Nav optimizing for supercharger time

[sowbug]

Hope this isn't a duplicate. I've searched the forums and found lots of Supercharger discussions, but none with this precise question.

Suppose I'm going from Point A to Point B, with Superchargers 1 and 2 between them. Supercharger 1 is closer to A, and Supercharger 2 is closer to B. I start at Point A (home, where I have a HPWC) with a full charge. The distances are such that I could go all the way from A to B, but I'll need a charge to get from Point B (the destination) back to Supercharger 2 on my way back home to A. Since there's no destination charger, I need to stop at at least one Supercharger on the way there.

Option 1: I could charge at Supercharger 1, which will charge at a slower rate (since I have a higher charge at that point, and Teslas with higher charges charge relatively slowly at Superchargers). That charge will give me enough to go past 2, to B, back to 2 again, where I can charge up for the trip home.

Option 2: I could charge at Supercharger 2, which will charge faster (since my battery will be closer to depleted, and low state-of-charge Tesla batteries charge at a high rate at Superchargers).

The Tesla nav system always seems to recommend Option 1 -- charging sooner than later. This makes sense from their perspective because (a) it probably reduces range anxiety, and (b) it likely ends up using less Supercharger electricity, since the next charge always has a nonzero chance of happening at a non-Supercharger outlet.

However, if given the choice, I'd prefer Option 2. Time spent charging is lower, though I admit that I'm running the car closer to empty and thus taking a higher risk of stranding myself. (See this comment in How do I arrive faster on a long trip? for similar reasoning.)

I believe that an equivalent way of stating this is that I'd always prefer to use as much range of my car as possible, because I'll spend more overall charge time in the low-charge/fast-power state at Superchargers.

My question is whether other owners ignore the nav system's recommendations to optimize for charge time rather than peace of mind. Do you? Please discuss (or refer me to the existing discussion if it exists).

 

[SoxFan2004]

I optimize for peace of mind... Range anxiety ruins an otherwise perfect drive

 

[jerry33]

Usually by the time I drive the distance between SCs it's time for a break anyway, so might as well stop and charge for a few minutes.

 

[gregincal]

Starting from home I would definitely always drive to the furthest supercharger I could reach without introducing range anxiety. Stopping at nearby superchargers when your car is still almost full doesn't make any sense.

 

[mikeash]

Don't attribute to malice what can be explained by stupidity.

The trip planner has been kind of stupid from the beginning. It's better than it was, but it's still not great. I think this is just a symptom of not being great.

For example, I'm in Fairfax, VA, just west of Washington. When heading to points northeast, towards NYC, the natural route is to take I-95 and stop at either the Newark, DE supercharger or Hamilton, NJ, depending on how much I want to push it. Heading to points northwest, I'd take I-270 to I-70 and stop at either Hagerstown, MD or Somerset, PA. In both cases, the trip planner often routes me to the Bethesda, MD supercharger first. This is a terrible idea, not only because of charging way too early, but because Bethesda is a temporary location with only two stalls located in a mall parking lot near the entrance. That makes it extremely busy and prone to abuse, and it's likely you'll end up waiting a long time to even get a spot. The last thing Tesla wants to do is encourage even more traffic to come there, but that's what the trip planner does.

I will always hit the most distant supercharger I can comfortably reach along my route. If the trip planner tells me to visit an earlier one, I'll ignore it, unless of course there's a need to stop for other reasons. But I think the trip planner's failure here is just because it's not very good (yet? I hope), not because of any attempt to manipulate our behavior.

 

[thimel]

I've certainlay had the Tesla navigation system choose to skip an SC when I have enough charge to reach the next one.

 

[Naonak]

You should always arrive at a Supercharger as close to 0 as possible. That is simply the most efficient method of driving and charging you can manage. If you always arrive at a Supercharger at 0%, you are as optimized as you can possibly get, assuming you maintain reasonable highways speeds during the trip. IF you have to slow down to 45mph to get there, then you should charge sooner and drive faster to get to the charger.

