EV range v speed
Discussion
Am interested in people's views / understanding of why EV range seems so much more speed dependent that the ICE equivalents. my e-golf will seemingly bimble along forever at 55-60 - I would reckon the range if about 180 miles fully charged, but at 70 I would estimate the range is a good 25% less. At 80, I would estimate it is not far off half - well under 100 miles. Now I know that our diesel Volvo mpg is also speed sensitive but nothing like as much. Is it simply a function of the lack of gearing in the EV or something else that I am missing?
Thanks in advance for enlightening the EV novice.
Thanks in advance for enlightening the EV novice.
The reason is simply because ICE's are HORRENDOUSLY in-efficient at all speeds!
You may see this as "my ICE doesn't get that much worse as it goes faster" but that's human nature...... ;-)
For example a typical ICE vehicle driving at 35 mph is using the same amount energy just to spin the engine around as that used to actually push the entire car down the road. With an EV, as speed falls, consumption falls pretty proportionally. And of course, as aero drag goes up with the cube of speed, what looks to be minor increases in average speed start to really increase the consumption!
(note: efficiency != consumption, even with a mythical 100% efficient system, if more power is required, then more "fuel" must be consumed, be that a liquid hydrocarbon fuel or energy stored in a chemical battery)
What you need to remember also is how much energy is stored in a fuel tank, and how wasteful your ICE actual is.
Take a fairly typical ICE with a 55 litre fuel tank. Gasoline has a specific energy density of 34 MJ per litre. That probably doesn't mean much to you, but that means a 55 litre tank contains 1.87 GJ, which is the equivalent of a battery that can hold 520 kWhr of electricity. For reference, the Tesla P100D, which has the largest battery yet fitted to an EV, has a 100 kWhr battery, or 1/5th of that normal fuel tank ( roughly 11 litres, about what you'd find in a motorbike fuel tank)
My little battery i3, has 18 kWhr that are useable, the same as just 2, yes 2, liters of fuel, and it can, under ideal conditions, go about 100 miles on that!
You may see this as "my ICE doesn't get that much worse as it goes faster" but that's human nature...... ;-)
For example a typical ICE vehicle driving at 35 mph is using the same amount energy just to spin the engine around as that used to actually push the entire car down the road. With an EV, as speed falls, consumption falls pretty proportionally. And of course, as aero drag goes up with the cube of speed, what looks to be minor increases in average speed start to really increase the consumption!
(note: efficiency != consumption, even with a mythical 100% efficient system, if more power is required, then more "fuel" must be consumed, be that a liquid hydrocarbon fuel or energy stored in a chemical battery)
What you need to remember also is how much energy is stored in a fuel tank, and how wasteful your ICE actual is.
Take a fairly typical ICE with a 55 litre fuel tank. Gasoline has a specific energy density of 34 MJ per litre. That probably doesn't mean much to you, but that means a 55 litre tank contains 1.87 GJ, which is the equivalent of a battery that can hold 520 kWhr of electricity. For reference, the Tesla P100D, which has the largest battery yet fitted to an EV, has a 100 kWhr battery, or 1/5th of that normal fuel tank ( roughly 11 litres, about what you'd find in a motorbike fuel tank)
My little battery i3, has 18 kWhr that are useable, the same as just 2, yes 2, liters of fuel, and it can, under ideal conditions, go about 100 miles on that!
Edited by anonymous-user on Monday 27th August 20:07
Edited by anonymous-user on Monday 27th August 20:09
Max has pretty much nailed it.
Wind resistance increases non linearly with speed. And your traditional engine is not particually thermally efficient (if it’s running at 40% it’s doing well)
Very large two stroke marine engines in container ships are probably about as efficient as reciprocating engines get.
Wind resistance increases non linearly with speed. And your traditional engine is not particually thermally efficient (if it’s running at 40% it’s doing well)
Very large two stroke marine engines in container ships are probably about as efficient as reciprocating engines get.
Max_Torque said:
Take a fairly typical ICE with a 55 litre fuel tank. Gasoline has a specific energy density of 34 MJ per litre. That probably doesn't mean much to you, but that means a 55 litre tank contains 1.87 GJ, which is the equivalent of a battery that can hold 520 kWhr of electricity. For reference, the Tesla P100D, which has the largest battery yet fitted to an EV, has a 100 kWhr battery, or 1/5th of that normal fuel tank ( roughly 11 litres, about what you'd find in a motorbike fuel tank)
That's a neat calculation Max. I guess it also infers we probably have a long way to go at improving battery chemistry to gain better energy density.jjwilde said:
EddieSteadyGo said:
That's a neat calculation Max. I guess it also infers we probably have a long way to go at improving battery chemistry to gain better energy density.
Well remember that EVs are ridiculously more efficient so the 'gap' isn't as big as it sounds. I wonder how far battery energy densities will improve over the next 20 years....
williaa68 said:
Brilliant (and fascinating) - thank you very much. So if I am going a long way them bimbling at 60 really is the way to go!
in fact, for an EV, the slower you go, the better, in terms of range, right down to about 10 mph, when the hotel loads of running the car (electrics)) start to dominate. On my commute, which is roughly 30miles of B road, when i get stuck behind some muppet who can't exceed 30mph, my i3 does over 6 miles per Kwhr, where as, if i just floor it and do 60 (or more....) it drops down to around 4 miles per kWhr. The thing that really hurts it, is to drive hard enough to start using the friction brakes, rather than doing all the slowing with regen, at that point, you are losing energy that isn't recoverablejjwilde said:
EddieSteadyGo said:
That's a neat calculation Max. I guess it also infers we probably have a long way to go at improving battery chemistry to gain better energy density.
