New EV Range Calculator

[gge5]

I've had requests for the calculator that I use to estimate the usable range of an EV. It takes into account aerodynamics and battery chemistries.

EV Range Calculator

The example above is for a lead-acid based bicycle, but it will work for lithium-based large cars and trucks as well.

You might be able to find values for your vehicle's Drag Coefficient and Frontal Area at Wikipedia

 

[SamTexas]

Excellent. Thanks Alan. I'm going to spend some time on it.

I still don't know why the results provided by your calculator is so vastly greater than http://bikecalculator.com/veloUS.html

The differences that are obvious to me is
1) Battery to wheel efficiency. But even 100% efficiency, the difference is still to great.
2) Drag coefficient. I use 0.9 instead of the default 1.0
3) Frontal area of 7sf instead of the default of 8

For a 200lbs bike at 15mph:
Your calculator: 18wh/mi
Bike calculator: 9.1wh/mi

Any idea why?

 

[gge5]

SamTexas,
That bike calculator that you link to seems like it's intended as an exercise guide. It does not take into account battery chemistry for one. Second, the power field is the total power output to the pedals so it has no concept of motor or controller losses.

I could add more complexity to my calculator to make it marginally more accurate, like the wire gauge used and losses due to wire heating. I could also make a lookup table of popular motors and their efficiency curves, but that would be a lot of work for a theoretical outcome.

 

[SamTexas]

Alan,

You are right, that calculator (http://bikecalculator.com/veloUS.html) was never designed for an electrified bike. Its sole purpose is to determine the human power necessary to achieve a certain speed on a bicycle. The results produced by that calculator are very close (within a few %) to numbers claimed by expert bicyclists, as far as I know. e.g. 115 watts for 15 mph, 45 watts for 10 mph.

So when a bicycle is electrified, around 20 to 30 lbs is added to its weight. If we were to assume the worst case of 50% efficiency for "battery to wheel", then the required power for the ebike (without rider assistance) would be just a little over 230 and 90 watts for 15 and 10 mph, respectively. Yet your calculator (and my own personal measurement) shows that the electrified bike consumes even more than that. In other words, the real efficiency is only 30 to 40%.

I'm not criticizing your calculator. In fact, I'm surprised at its relative accuracy to the real world numbers. I'm just wondering where the inefficiency is. I am only concerned about power, not range. So battery chemistry is not a variable. Of course, the premise of all this is that the http://bikecalculator.com/veloUS.html calculator and all expert bicyclists were right in their numbers.

Sam

 

[gge5]

I believe that the inefficiency lies in the electromechanical resistance within the motor and transmission. In the bicycle calculator I put in 200lbs combined weight, MTB tires, and 75% system efficiency. It gave me 13.5 MPH at 160 watts.

Is the 160 watts the amount of energy being consumed by our muscles, or the mechanical energy required at the pedals? I imagine there is some form of losses in converting ATP to muscle movement, just as there is converting chemical potential energies to electricity to electromagnetic work.

The default value of 95% for the transmission efficiency is only the friction losses from the chain and ball bearings, and I don't believe it takes into account the energy losses in our own body. In short, I believe the bike calculator is using idealized equations which ignore how that work is created.

 

[SamTexas]

Is the 160 watts the amount of energy being consumed by our muscles, or the mechanical energy required at the pedals?

I'm relatively confident it's the power required at the pedals.

I imagine there is some form of losses in converting ATP to muscle movement,

Yes. I believe it's around 1:4 ratio. i.e. human consumes about 4wh in order to produce 1wh. Yeah, about the same efficiency as an ICE.

The default value of 95% for the transmission efficiency is only the friction losses from the chain and ball bearings,

Yes. And 95% seems reasonable to me. May 90% is more realistic and more conservative, but definitely nothing lower than that unless the bike is badly maintained.

 

[gge5]

I imagine there is some form of losses in converting ATP to muscle movement,

Yes. I believe it's around 1:4 ratio. i.e. human consumes about 4wh in order to produce 1wh. Yeah, about the same efficiency as an ICE.

I can't see the code behind their calculator, or have a complete understanding of the equations they're using. However, it seems to me that the key to the problem is that you're measuring losses in the conversion of electricity to work, where in a human powered bike those losses are ignored.

So, going with what you're saying:
Human: 4 watt-hours input to 1 watt-hour output. (25% efficiency from ATP to muscles to the pedals)
Motor: 4 watt-hours input to 3 watt-hours output. (75% efficiency through the controller and motor to the drive shaft)

75% / 25% means you should multiply the output of the bike calculator by 3X.

Can that be true that an electric motor is more efficient than a human?

 

[SamTexas]

So, going with what you're saying:
Human: 4 watt-hours input to 1 watt-hour output. (25% efficiency from ATP to muscles to the pedals)
Motor: 4 watt-hours input to 3 watt-hours output. (75% efficiency through the controller and motor to the drive shaft)

75% / 25% means you should multiply the output of the bike calculator by 3X.

Can that be true that an electric motor is more efficient than a human?

Of course if you begin at the energy from the battery and end at the wheel.

But that's not the whole story as far as battery energy is concerned. For example, let's say electricity in your region is produced by burning natural gas. So the various steps are:

- Burn natural gas to produce steam
- Use steam to turn the turbine to generate electricity
- Convert electricity to high voltage for transmission
- Convert electricity to low voltage for household consumption
- Convert from AC to DC to store in battery
- Draw energy from battery to feed the controller
- Finally we are now feeding the electric motor of our ebike

And I'm certain I have missed several steps along the way. All things considered, we'd be lucky if we get 25% of the energy stored in the original natural gas.

Motor: 4 watt-hours input to 3 watt-hours output. (75% efficiency through the controller and motor to the drive shaft)

I would be very pleased at that 75% efficiency. But so far, it appears to be a lot less. Could be as low as 33%.

 

[gge5]

Haha of course we could go back to the processes of farming and food production as well, but the farther back we go the more unknown variables...

I'm just trying to determine the validity of the bicycle calculator, and why there is a discrepancy between it and my calculator.

I guess you could use theirs for exercise, and mine for planning an EV and call it good.

 

[SamTexas]

Actually it takes very little to get to the bottom of this inconsistency, given access to a dyno.

 

[AussieJester]

Be nice if it was in metric

KiM