Basically, drive as fast as you can (within ... err... legal limits?) to the next Supercharger that will bring you as close to 0 without going under. Anything else and you're wasting time. Superchargers replace miles faster than you can drive them off at any speed.

 

[MorrisonHiker]

Basically, drive as fast as you can (within ... err... legal limits?) to the next Supercharger that will bring you as close to 0 without going under. Anything else and you're wasting time. Superchargers replace miles faster than you can drive them off at any speed.

Last week, I used EV Trip Planner to calculate times for a 500 mile route (200 miles in Colorado, 300 miles in Kansas) and found there wouldn't be a huge decrease in time to drive as fast as possible. If I increased speed 10%, I could cut 8 minutes off the trip. Increasing speed by 20% actually increased the length of the trip slightly. Increasing speed 30% only cut a 3 minutes off the entire 500 mile trip. I haven't tried this out yet but will be making the trip later this year.

From the numbers, it doesn't look like going as fast as you can will always be the fastest way to make the trip.

 

[Johann Koeber]

I always plan to arrive at the SC with 10-5 % charge left. This is (for me) the comfortable buffer. The other reason I don't want to go to zero is that is bad for the longevity of the battery.

Last time I dearly needed the buffer because the exit where the SC is located (Schweitenkirchen) is closed due to construction. Luckily I had enough buffer to use the next exit and return back to the SC. Close call it was.

Otherwise I optimise time; unfortunately the trip planner does not allow planning for return trips or waypoints, so yes, some guessing and experience is needed.

Fortunately in 152,000 km I was never left stranded.

 

[Johann Koeber]

...

Basically, drive as fast as you can (within ... err... legal limits?) to the next Supercharger that will bring you as close to 0 without going under. Anything else and you're wasting time. Superchargers replace miles faster than you can drive them off at any speed.

I believe this is not the case.

Yes, SC replace the electrons fast. But the time it takes to charge must not be compared with the driving time, but the "saved" driving time. Driving faster saves only incremental driving time, which may then be used to charge. The incremental energy consumption to save 1 minute of driving time will be very high, essentially discouraging faster travel.

I have tried this on the German Autobahn, where velocities of over 200 km/h are possible (and legal), traffic permitting.

This has to do with derivatives. Maybe some of the more mathematically inclined can come up with the correct function for minimizing travel time. I haven't discovered it here yet.

Another aspect is that even when we got a function for the steady state optimum travel velocity, there are 2 more aspects to consider:

- starting with a full battery shifts the optimum - no problem to arrive back home empty; this makes the iptimum more granular

- real world driving includes constant acceleration and deceleration - much more so when travelling at high speeds. This pops the question not only of the optimum speed but also (derivatives anyone?) the optimum acceleration

 

[jerry33]

This has to do with derivatives. Maybe some of the more mathematically inclined can come up with the correct function for minimizing travel time. I haven't discovered it here yet.

This was calculated in the very early days of the Model S (it's in the forum someplace, but a quick search didn't find it). The conclusion was that 90 mph between SCs was the fastest way. Note that it was calculated off the energy curve that Tesla supplied (it's in the Tesla Blogs) using the SC charging rates given at the time. It probably doesn't equate that well to the latest SCs and Model S.

 

[daxz]

Also Consider
1. Potential queue or shared slot likelihood at Supercharger 2 vs SC1.
2. What amenities are located at each spot. (30 mins doing something I need anyway vs 15m of sitting in car)

 

[Rocky_H]

I don't see my answer as an option in your poll:
"I disabled that brainless piece of $#!^ beta trip planner shortly after it came out."

 

[Electric700]

I always plan to arrive at the SC with 10-5 % charge left. This is (for me) the comfortable buffer. The other reason I don't want to go to zero is that is bad for the longevity of the battery.

Last time I dearly needed the buffer because the exit where the SC is located (Schweitenkirchen) is closed due to construction. Luckily I had enough buffer to use the next exit and return back to the SC. Close call it was.

Otherwise I optimise time; unfortunately the trip planner does not allow planning for return trips or waypoints, so yes, some guessing and experience is needed.

Fortunately in 152,000 km I was never left stranded.

Great points.

