I just asked chatGTP an interesting question that I've never been able to calculate on my own. I gave it the basic specs of an electric car with 74kw battery, ability to charge 10% to 80% in 18 minutes, efficiency of 3.6kw/mile at 70mph, efficiency of 3.0kw/mile at 80mph. The efficiency numbers are ones that I have experienced myself, assuming each trip was done at the same temperature and terrain.

I then asked it the length of a trip when driving at each speed would take the same amount of time and which speed would be faster in a longer trip. The surprising answer was shorter than I expected and is listed below:

"chatGTP: Thus, the crossover distance happens approximately at 309.6 miles. Beyond this distance, driving at 70 mph will be faster considering the necessary recharge times."

I also gave me a long list of formulas and calculations that were required to achieve at that answer. Of course, we can't always charge to 80% when we are at 10% because charger location (especially working ones) are not always available, but that is true at either speed. And at 80mph, you would have to make more stops which would make charging time even more risky.

So if my trip is longer than 309 miles, it is faster for me to drive slower.

There are other factors, but I generally agree. Many charging stops take time to navigate to, some/many have slow or out of service units, and there can be queues at chargers.

So the answer, which AI can’t really quantify is the likelihood issues at charging sites will add time would favor slower speeds and thus fewer stops.

I’m currently on a trip, first stop no issues, second was 125kW ChargePoint, third no issues (4 ioniqs at the site), 5th were all 150kw, 6th was Francis Energy requiring installing an app. Next three no issues but staying two days at the third and got 65kW from a 350 unit until I restarted a session using the second handle.

Conclusion: charging is still unpredictable. But the good news is despite the Holiday weekend, no queues yet. We head to SoCal tomorrow, who knows what we’ll experience there.

 

So the answer, which AI can’t really quantify is the likelihood issues at charging sites will add time would favor slower speeds and thus fewer stops.

But charging issues are basically the same for both speeds... and perhaps one extra stop for 80mph if driving over 309 miles. So the longer the trip, the more likely the slower speed will get you there faster with less stops. The shorter trips are faster at 80mph. Of course the math changes if you charge to 100%, if you're car is more aerodynamic like the I6, or if you have to stop at a charger when you are at 50% SOC because the next charger is too far away, etc. But again, that would be the similar for both speeds.

 

Assume a 150kW charger and it will take 0.35 hrs to charge
At 80 you are going 155 miles between stops and taking 1.9 hrs. Add charging time to get 2.25hrs and an average of 69mph.
At 70 you are going 185 miles between stops and taking 2.6 hrs. Add charging time to get 2.95hrs and an average of 63mph.

Now let's look at the best case scenario for 70mph which is a journey of 266 miles involving no stops and taking 3.8hrs.
At 80 we would have to stop once so it would take 3.33 hrs plus 0.35hrs which is 3.68hrs.
So even in the best case it is better to go at 80mph assuming you can find an empty charger of 150kW of more. You will also feel a lot more relaxed not arriving with 0% left and no margin for error, and not desperate for the loo/food.

 

Yes, there is a lot of variables. ChatGPT indicated that 70 or 80 mph was equal at the 309 mile mark. Over that it would depend greatly on the charging stops. I would think the longer the trip the more likely less stops would be an advantage over more stops. Each trip would have to be calculated separately. And as you indicated, bathroom stops are sometimes necessary even if you had a 300kw battery pack.

 

On the charger sparse route from my home to relatives in another state, two of the charging locations require a detour. One is about 10 minutes from the freeway, the other adds about 10 miles and maybe 20 minutes. If going a bit slower means you can skip those then your trip time could well be quicker.

So that calculation probably needs to be trip dependent.

As more charging stations are built the calculation will change, probably in favor of a higher cruise speed.

 

just drive ... put up a smile ... enjoy ...