Tuesday, June 15, 2010

Thoughts about eBike Range Measurement

As I've been testing new ebikes, and planning more tests in the future, I've been trying to think of the best gauge of range. It's a common question among people shopping for ebikes, and it's one of most often asked of me by the e-curious. Usually, battery capacity is measured in amp-hours; I've also seen measurements in watt-hours and "up to" mile ranges. Amp-hour is a fairly straightforward metric, but it's mile range I've been thinking about; after all I don't want a 10ah battery, I want an effective range. A range estimate usually includes caveats such as "if you pedal as well, and work more at starts and stops, you could get up to XX miles out of a charge". If I never actually turned the motor on, I would get an infinite range from a single charge. But that's not why we ride ebikes, is it?

My current thinking is that the best measure of range is a "worst case scenario" (hereafter WCS). Put another way, if I do as little work as possible and use the battery as much as possible, what's the lowest range I can expect?

Upper distance measurements proffered by ebike manufacturers often have a lot of caveats (flat route, 150lb rider, full tires). Instead of applying these caveats in order to get to that upper range, I like to think of it as applying the caveats to extend the WCS range.

Finally, don't forget, if you ride an ebike, you'll eventually ride on a dead battery. The best way to prevent this is ABC - "Always Be Charging". Your battery will thank you!

And finally-finally, what's your assist type (pedal assist only, pedal assist/throttle combo, throttle only, other), voltage, amp-hours, and WCS scenario range? My primary ride is throttle/36v/10ah, and I get about 18 miles, WCS. Throw your answer in comments, please!

Shortly after publishing this, I found an iPhone app "Ebike Calculator" that claims to calculate ebike range. Sadly, it's for a later version of the iPhone software than I have, so I can't attest to its accuracy. Anyone have a newer iPhone want to download it and check it out? It's free!


  1. Just to reiterate, my primary ebike is a throttle-only with 36v 10ah. I get 18 miles, WCS.

  2. as for range, i usually give people a range of range. on my pheonix racer ebike, i tell people that "balls out" it will do 12 miles and with "some peddling" with speeds around 20 mph, range is 20 miles. Hills kill range regardless.

  3. Justin, of ebikes.ca uses the formula of 10-13 watt hours per mile for light motor use, 14-20 for average throttle use and 21-32 for all out huge motor applications.

    So, to figure out how many watt hours you have, multiply your battery's voltage by its amp hours. eg, a 36v battery with 10ah has 360 watt hours, so it looks like for full throttle, your 12 mile range means that you are burning the upper end of that range at 30 watt hours per mile.

  4. You really need on of Justin's (ebikes.ca) Cycle Analyst units, or something similar to do a proper measurement. Measuring the rider weight, distance traveled, amp hours used, and average speed attained will give you a more accurate "WCS".

    Most E-Bike controllers use a simple PID ( http://en.wikipedia.org/wiki/PID_controller ) that always uses the maximum current to reach the desired speed, as set by the throttle. Only a "smart" controller will allow you to "dial back" that algorithm to trade off range and performance. There are "soft start" controllers, but usually that only applies to the initial motor spin up.

    BTW, are you using Lithium or SLA?

    ;-) Tom

  5. Thanks for the comment Tom! I'm running a Ping lithium pack - a very nice upgrade from SLA's.

    My thinking with the WCS range concept is that it's a quick, easy, and mathless way to estimate functional range. I find that eyes glaze over when the math comes out, at least outside the hardcore enthusiast range. It's kind of like the old adage "I'm not buying a drill, I'm buying holes." I'm not buying amp-hours, I'm buying miles.

    (And hopefully, at least for me, rider weight will be mostly constant over the range of my testing!)

  6. This is great, but does someone know the relation between speed and range? I believe 36V and 10Ah might get you 18miles WCS if you have a maximum speed 20mph, but at 30mph what would the range be?

  7. Air resistance increases at a more-than-linear rate with increase in speed, so I'd expect it would significantly adversely affect range. That could have been part of the extended range in pedelec-only mode on the Pathfinder tested recently - I was averaging a couple miles an hour lower in pedelec mode than when in throttle mode(say 17mph in pedelec, 20 in throttle). I don't think that accounts for all of the difference I observed, though.

  8. Is there a calculator somewhere on the Internet, that would give me the the wattage needed based on all or most of the variables?

    Say: 20mph, 2% uphill, 200lbs me + bike, no wind, no assistance = at least 400W motor/controller/battery combo.

  9. Found it! It's not for ebikes, but it does calculate HP/Watts necessary.


  10. That is a comprehensive calculator, Emil, thanks for the link. If you apply it to an ebike design, let us know what your experience is!

  11. I tried out the iPhone app, by the way -- it's really stupid and misleading. No weight, grade, or even speed inputs. It's basically an ad for some e-bike shop that tries to show how your bike is inferior to their BionX setups.

  12. nice blog.. check this out ebike 48v 15ah battery, affordable and very reliable batteries can be found here

  13. You can always use the calculator which is in my site, it's quite preliminary but i've big plans for the future (: