7 ms·

Slightly off topic, but tengential : EV's range is usually measured using some sort of "mixed" cycle like WLTP where you do a bit of slow speed, a bit of high s

Slightly off topic, but tengential : EV's range is usually measured using some sort of "mixed" cycle like WLTP where you do a bit of slow speed, a bit of high speed etc. Is there a "dumb" test that just fill up the battery, put the car on a bench, goes up to 130km/h (or whatever a sensible value for "highway speed" is), and waits for the battery to be empty ? If it's a completely stupid idea, why ?

grecy 3y ago
Wind resistance is by far the biggest use of energy when moving at highway speed. Without it, you're not testing anything useful in the real world.
- mschuster91 3y ago
  Increase the drag resistance on the bench appropriately, that's it.
- eightysixfour 3y ago
  This is partially it, but it isn’t hard to calculate the CdA of a car and then add resistance to the rollers equivalent to the expected wind resistance, however the bigger reason is that cars operate in a dynamic environment and you don’t actually want to know how far they’d drive at 100kph with no additional wind in a straight line. It just isn’t helpful. The driving cycles have a number of issues but they purposefully try and capture more of the range of dynamics to help consumers better understand what they should actually expect from vehicles. It is like comparing CPUs only on their ability to multiply quickly vs a suite of software benchmarks.
  - phtrivier 3y ago
    Thanks, I would not have suspected drag. Still, it seems like you're saying "the number would be unrealistic", but I'm not sure people want a realistic number, just a "worst case scenario"... I understand that standard tests try to measure something "realistic", but they end up giving a value that is not what people are waiting for - which is, again, and I can't stress it enough because it's 100% of the conversation i still have about EVs with every single person I talk to - "insurance that I will be able to visit my mum on a single charge at the other end of the country". I understand it's a bad question to ask and people don't really need _that_ range, but apparently they need _that_ info :/
    - eightysixfour 3y ago
      Right, but those people would then get their EV, go for a 20km drive in bad EV conditions, see their range decrease by 30km, and call the manufacturer a liar. Been there, done that. Family member sold theirs because they weren’t “confident in the range” despite never driving more than 80km in a day. Truth is I think some people just don’t want them and they look for a “logical” out since that conflicts with their desire to be less impactful to the environment. A 75kwh battery pack has the same energy as ~8.5 liters of gasoline. While the conversion of that energy to movement is far more efficient than a gasoline engine, that’s still a small amount of energy comparatively, so the variance is going to be very, very high.
edent 3y ago
You could say that 1W can move a mass of 1Kg a distance of 1m in 1s. The battery can store NN kWh and the car weights XX Kg. How many Km can it travel? It ignores wind resistance, road incline, use of air con, external temperature, number of passengers etc. So it would even more meaningless than the somewhat synthetic tests already being done.
- out_of_protocol 3y ago
  Also depends on speed - for electric cars most economic speed is kinda low, due to atmospheric drag. So in city cycle electric cars will do much much better than gasoline ones.
  - r00fus 3y ago
    Not to mention in traffic. Every time I'm stuck in traffic in my EV I just wonder how many gallons of gas are wasted by idling ICE vehicles.
- t0mas88 3y ago
  Ignoring wind resistance is what he wrote, but I think it's the opposite of what he meant. A highway test that accelerates once to a high speed and maintains that speed is mostly about aerodynamic drag.
Rebelgecko 3y ago
Your test won't handle air resistance, which is a big part of why US EPA and Chinese range estimates tend to be so different
mensetmanusman 3y ago
It's not stupid, but the test would be complex. E.g., one would have to model and output the relevant solar spectrum to heat the vehicle and see how it responds to the IR heat load in maintaining human and battery operating temperature. Same with cold cycles, and the details get complicated when you try to average out the experienced conditions. One of Tesla's mileage advantages comes from using the most advanced multilayer mirrors in their glass to minimize the effect of radiant heating/heat loss. Fun aside: An interesting side effect of the technology is the appearance of red droplets when rain or dew is on the glass with the proper sunlight conditions: https://www.reddit.com/r/oddlysatisfying/comments/9ustsa/refraction_of_light_through_raindrops_on_the/ https://www.reddit.com/r/oddlysatisfying/comments/9ustsa/ref...
slaw 3y ago
Car and Driver does range test at 75mph. https://www.caranddriver.com/shopping-advice/a32603216/ev-range-explained/ https://www.caranddriver.com/shopping-advice/a32603216/ev-ra...
r00fus 3y ago
Youtube has some videos of highway road tests at 70mph (just search "highway range test EV 70 mph") if you're curious. Actual road tests are nice because they give you a good idea of wind resistance + some elevation variance - which is what you'll likely encounter doing road trips which is when range is most important.