 

[Naonak]

Last week, I used EV Trip Planner to calculate times for a 500 mile route (200 miles in Colorado, 300 miles in Kansas) and found there wouldn't be a huge decrease in time to drive as fast as possible. If I increased speed 10%, I could cut 8 minutes off the trip. Increasing speed by 20% actually increased the length of the trip slightly. Increasing speed 30% only cut a 3 minutes off the entire 500 mile trip. I haven't tried this out yet but will be making the trip later this year.

From the numbers, it doesn't look like going as fast as you can will always be the fastest way to make the trip.

I believe this is not the case.

Yes, SC replace the electrons fast. But the time it takes to charge must not be compared with the driving time, but the "saved" driving time. Driving faster saves only incremental driving time, which may then be used to charge. The incremental energy consumption to save 1 minute of driving time will be very high, essentially discouraging faster travel.

I have tried this on the German Autobahn, where velocities of over 200 km/h are possible (and legal), traffic permitting.

This has to do with derivatives. Maybe some of the more mathematically inclined can come up with the correct function for minimizing travel time. I haven't discovered it here yet.

Another aspect is that even when we got a function for the steady state optimum travel velocity, there are 2 more aspects to consider:

- starting with a full battery shifts the optimum - no problem to arrive back home empty; this makes the iptimum more granular

- real world driving includes constant acceleration and deceleration - much more so when travelling at high speeds. This pops the question not only of the optimum speed but also (derivatives anyone?) the optimum acceleration

You are both correct ... The speed curve actually levels off around 75 MPH (I have never seen the 90 MPH figure quoted above?) as being the optimum speed. Once you are above a certain speed, yes, it's more efficient time wise to drive slower than it is to charge. I was more speaking to driving legally (in the US) at or around the speed limits on the Interstates here. It maybe that above 90 MPH you seem some minor gains to a certain point, but I'm not sure what those gains would be.

 

[jerry33]

You are both correct ... The speed curve actually levels off around 75 MPH (I have never seen the 90 MPH figure quoted above?) as being the optimum speed. Once you are above a certain speed, yes, it's more efficient time wise to drive slower than it is to charge. I was more speaking to driving legally (in the US) at or around the speed limits on the Interstates here. It maybe that above 90 MPH you seem some minor gains to a certain point, but I'm not sure what those gains would be.

Some graphs are here. I couldn't find the original post I was looking for though. Sorry about that. There was a spreadsheet on it that had a number of different cases.

 

[mikeash]

Won't the optimal speed depend heavily on how far you're going? The supercharging taper means you're looking at adding miles very slowly if you need to use most of the battery charge.

For example, if you're at a Supercharger and it's 90 miles to the next one, then you need well under 50% to make that distance. If you make the trip at 80MPH instead of 60MPH, you'll need more miles in the battery, but you'll add those extra miles at probably well over 300MPH.

On the other hand, if it's 200+ miles to the next stop, then you may well need a 100% charge to go fast. Those extra miles will be added very slowly, so you're probably better off stopping the charge earlier and driving more slowly.

 

[Boatguy]

I charge when my wife needs to stop.

 

[David99]

I agree with the others, I charge later than earlier, to get a higher charge speed = shorter charging time. But when traveling, sometimes other factors play in. If you need a bathroom, you stop so you might as well charge. If you have sleeping kids with you, you might want to go slower, skip a Supercharger and drive a longer stretch so they don't wake up.

As for ideal driving speed, that depends on the distance between Superchargers. The shorter the distance between them, the higher your ideal speed. Technically you would have to account for weather as well and each leg would be slightly different. Either way, the differences are within a few minutes, so just drive as you feel comfortable. Arriving at near zero battery is the single most important thing to save time on a long road trip. Arriving at 0% vs 30% and charging 120 miles to get to the next SuC takes twice as long when arriving at 30%.

 

[TaoJones]

I charge when my wife needs to stop.

Ah, yes. The DHF* cannot be overstated during the planning and execution stages of any proper road trip.

*DHF. The Domestic Harmony Factor. A critical path component that is disregarded At One's Peril for the rest of the trip, and the one after that