EBike Efficiency

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Efficiency

Every eBiker faces it ...

etc.

--Drkangel 14:37, 13 February 2014 (UTC)

Electric Watts in vs Power Watts Out

Thought I'd graph a direct comparison of the electrical watts supplied into a motor and the actual motor output (watts) power.

Based on a 750w motor at full throttle ...

Watts in vs Watts out.jpg

Pedal assist getting started ... and up to more efficient speed?

Aerodynamics

Bicycle type, (largely aerodynamics), determines the amount of power to acquire various speeds. Watts = Speed HP.jpg

Frontal area is not as much a factor as the shape of the frontal area!

AeroShapes.JPG

See - Aerodynamic Factors

Speed vs Range

Everyone likes more speed! Well ... most everyone.

What most don't realize is the cost of more speed. Pulled from the ebikes.ca simulator ...

I noted the various ranges supplied at different speeds.

Generic Mountain bike - motor only. 665w peak output motor w/48V 10ah battery = similar to a 24V 450w peak output eZip motor - pushed to 36V 675w peak output motor.

Anyway

Wow! ... calculator - click click click ... Every 5mph increase in speed decreases range by 33%!

Most are shocked at the affect wind resistance plays. Wind resistance is the major factor, but road load, tires, drive train etc. also contribute.

A differing summation: Watts required for a mountain bike to maintain various speeds.

30mph requires almost precisely 300% the energy as 20mph.

Of course, it does not require 3x the energy per mile.

Still 30mph require 2x the energy per mile, or ...

Same ~36V 10Ah battery ...

See - Speed vs Range

Rating By Watts Input Or Watts Output

Sadly ... It seem a goodly percentage of members still rate their systems (motor-controller-battery) by the watt input rather than the motor output watts.

The same 1500w input could output anywhere between 50w- and 1300w+ usable ...

Well ... some claim that their "system" can run 1500w continuous input without overheating, and that is why they claim 1500w.

What actually happens is that, at a maintainable speed, level travel, consistent road surface and wind, watt input is 1500w, with no overheat. So they are likely cruising near peak efficiency ... possibly 80% efficient. That would be 1200w usable and 300w damaging heat. The same 1500w input at slower speed, up a hill?, might output 500w usable power and 1000w damaging heat. To keep the motor from overheating they would have to reduce heat to ~300w or - by reducing watt input to 1/3rd?

So their "1500w" is now a 500w ... by their same reasoning ... ?

The only way to accurately rate their motor system, using their reasoning, would be to rate the motor by its continuous heat dissipation potential ... 300w?

Determining Peak Motor Output - Simple-Cheap Method

Observing the ebike.ca\simulator, I notice a consistent ~50% relation of input watts to watts output occurring at maximum-peak motor watt output, at ~40% of no load motor speed. Using this, it is possible to determine peak motor output.

(Precise 50% of input occurs at 38.6% of no-load speed- 13.9mph (this EG) ... but 40% is a fairly close, easy to figure number )

40% of no-load speed intersects 50% efficiency near the center of peak watt output nicely! 50% input at 40% no-load speed.jpg

Surprisingly, this seems fairly consistent among all the simulations I have tried and looks to be a reasonably reliable measure.

Not precise but accurate within a few percentage.

(The exception is if the controller is of inadequate amps to allow maximum peak watt output. Update - I just sampled various-more motor types and found a variance of ~ 40% to 50% (for some Clyte motors) of no load speed as the peak watt output and 50% efficient point.


Current Limiting?

I did make note of this ...

(The exception is if the controller is of inadequate amps to allow maximum peak watt output.)


Current Limited.jpg

Please Note!

450w @ 50% efficiency 900w/24v = 37.5A controller will provide rated peak 450watts

500w @ 50% efficiency 1000w/36v = 27.7A controller will provide rated peak 500watts

750w @ 50% efficiency 1500w/36v = 41.6A controller will provide rated peak 750watts

750w @ 50% efficiency 1500w/48v = 31.25A controller will provide rated peak 750watts


Input - Output Watt Comparison 50A vs 30A 48V controller (54.6V full charge) Watts - 50A vs 30A.jpg

Tuning controller to optimal might be possible using shunt mods

Attack Them Hills!

For climbing hills,

It is important to keep speed-rpm above 40-50% of no-load motor speed! (Percentage dependent on motor design.)

This is the point of maximum watt output and 50%+ efficiency.

Above this point efficiency increases, with a diminishing output - Below this point output efficiency decreases and output diminishes - quickly!


Attack the Hill!

Get a good run at it, faster is better!

Pedal assist at a moderate strength, so as to maintain higher speed-efficiency for as long as possible.

As, speed diminishes towards maximum watt output, increase your pedal assist effort.

The object is to keep motor rpm-speed as high as possible,

more importantly,

to maintain above the maximum watt output point! (Effort might need to be rationed, dependent of length of hill.)

From top "attack speed", efficiency decreases with speed loss, but motor output increases.

It is important to maintain above the 40-50% of no load speed... because ...

Below this maximum motor output, efficiency decreases and motor output decreases as well!

A double whammy!


If maintainable speed drops noticeably below this efficiency point, due to heat production, it becomes important to reduce throttle-watts input.

Sadly, while doing this decreases heat production it also decreases motor output, putting you in a downward spiral of speed loss and heat production ...


So ... Attack them hills! --Drkangel 14:49, 13 February 2014 (UTC)

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