Well remember that EVs are ridiculously more efficient so the 'gap' isn't as big as it sounds. Current battery chemistry uses "heavy" elements, so fundamentally, if we can find a way to make a battery our of light ones (hydrogen etc) then even using a similar electrochemistry, the specific energy density (kWhr/Kg) can be improved by a maximum of around 3 to 4. However, before that point the volumetric density starts to be a concern, and it'll take a different physical architecture to deliver a higher kWhr/m3 to enable that bigger battery to be packaged.
EddieSteadyGo said:
True, but it's an interesting fact nevertheless. I just checked (for curiosity) the energy density of human fat which is about 37 MJ/kg. If I've got my maths right, a good current lithium ion battery seems to be around 0.5 MJ/kg. So it puts into context how beautifully efficient nature is at storing energy.
I wonder how far battery energy densities will improve over the next 20 years....
To be fair we can have far more energy dense storage but it wouldn't give us the power we need or the fast charging I wonder how far battery energy densities will improve over the next 20 years....
RobDickinson said:
To be fair we can have far more energy dense storage but it wouldn't give us the power we need or the fast charging
We currently have people claiming thousands for whiplash after innocuous shunts. What could go wrong?
https://www.damninteresting.com/the-atomic-automob...
With my business I have three Leafs and an eNV200.
The Leafs (all 24kW ones, two with 8 bars and one with 10), generally use about 10% less power than the van, even though I drive the van like a nun.
High speed is the killer - the difference in the range can be up to 50%.
Round town the range can be 1.5k per 1% battery charge, open road at 60 (mph) is 1k per 1%.
I'm getting used to driving slow these days, for short journeys the time saved by driving/accelerating fast makes very little difference.
I do around 1000 miles a month in the van, savings (depends how you work things out) are at least 6p a mile over dinosaur technology.
It makes you realize just how inefficient all internal combustion engines are, I'll never go back, doubt I will ever buy another ICE vehicle.
caziques said:
With my business I have three Leafs and an eNV200.
The Leafs (all 24kW ones, two with 8 bars and one with 10), generally use about 10% less power than the van, even though I drive the van like a nun.
High speed is the killer - the difference in the range can be up to 50%.
Round town the range can be 1.5k per 1% battery charge, open road at 60 (mph) is 1k per 1%.
I'm getting used to driving slow these days, for short journeys the time saved by driving/accelerating fast makes very little difference.
I do around 1000 miles a month in the van, savings (depends how you work things out) are at least 6p a mile over dinosaur technology.
It makes you realize just how inefficient all internal combustion engines are, I'll never go back, doubt I will ever buy another ICE vehicle.
Of course what has been posted in this thread is basic and well known stuff. Everybody gets that an EV, at point if use, is more efficient than an ICE. The Leafs (all 24kW ones, two with 8 bars and one with 10), generally use about 10% less power than the van, even though I drive the van like a nun.
High speed is the killer - the difference in the range can be up to 50%.
Round town the range can be 1.5k per 1% battery charge, open road at 60 (mph) is 1k per 1%.
I'm getting used to driving slow these days, for short journeys the time saved by driving/accelerating fast makes very little difference.
I do around 1000 miles a month in the van, savings (depends how you work things out) are at least 6p a mile over dinosaur technology.
It makes you realize just how inefficient all internal combustion engines are, I'll never go back, doubt I will ever buy another ICE vehicle.
but when you do your sums remember that the ‘dinosaur’ technology is much more highly taxed, at the moment, than EVs.
Your savings won’t slways be that significant, the tax take on ICE fuel will need to be replaced.
I’d bet that, in 10/15 years the proportionate cost of running your vehicles in your business will be little different to what it always was.
REALIST123 said:
I’d bet that, in 10/15 years the proportionate cost of running your vehicles in your business will be little different to what it always was.
And you think you are being insightful? 
Of course the balance of tax will switch to EVs as they become the main form of transportation. But not only will the cars be better, they won't be chucking out loads of toxic fumes which people in the city currently have to breathe in.
I guess one shouldn't under-estimate the ingenuity of politicians when it comes to taxes but taxing the "fuel" for EVs strikes me as inherently difficult, given it is the same stuff that people use for cooking, heating, lighting etc and at the moment is massively concessionally taxed (5% VAT not 20%). If politicians try and tax the electricity that goes into cars, that will alter the dynamics of things like PVs and storage.
I imagine the impact on the grid overall is far more significant and I am sure that over time we will see some attempts to regulate or restrict charging hours,
I imagine the impact on the grid overall is far more significant and I am sure that over time we will see some attempts to regulate or restrict charging hours,
williaa68 said:
I guess one shouldn't under-estimate the ingenuity of politicians when it comes to taxes but taxing the "fuel" for EVs strikes me as inherently difficult, given it is the same stuff that people use for cooking, heating, lighting etc and at the moment is massively concessionally taxed (5% VAT not 20%). If politicians try and tax the electricity that goes into cars, that will alter the dynamics of things like PVs and storage.
I imagine the impact on the grid overall is far more significant and I am sure that over time we will see some attempts to regulate or restrict charging hours,
I suspect the preferred method of taxing will switch over from fuel taxing to some form of road pricing. I imagine the impact on the grid overall is far more significant and I am sure that over time we will see some attempts to regulate or restrict charging hours,
williaa68 said:
I guess one shouldn't under-estimate the ingenuity of politicians when it comes to taxes but taxing the "fuel" for EVs strikes me as inherently difficult, given it is the same stuff that people use for cooking, heating, lighting etc and at the moment is massively concessionally taxed (5% VAT not 20%). If politicians try and tax the electricity that goes into cars, that will alter the dynamics of things like PVs and storage.
Have you installed a free smart meter yet? Ever wondered why the government are pushing them?
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