in the adverse condition and achieve a better glide angle over the ground. An EV in a headwind has a very different problem (with wheels on the ground to provide motive force, a headwind does not directly affect your speed, it just affects the current draw required to maintain that speed), but my engineer's gut tells me that it's backwards from the glider case: optimum speed in an EV rises with an increasing tailwind (but the increase is always less than the speed of the tailwind) and diminishes (slowly) with a headwind.
Useful rules of thumb:
1. If you plan a multi-leg trip utilizing chargers of differing rates, then:
a) if the charger you're at has a higher rate of charge than the next charger you'll encounter, leave for the next charger only when the charge rate tapers off to the rated charge rate of the next location;
b) if the charger you're at has a lower rate of charge than the next charger, leave as soon as you have sufficient range to make it to the next charger (plus your personal minimum reserves).
2. If the charger you're at has the same charge rate as the next one, you may head out at any time after you have sufficient range, but in any case as soon the charge rate starts to taper off.
3. If you're charging at the last location before an extended stop, leave for your destination as soon as you have sufficient range (plus reserves). Of course, this only holds if you can get a full charge during your planned extended stop: otherwise, see the rules above.
I'll have to see if I can noodle through the math behind all these claims and quantify things. Charts and tables, anyone?
