I'm looking for an efficiency formula. Drive speed vs charging time.

[gad1976]

I'm looking for a math wiz smarter than I am to help with what is probably a simple formula.

Example:

I have a 6 hour 360 mile trip. I will have to stop once to charge at a super charger.

If I drive half the trip at 60 mph for 3 hours rather rather than 55 mph I arrive at the charger 15 minutes early. Is it possible that the time saved by more speed could use up enough extra battery power that I would have to charge for an extra 5, 10, or 15 minutes thus lessening or negating the time I saved by driving faster?

Any math wiz's willing to take a whack at this?

Thanks.

 

[jjrandorin]

I dont know any formulas, but I remember reading that the math says its fastest to drive as fast as possible and charge only enough to make the next charging station. So, im almost (but not 100%) positive that it would be faster for you to drive that trip at 80 MPH and then charge to enough to recover, vs driving at 55 or 60 (for example).

I also think this math is already built into websites like abetterrouteplanner so you can simply plan your trip there and input speed you are traveling.

ABRP

ABetterRouteplanner is the world's most popular consumer EV routeplanner - both for beginner and experienced EV drivers. And of course for anyone curious about EVs.

abetterrouteplanner.com

 

[dafish]

I’m away from it at the moment, but I did build a spreadsheet and model this. From memory:

As a general statement even 80 is still in the “better to run fast and recharge” area. However, it’s really close at that point. ANY modeling of this depends on relatively low SoC when you plug in and relatively conservative “next leg” lengths. trying to charge past 75% or so gets pretty slow. Suddenly you’re better off slowing down and not charging so long.

the return (range increase for slowing drops as you slow. I would not drop below 73 myself, although I did have to come down to 63 recently while towing. Whole different drag bucket there

obviously being on a fast super charger is needed too. you need to see an average charge speed of 85-90kw or more for 80mph to make sense

also, for me, the best way to do this is on the fly. Use the energy graph, your minimum and maximum SoC levels, (against pack capacity) and decide. This as wind can really screw thing up, or really help things, on the fly. For leg lengths of 180 miles I would depart at 80%, run 74 for 15 miles, and decide if I could move up to 78-79. Personally i increase speed to a goal of 15% SoC on arrival. If the headwind is insane you may need 65, but that’s rare.

tesla’s nav projections helps with that, but I’ve seen it be off by a lot too. I’m usually more accurate.

ymmv

 

[Black306]

I'm looking for a math wiz smarter than I am to help with what is probably a simple formula.

Example:

I have a 6 hour 360 mile trip. I will have to stop once to charge at a super charger.

If I drive half the trip at 60 mph for 3 hours rather rather than 55 mph I arrive at the charger 15 minutes early. Is it possible that the time saved by more speed could use up enough extra battery power that I would have to charge for an extra 5, 10, or 15 minutes thus lessening or negating the time I saved by driving faster?

Any math wiz's willing to take a whack at this?

Thanks.

6hr (drive time @60mph) + 0hr (low est. charge time needed; unlikely) > 4.5hr (drive time @80mph) + 1hr (high est. charge time needed; only need enough to get to the destination)

Don’t drive fast, fly low.

 

[Goobers]

I'm looking for a math wiz smarter than I am to help with what is probably a simple formula.

Example:

I have a 6 hour 360 mile trip. I will have to stop once to charge at a super charger.

If I drive half the trip at 60 mph for 3 hours rather rather than 55 mph I arrive at the charger 15 minutes early. Is it possible that the time saved by more speed could use up enough extra battery power that I would have to charge for an extra 5, 10, or 15 minutes thus lessening or negating the time I saved by driving faster?

Any math wiz's willing to take a whack at this?

Thanks.

The problem is, you need to know just how much of a difference in battery SoC you'll have left driving at 60 vs 55 mph and what speed you'll be able to charge at the supercharger. And those numbers have HUGE variances.

For myself, I drive the same 4 mile stretch of highway everyday to and from work (close to 80 mph average). And I always get a message saying if I drove less that 70 mph, I could save 0.2%. Let's stretch this and make it worse at 0.4%, so that it's 0.1% per mile.

So, if we mix in your numbers and drive at 80 mph over the course of 180 miles (60 mph for 3 hours), it would use an additional 18% (180 x 0.1) and get there in 2.25 hours... a supercharger won't take 0.75 hrs (or 45 minutes) to charge just 18% (to cover the difference). If we use the slowest 50kW speed and converting the 18% to about 14.4 kW (assuming 80-ish kW battery pack), it would take it a little over 17 minutes, leaving you ahead by 28 minutes to now charge up the rest of the battery pack (about 55% or so to cover 180 miles used). Actually, if you charge to 75% (from 0%), it would take about 1.5 hrs and get you to your destination (with about 3% remaining), meaning you would be halfway through your charge time by the time a person going 60 mph ENTERS.

Stepping back a bit, using a 50kW charger allows you to charge 12.5 kW in 15 minutes. And 12.5 kW is almost 16% of the MY's battery. Then tack on the assumption, there's no way you would use 16% more of the battery going 60 mph instead of 55 mph over the 180 miles, you'll be ahead going 60 mph.

I think for most drivers, faster speed outweighs the extra % used.

At least, right up until you get pulled over and a ticket.

 

[MY1stEV]

@gad1976 This is what Teslafi shows for the efficiency of my car at different speeds. From this I would infer that the difference between the speeds in the range of 55 and 75 are negligible. It's only when you push above that it drops off. Appears to be a 15% drop in efficiency over 75.

 

[kgvickers]

Is that supercharger stop at exactly the 180 mile mid-point in your trip? I'm betting "no", and the longer leg will dictate how fast you can drive for that portion. For the shorter leg, sure, drive as fast as you can get away with.

I have a 940 mile one-way trip I make to a vacation spot several times a year. I've done it several times all in one shot with the necessary supercharger stops. For me, what seems to work best is to just drive maybe 5 mph over the limit (mostly 70 mph limit) and just follow the stops recommended by Elon. I've tried doing the math and minimizing charging stops. That plan works until a line of thunderstorms hits me in the mouth and suddenly I'm really slowing down to make the next charger.

 

[SSonnentag]

I think Bjorn did a video on that years ago using a Model S. His results were something like 120 mph was optimal for total time. This was using V2 chargers and less efficient motors, so the results may be a bit different with today's faster chargers and more efficient vehicles.

 

[Feathermerchan]

The car can tell you what you need to know. While charging watch the display of the remaining charge at your next stop. I leave when it says ~10%. Then when driving, if it starts dropping below 10%, slow down. If it climbs above 10% speed up. The 10% is only a suggestion. You may be more comfortable with 15% or 5%. But the goal is to arrive at a low state of charge because 1) it will charge faster, 2) you'll spend less time charging when you leave with the next stop being at a low state of charge.

 

[gad1976]

Thanks for your input. As many on this forum predicted, the longer I drive my Tesla, the more I simply drive and don’t worry about these things. I still enjoy the ”math” of driving a Tesla, but I find I just wanna drive the way I wanna drive and charge accordingly.

 

[Feathermerchan]

I just drove a 560 mile round trip. Followed my routine outlined above. Had a great time on the back roads and never thought about range. Only Supercharged 4 times.