Go back to Electric Bicycles
Every eBiker faces it ...
- How far can I go?
- How big a battery do I need?
- Can I go further ... faster?
--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 ...
- At 1mph energy efficiency is ~3% = 97% wasted-damaging heat.
- Flat watts in line at low speed is the Amp limit of the controller.
- A lower Amp controller reduces waste at low speed without affecting higher speed performance!
- Best ... methodology?
Pedal assist getting started ... and up to more efficient speed?
Frontal area is not as much a factor as the shape of the frontal area!
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.
- 10mph = 46 miles range
- 15mph = 30 miles range
- 20mph = 20 miles range
- 25mph = 13 miles range
- 30mph = 8 miles range
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.
- 5mph = 22w
- 10mph = 68w
- 15mph = 163w
- 20mph = 333w
- 25mph = 601w
- 30mph = 993w
- 35mph = 1532w
- 40mph = 2247w
- 45mph = 3147w
- 50mph = 4280w
- from ebike.ca simulator
30mph requires almost precisely 300% the energy as 20mph.
Of course, it does not require 3x the energy per mile.
- 163w/15mph = 10.86wh/mile
- 333w/20mph = 16.66wh/mile
- 993w/30mph = 33.1wh/mile
Still 30mph require 2x the energy per mile, or ...
Same ~36V 10Ah battery ...
- 15mph = 30+ mile range or - 92.08 miles per kWh = 6 hour cruise time
- 20mph = 20 mile range or - 60.02 miles per kWh = 3 hour cruise time
- 30mph = 10 mile range or - 30.21 miles per kWh = 1 hour cruise time
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.
- 1. Determine no-load maximum motor speed - digital speedometer on motor wheel, blocked up (EG - 36mph)
- 2. Multiply no-load speed by 40% (EG - 36mph x .40 = 14.4mph)
- 3. Cruise near 14.4mph, Apply full throttle and note watt usage as speed hits-passes 14.4mph (1498w x .50 = 749w 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 )
Surprisingly, this seems fairly consistent among all the simulations I have tried and looks to be a reasonably reliable measure.
- "Reasonably" accurate!
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.
- The 50% efficiency point still remains firmly within the peak watt output region!
I did make note of this ...
(The exception is if the controller is of inadequate amps to allow maximum peak watt output.)
- In my example, the motor shows capable of using >90A at low rpm.
- However, even with current limited to <35A, it is still capable of full Amperage at peak watt output point:
- Limiting current, (controller amps), substantially ... does not limit maximum motor output or top speed!
- Current limiting does reduce waste and heat production in the motors most inefficient region!
- 24V 450w motor
450w @ 50% efficiency 900w/24v = 37.5A controller will provide rated peak 450watts
- 36V 500w motor
500w @ 50% efficiency 1000w/36v = 27.7A controller will provide rated peak 500watts
- 36V 750w motor
750w @ 50% efficiency 1500w/36v = 41.6A controller will provide rated peak 750watts
- 48V 750w motor
750w @ 50% efficiency 1500w/48v = 31.25A controller will provide rated peak 750watts
- Lower amperage controllers will not be capable of providing the motors rated output!
- Higher amperage controllers provide more power in the heat production arena!
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,
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)