# Electric bicycle, use a kit or build from scratch?



## bomber991 (Feb 15, 2008)

Not sure how I should start this thread so I guess I'll state what I want first.

What I want is a bicycle that will go up to 20mph off of the electric power alone, and have a range of 15 miles on slightly hilly terrain. Seems like 15 miles on flat terrain would be the same since I can just coast down all of the hills.

Right now I'm debating between using the front hub motor kit from amped bikes.com
http://ampedbikes.com/buynow.html

Looks like it's about $400 shipped, and pretty easy to setup. 36V 500W Brushless motor, whatever all those numbers mean, I really don't know. Except that brushless is supposed to be better than brushed.

I'm not sure what kind of batteries to get, they sell a 3 pack of 12V 10Ah batteries for about $100 shipped from ampedbikes too. My question is, how many amp-hours will I need to travel 15 miles?

The other option is to build an electric bicycle from scratch, doing something like this guy: http://www.electricycle.com/

Seems like that would be cheaper, but I'm gonna have a helluva hard time figuring out how to mount the motor. I've never built any EV thing before, and I never mounted motors onto things that never had motors to begin with. Pretty much all the tools I have available is just some screwdrivers, wrenches, and a hammer.

Thanks for the help everyone.


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## kingofalldrunks (Sep 16, 2008)

try out this site--http://www.atomiczombie.com and then give the forum a look. sorry to pimp another message board, but they have a lot of cool builds and a bunch of suggestions. brad does good work with bikes and is more than happy to answer questions.


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## irfree (Sep 14, 2008)

I'm in pretty much the same position Justin is in. I'm drawn to the hub motors cause they look so simple. I have a factory three wheel recumbent with about the same terrain to travel to work, but don't know how to size them. I hate to do just trial and error but $400-600 bucks for a kit wouldn't be the most expensive mistake I've ever made.


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## bomber991 (Feb 15, 2008)

Thanks for the link to the other forum. I actually found this one forum that seems more focused on what I'm trying to do, the whole forum is focused on motored bikes.

I completely forgot that you can also mount little weed-eater like motors to bikes too to get the same kind of power assist you get from electric. Looks about similar in cost, only real advantage seems to be the range is slightly further. Looks like you can get more like 20 to 30 miles on a "tank" of gas. Supposed to get like 200 mile per gallon. However I think it would be sucky to have a loud, fumy, hot engine on a bicycle all the time, and also it would be real sucky to have to fill it up with gas everyday. Also you gota mix in either 4cycle or 2cycle oil everytime you put in gas.

I think I'm still going to go with electric though, cause thats the future.

Anyways, that forum was http://www.motoredbikes.com/


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## JohninCR (May 6, 2008)

Justin,

With hills in play, I'd suggest a geared hub motor for more efficiency and torque while climbing the hills. The non-geared hubbies shine on the flats, and brushless you can over-volt. One of the guys at the EndlessSphere E-bike forum reports averaging 9 watt hours per miles with some leisurely pedalling with a Bafang geared hub motor. 

With hills in the picture (and not big ones), I'd budget 30 watt hours per mile to be on the safe side. Amp hours is the measure of capacity, but it's not a meaningful number till you know the voltage. 

I use about 40wh/mile, but I weigh 240lb and have my 500W brushless hub running at over 80V and a run around everywhere wide open at about 35mph. Even pumping almost 2kw into the motor on hills I still have to help pedal up them slowly. It's a front hub and I'm in the process of adding a geared hub to the rear to improve hill climbing and acceleration off the line. I want those smoker scooters eating my dust at the stoplights.

John


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## bomber991 (Feb 15, 2008)

Cool, I've actually been estimating between 30 to 50wh per mile, and it's great to hear that 30 should be the max for me. 

I read up a bit here on how to figure out the watt hours based on the amp hours and the voltage. Looks like those 3 12v 10ah batteries I was gonna get should be run in 36v mode, so 36v*10ah=360watt hours. Looks like that gives me a 12 mile range. I might pick up one more battery, and run in 24v mode instead. Have 2 batteries wired in parallel, and then those 2 packs wired in series. So that would give me 24v*20ah=480 watt hours, so that would be a 16 mile range which is over the 13 mile range I require.

One question though, if it's a 36v motor, I can still power it with 24v right? That just means I won't get as much pulling force out of it correct?


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## JohninCR (May 6, 2008)

Justin,

A 36V hub motor itself of course will run at 24v, but I promise you won't want to. The controller probably has an LVC, low voltage cutoff, anyway that would prevent use at 24v. RPMs vary directly with voltage, so at 24v you would go 1/3 slower.

Regarding batteries, with lead acid batteries you can really only count on 80% at most of their rated capacity, and you should really restrict yourself for using only 50-60% of it for best battery life. Those batteries are typically rated based on a 20hr discharge, but when you drain them more quickly they have less capacity. Running lead batteries to lower than 20% remaining capacity kills it quickly, and remember to put lead on the charger as soon as possible. Their guts disintegrate much more quickly when left in a discharged state.

John



bomber991 said:


> Cool, I've actually been estimating between 30 to 50wh per mile, and it's great to hear that 30 should be the max for me.
> 
> I read up a bit here on how to figure out the watt hours based on the amp hours and the voltage. Looks like those 3 12v 10ah batteries I was gonna get should be run in 36v mode, so 36v*10ah=360watt hours. Looks like that gives me a 12 mile range. I might pick up one more battery, and run in 24v mode instead. Have 2 batteries wired in parallel, and then those 2 packs wired in series. So that would give me 24v*20ah=480 watt hours, so that would be a 16 mile range which is over the 13 mile range I require.
> 
> One question though, if it's a 36v motor, I can still power it with 24v right? That just means I won't get as much pulling force out of it correct?


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## TX_Dj (Jul 25, 2008)

John, don't take any offense to my cutting up your post- I am only bolstering what you're saying with further explanation. 



JohninCR said:


> A 36V hub motor itself of course will run at 24v, but I promise you won't want to. The controller probably has an LVC, low voltage cutoff, anyway that would prevent use at 24v. RPMs vary directly with voltage, so at 24v you would go 1/3 slower.


Of course, if you were to use a 24v controller, this would solve the issue that John is talking about above. Though, not only will you reduce speed, you will also reduce the power output of the motor. Lets say the motor is rated at 36v & 1000w, at 24v, you'll only get about 666w, assuming that the controller has the same amperage rating.



> you should really restrict yourself for using only 50-60% of it for best battery life.


Again, absolute truth... The "50% Rule" has a very good scientific basis to it. See here for more information.



> Those batteries are typically rated based on a 20hr discharge, but when you drain them more quickly they have less capacity.


This is, in summary, the definition of Peukert's Law. Peukert determined that the more current you try to draw from a battery within a period of time, the less apparent energy is stored in that battery. This is why the C/20 rate is a very unreliable indicator of how your battery will perform. Some batteries are constructed in such a way that the effect on the capacity of the battery may actually be less at high discharge rates compared to other batteries at the same C/20 rate. That is to say, if the Peukert's exponent for Battery A is different than the Peukert's exponent for Battery B, they may have the same 20-hour rate, but one battery may perform more favorably at the 1-hour rate.

Here are some handy links to help explain Peukert's equation:
http://www.smartgauge.co.uk/peukert3.html
http://www.smartgauge.co.uk/peukert2.html
http://www.smartgauge.co.uk/calcs/peukert.xls


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