Speak EV - Electric Car Forums banner

21 - 40 of 58 Posts

·
Super Moderator
Joined
·
7,455 Posts
Why use such a counter intuitive measure as power/ mile (or km) ??

Surely we want to know how much distance we can cover per KWh as we know the size of battery ?

In other words if my 64KWh battery is 50% full I have 32kwh. If I know I do 4 m per KWH I have a range of 128m - can do that in my head easy.

But if I know I do 13.82kwh m/ 100km - how on earth does that help me?

First I have to convert the battery percentage to KWh (easy and same as other calculation) so using the 50% thing as above I have 32kwh in my battery.

But then I have to calculate 32 / 13.2Kwh (less easy) then multiply that through to another figure to get a range - totally counter intuitive.

Never understood why some people use such a weird way of measuring it.
4mpkwh compared to 6mpkwh is a difference of two. 2mpkwh compared to 4mpkwh is a difference of two. But the difference in consumption between 6 and 4 is an increase of 50%, and between 4 and 2 it’s 100%. Whereas the difference between 2kwh/100km and 4kwh/100km, and between 20kwh/100km 22kwh/100km is the same.

the miles per “thing” makes you focus on increasingly less important numbers, a car dropping from 10mpkwh to 9mpkwh is unimportant, but going from 3mpkwh to 2mpkwh is catastrophic.
 
  • Like
Reactions: Todor

·
Registered
Kia Soul EV 2020
Joined
·
2,503 Posts
Why use such a counter intuitive measure as power/ mile (or km) ??

Surely we want to know how much distance we can cover per KWh as we know the size of battery ?

In other words if my 64KWh battery is 50% full I have 32kwh. If I know I do 4 m per KWH I have a range of 128m - can do that in my head easy.

But if I know I do 13.82kwh m/ 100km - how on earth does that help me?

First I have to convert the battery percentage to KWh (easy and same as other calculation) so using the 50% thing as above I have 32kwh in my battery.

But then I have to calculate 32 / 13.2Kwh (less easy) then multiply that through to another figure to get a range - totally counter intuitive.

Never understood why some people use such a weird way of measuring it.
 

·
Registered
Kia Soul EV 2020
Joined
·
2,503 Posts
In other words if my 64KWh battery is 50% full I have 32kwh. If I know I do 4 m per KWH I have a range of 128m - can do that in my head easy.

But if I know I do 13.82kwh m/ 100km - how on earth does that help me?

First I have to convert the battery percentage to KWh (easy and same as other calculation) so using the 50% thing as above I have 32kwh in my battery.

But then I have to calculate 32 / 13.2Kwh (less easy) then multiply that through to another figure to get a range - totally counter intuitive.
Sorry to bring this one but your calculations here are very flawed. It is clear that you have taken steps to make the m/kWh calculations easy, such as rounding the numbers. I will give you a couple of options:
  • #1: do the same conversion but this time use 3.68mi/kWh? How did you do?
  • #2: now lets simplify the kWh/100km conversion to the same level. I like 10kWh/100km value. And I'm even going to use 32000Wh/100Wh/km. Wow, even I can do that in my head: 320km.
  • #2.1 Or if you don't like km, let's work with 320Wh/mile. Your range at 50% is then 32000Wh / 320Wh/mile = 100miles.
 

·
Registered
Hyundai Ioniq 28
Joined
·
7,702 Posts
I have always worked from a basic initial calculation of how many miles per % of battery the car will normally drive on average. Then round down a bit. For instance, my Leaf 24 was good for 0.8 of a mile per % using that metric. Or 8 miles per 10% for ease of working. Then a glance at the % remaining and a quick and easy mental calc gives a rough enough indication of the remaining range.

34% = approx 25 miles
66% = approx 50 miles
12% = look for a charger within the next 5 miles.

In my classic Ioniq it is much easier as I can use 1 mile per % - or 10 miles per 10%. And that gives a good margin of safety in all cases.

62% =62 miles etc

All very basic info but often better than the GOM.

Nothing to do with economy of course as a larger battery gives more miles per % but still a useful 'tool' to compare with the GOM if range is a critical factor at that time.

On the subject of the selection of a measure for economy, I have always used the miles /kWh and intuitively know the difference between a poor figure of say 2.5 against an excellent one of 5.2. And as long as the figure displayed is within my normal experience for the weather and conditions it becomes just another cross check that drivers used to do such as scanning the dash to see that the ammeter is showing positive, that the oil pressure is in range, temperature OK, and fuel level fine. Interesting to think back to how many times I used to carry out such a visual scan during a drive.
 