grecy 3y ago

Wind resistance is by far the biggest use of energy when moving at highway speed. Without it, you're not testing anything useful in the real world.

mschuster91 3y ago
Increase the drag resistance on the bench appropriately, that's it.
eightysixfour 3y ago
This is partially it, but it isn’t hard to calculate the CdA of a car and then add resistance to the rollers equivalent to the expected wind resistance, however the bigger reason is that cars operate in a dynamic environment and you don’t actually want to know how far they’d drive at 100kph with no additional wind in a straight line. It just isn’t helpful. The driving cycles have a number of issues but they purposefully try and capture more of the range of dynamics to help consumers better understand what they should actually expect from vehicles. It is like comparing CPUs only on their ability to multiply quickly vs a suite of software benchmarks.
- phtrivier 3y ago
  Thanks, I would not have suspected drag. Still, it seems like you're saying "the number would be unrealistic", but I'm not sure people want a realistic number, just a "worst case scenario"... I understand that standard tests try to measure something "realistic", but they end up giving a value that is not what people are waiting for - which is, again, and I can't stress it enough because it's 100% of the conversation i still have about EVs with every single person I talk to - "insurance that I will be able to visit my mum on a single charge at the other end of the country". I understand it's a bad question to ask and people don't really need _that_ range, but apparently they need _that_ info :/
  - eightysixfour 3y ago
    Right, but those people would then get their EV, go for a 20km drive in bad EV conditions, see their range decrease by 30km, and call the manufacturer a liar. Been there, done that. Family member sold theirs because they weren’t “confident in the range” despite never driving more than 80km in a day. Truth is I think some people just don’t want them and they look for a “logical” out since that conflicts with their desire to be less impactful to the environment. A 75kwh battery pack has the same energy as ~8.5 liters of gasoline. While the conversion of that energy to movement is far more efficient than a gasoline engine, that’s still a small amount of energy comparatively, so the variance is going to be very, very high.

edent 3y ago

You could say that 1W can move a mass of 1Kg a distance of 1m in 1s. The battery can store NN kWh and the car weights XX Kg. How many Km can it travel? It ignores wind resistance, road incline, use of air con, external temperature, number of passengers etc. So it would even more meaningless than the somewhat synthetic tests already being done.

out_of_protocol 3y ago
Also depends on speed - for electric cars most economic speed is kinda low, due to atmospheric drag. So in city cycle electric cars will do much much better than gasoline ones.
- r00fus 3y ago
  Not to mention in traffic. Every time I'm stuck in traffic in my EV I just wonder how many gallons of gas are wasted by idling ICE vehicles.
t0mas88 3y ago
Ignoring wind resistance is what he wrote, but I think it's the opposite of what he meant. A highway test that accelerates once to a high speed and maintains that speed is mostly about aerodynamic drag.

Rebelgecko 3y ago

Your test won't handle air resistance, which is a big part of why US EPA and Chinese range estimates tend to be so different

mensetmanusman 3y ago

It's not stupid, but the test would be complex. E.g., one would have to model and output the relevant solar spectrum to heat the vehicle and see how it responds to the IR heat load in maintaining human and battery operating temperature. Same with cold cycles, and the details get complicated when you try to average out the experienced conditions. One of Tesla's mileage advantages comes from using the most advanced multilayer mirrors in their glass to minimize the effect of radiant heating/heat loss. Fun aside: An interesting side effect of the technology is the appearance of red droplets when rain or dew is on the glass with the proper sunlight conditions: https://www.reddit.com/r/oddlysatisfying/comments/9ustsa/refraction_of_light_through_raindrops_on_the/ https://www.reddit.com/r/oddlysatisfying/comments/9ustsa/ref...

slaw 3y ago

Car and Driver does range test at 75mph. https://www.caranddriver.com/shopping-advice/a32603216/ev-range-explained/ https://www.caranddriver.com/shopping-advice/a32603216/ev-ra...

r00fus 3y ago

Youtube has some videos of highway road tests at 70mph (just search "highway range test EV 70 mph") if you're curious. Actual road tests are nice because they give you a good idea of wind resistance + some elevation variance - which is what you'll likely encounter doing road trips which is when range is most important.