·
Registered
Joined
·
55 Posts
Never understood why some people use such a weird way of measuring it.
Smoke and mirrors? I've always assumed it's the same reason we use MPG in a country which sells fuel per L and rates prices in £/L. Very difficult to see just how much rock juice we're putting in our machines and how that equates to distance travelled.
 

·
Registered
Joined
·
616 Posts
Why use such a counter intuitive measure as power/ mile (or km) ??

Surely we want to know how much distance we can cover per KWh as we know the size of battery ?

In other words if my 64KWh battery is 50% full I have 32kwh. If I know I do 4 m per KWH I have a range of 128m - can do that in my head easy.

But if I know I do 13.82kwh m/ 100km - how on earth does that help me?

First I have to convert the battery percentage to KWh (easy and same as other calculation) so using the 50% thing as above I have 32kwh in my battery.

But then I have to calculate 32 / 13.2Kwh (less easy) then multiply that through to another figure to get a range - totally counter intuitive.

Never understood why some people use such a weird way of measuring it.
It's an odd one. If you really think about it, mpg or m/kWh is actually the "weird" way to measure it. As an analogy, prices in supermarkets are in £/kg, i.e. currency per commodity. This allows you to think "OK it's £2/kg and I want 2kg, that means it's going to cost me £4". Imagine if prices in supermarkets were the other way around, kg/£. That would be weird.

Similarly, miles is the commodity you want and kWh is the currency you have. Knowing kWh/mi lets you know how much it's going to "cost" you to go a particular distance. For example, I want to go 50 miles and my efficiency is 25 kWh/100mi, thus I'll need 12.5 kWh, or ~20%.

However, the car never tells you how many kWh you have left so either way it's unnecessary complicated to calculate. That's why we have a GOM.
 

·
Registered
Joined
·
872 Posts
I see what you are saying, but I don’t want to know that I don’t think.

Within reason, I don’t give a monkeys “how many KWh” my 80mile trip will “cost” me while I am doing it. I want to know, how far I can get either “with what I have left” for a particular trip or “with a full battery” for general musings.

So if I have 130m to go and a 50% battery, I don’t give a stuff about anything other than “will I get there” and will I have some range to spare... so the kw/m measurement doesn’t give me that - the m/KWh does provides I can make a sensible guess at what my KWh is - which I can, cos I work on 10% being 6kwh (gives me 4kw at the end as “spare” so if my battery says 23% I can work out in my head 2.3 x 6 = 13.8 and if I work on 4 m per kw then I have 4 x 13 = 52m (and a tiny reserve) - so if I have 40m to go i’m fine.

I couldn’t do that with a KWh/m calc.

“Range” is the issue with EV’s not “cost in KWh” of getting there - I know it’s the same thing but you have to then add more calculations in to get a sensible result
 

·
Registered
Joined
·
616 Posts
So if I have 130m to go and a 50% battery, I don’t give a stuff about anything other than “will I get there” and will I have some range to spare... so the kw/m measurement doesn’t give me that
Hmm I think it does. If you assume 50% battery means 32 kWh, you're looking for anything under 32/130 kWh/mi, or anything over 130/32 mi/kWh.

Either way involves some mental maths and estimation/rounding, I'm not sure you can really argue that one is useful and the other is not. Plus as the video posted above shows, kWh/mi (or variants thereof) make comparing efficencies much more sensible, be it between journies or between vehicles.
 

·
Registered
Hyundai Ioniq 28
Joined
·
7,702 Posts
I want to know, how far I can get either “with what I have left” for a particular trip or “with a full battery” for general musings.

Range” is the issue with EV’s not “cost in KWh” of getting there...........
I agree. But I tend to use the indicated battery % left as my main baseline. I always work out roughly the miles per % of battery on average and round down to create a margin. Then it is a simple calc to multiply that miles number by the % remaining.

In a Soul, being conservative, that would be about 2 miles per %. Assuming a safe mileage of 200 in all conditions.

So that a % showing in the car in your example above of 23% would calculate as 23 x 2 = 46 miles available. Your own figure of 52 miles is probably nearer but my method is more conservative, as well as being childishly simple to calculate.
 

·
Registered
Hyundai Ioniq 28
Joined
·
7,702 Posts
So we're having this discussion about what's the best way of calculating a number that is in effect supplied by the GOM?
Not really. Just a quick way of cross-checking the unreliable GOM. It's called the Guess-o-meter for a good reason. If a mental arithmetic check happens to confirm a GOM reading in a critical range situation then it has served its purpose and the driver can breathe again.
 

·
Registered
Joined
·
872 Posts
I agree. But I tend to use the indicated battery % left as my main baseline. I always work out roughly the miles per % of battery on average and round down to create a margin. Then it is a simple calc to multiply that miles number by the % remaining.

In a Soul, being conservative, that would be about 2 miles per %. Assuming a safe mileage of 200 in all conditions.

So that a % showing in the car in your example above of 23% would calculate as 23 x 2 = 46 miles available. Your own figure of 52 miles is probably nearer but my method is more conservative, as well as being childishly simple to calculate.
I really like this - it’s often all a guess anyhow.

Yes, 2m per 1% works in winter, 2.5 in summer would also work - I will use this, so super simple
 

·
Registered
Joined
·
34 Posts
Not really. Just a quick way of cross-checking the unreliable GOM. It's called the Guess-o-meter for a good reason. If a mental arithmetic check happens to confirm a GOM reading in a critical range situation then it has served its purpose and the driver can breathe again.
In your opinion. Does the GOM fail on estimating, as you call it unreliable?
It surprises me a bit that your intuitive and simple way of estimating would outperform the computing power and algorithms used by the manufacturers in the GOM.
 

·
Registered
E-Niro 64kWh '4' since 9/20 (was Prius)
Joined
·
865 Posts
I really don’t get all this trying to predict the range better than what the the GOM can. Trust the GOM, but remember it’s a prediction range to zero SOC. Nobody wants to go anywhere near zero because the car will stop (and the available acceleration power will seriously reduce too). So take the GOM value and deduct ~20%. That is how far you can probably safely go without recharging (with a bit of safety reserve).

Basing things on the indicated percentage SOC is not good anyway as I believe it is nonlinear. 5% near the top can go further than 5% near the bottom.
All IMHO of course.
Peter
 

·
Registered
Joined
·
91 Posts
Why use such a counter intuitive measure as power/ mile (or km) ??

Surely we want to know how much distance we can cover per KWh as we know the size of battery ?

In other words if my 64KWh battery is 50% full I have 32kwh. If I know I do 4 m per KWH I have a range of 128m - can do that in my head easy.

But if I know I do 13.82kwh m/ 100km - how on earth does that help me?

First I have to convert the battery percentage to KWh (easy and same as other calculation) so using the 50% thing as above I have 32kwh in my battery.

But then I have to calculate 32 / 13.2Kwh (less easy) then multiply that through to another figure to get a range - totally counter intuitive.

Never understood why some people use such a weird way of measuring it.
That will be a hang over from the ICE metric: litres/100km. Since you can't really equate litres to kWh it seems pretty pointless as you say.
 

·
Registered
Joined
·
872 Posts
I even see that as “the wrong way round” but it’s not that important.


The reason I like m/KWh is its simple reference point - I feel if I’ve done 4+ I have done “well” and under 4 not so well on a trip. 3.9 in spring or autumn if it’s chilly and 3.8 in the winter.

Just gives me a feel for how well (economically) I have driven.... or if I drove it like I stole it, how much worse it was (often surprisingly little!)
 

·
Registered
Joined
·
872 Posts
Just think he is wrong there.... I want to know how many miles I can get for each gallon ( I should use litres I know, but gallons is what we are all used to) liters / mile has no context in my brain at all, albeit I know they are mathematically related.
 

·
Registered
Joined
·
616 Posts
I even see that as “the wrong way round” but it’s not that important.


The reason I like m/KWh is its simple reference point - I feel if I’ve done 4+ I have done “well” and under 4 not so well on a trip. 3.9 in spring or autumn if it’s chilly and 3.8 in the winter.

Just gives me a feel for how well (economically) I have driven.... or if I drove it like I stole it, how much worse it was (often surprisingly little!)
You could do exactly the same the other way around. Under 0.25 kWh/mi (or 25 kWh/100mi) is "good".

It sounds like you just have a preference for the system you know, which is fine. Not much point coming up with not-very-strong justifications for that opinion. :p

At the end of the day it doesn't matter because, as far as I know, you can't change the way the car reports efficiency without shifting from miles to kilometers.
 

·
Registered
Hyundai Ioniq 28
Joined
·
7,702 Posts
It surprises me a bit that your intuitive and simple way of estimating would outperform the computing power and algorithms used by the manufacturers in the GOM.
That's probably because you haven't fully thought through the way the GOM 'guesses' the figure it displays. It is not Mystic Meg. It is Capt Hindsight. It reports the figure based on its experience of the last journey. And assumes that the next journey, and road and weather conditions, will be identical to the last time the car was used. So that if one driver is lightfooted, the day was warm and dry, and they drive a sedate route on level ground in a rural area, with low-speed limits, then they would have shown excellent economy figures. The GOM, therefore, shows the same efficiency figure multiplied by the SOC. It's a guess based on previous data.

Yesterday I drove 40 miles in my classic Ioniq 28. It was a warm day. The route was level and low speed. The car showed figures of 5.1 miles per kWh for that trip. I had a much longer journey next day so filled the car overnight. This morning it was showing 143 miles on the GOM. How did it know that today would be just as warm? That it wouldn't be raining hard. And that today's trip would be identical to yesterday?

Obviously, it couldn't know that. So it showed me 143 miles. However, today I would be driving high into the Pennines after 50 miles of motorway. My own method of checking the GOM figure showed the battery at 100%. And at my 1 miles per % that gave me 'safe' miles of 100. Or on today's journey an average of around 3.5 miles per kWh. Now the GOM may well have adjusted itself well into today's journey. But at the start, I really need to know the true range so that if necessary a charge stop can be planned. And if I had trusted the 143 at the start it could have caused problems.

This is why I check the % figure showing in the car and plan on a worst case scenario of 1 mile per % whatever the GOM tells me. If the trip is well inside this figure there is no drama. If the journey matches the % figure it's probably still OK. If the planned trip is further than the % showing I can plan a stop or drive slower, or take a calculated risk based on the knowledge that my % estimate is mega cautious anyway. But I have real information rather than guesswork.

Don't let me discourage you from trusting the GOM. Just be aware of how it bases its guesswork and you won't be disappointed when it gets it wrong sometimes. It really isn't the sophisticated algorithm by a supercomputer that you attribute to an OEM. It's actually quite stupid if you think about it.
 

·
Registered
Joined
·
34 Posts
That's probably because you haven't fully thought through the way the GOM 'guesses' the figure it displays. It is not Mystic Meg. It is Capt Hindsight. It reports the figure based on its experience of the last journey. And assumes that the next journey, and road and weather conditions, will be identical to the last time the car was used. So that if one driver is lightfooted, the day was warm and dry, and they drive a sedate route on level ground in a rural area, with low-speed limits, then they would have shown excellent economy figures. The GOM, therefore, shows the same efficiency figure multiplied by the SOC. It's a guess based on previous data.

Yesterday I drove 40 miles in my classic Ioniq 28. It was a warm day. The route was level and low speed. The car showed figures of 5.1 miles per kWh for that trip. I had a much longer journey next day so filled the car overnight. This morning it was showing 143 miles on the GOM. How did it know that today would be just as warm? That it wouldn't be raining hard. And that today's trip would be identical to yesterday?

Obviously, it couldn't know that. So it showed me 143 miles. However, today I would be driving high into the Pennines after 50 miles of motorway. My own method of checking the GOM figure showed the battery at 100%. And at my 1 miles per % that gave me 'safe' miles of 100. Or on today's journey an average of around 3.5 miles per kWh. Now the GOM may well have adjusted itself well into today's journey. But at the start, I really need to know the true range so that if necessary a charge stop can be planned. And if I had trusted the 143 at the start it could have caused problems.

This is why I check the % figure showing in the car and plan on a worst case scenario of 1 mile per % whatever the GOM tells me. If the trip is well inside this figure there is no drama. If the journey matches the % figure it's probably still OK. If the planned trip is further than the % showing I can plan a stop or drive slower, or take a calculated risk based on the knowledge that my % estimate is mega cautious anyway. But I have real information rather than guesswork.

Don't let me discourage you from trusting the GOM. Just be aware of how it bases its guesswork and you won't be disappointed when it gets it wrong sometimes. It really isn't the sophisticated algorithm by a supercomputer that you attribute to an OEM. It's actually quite stupid if you think about it.
Do you as a matter of fact that the GOM doesn't include some of the parameters you mention? The car knows about temperature at least, so I presume that KIA's engineers have included that parameter in the calculation. Furthermore, if you change driving style I presumed that in a matter of some time the GOM would update based on that.

Probably you're right, but driving around in a super-efficient marvel of technology, that still it can't do an estimation better than a human, that's based on parameters that it has at least some access to.
 
21 - 40 of 58 Posts
Top