Questions about Electric Bikes
Hi to all forum members :) I´m so sorry but i have soooo many questions :D (i´m trying to make a new vehicle (patent) but it will require some hard core info)
1.1)Weight - i would like to know the weigh of all motors if someone has found a list OR at least the weighs of motors that would go 20,30,40 [Km/h] with 100[Kg]/220[lb] person
1.2)Can those weight less than 3 Kg?
1.3)Do this motors have breaks and gears or they actually do everything by power direction (slowing,speeding,stopping etc...) ?
1.4)Why are Wattage AND Voltage differences "needed" ?
1.5)Can 250W/48V Engine be better than Higher Wattage but 12V Engine ?
1.6)Would additional gears on Ebike work? Ok lets just say i can change gears for 1x to 2x Speed so if it goes 5 RPM/Sec it should go 10 RPM/Sec (to the wheel) and reach double speed (or lets say a have motor attached to pedals)
2.0)Batteries (searching for low-weigh batteries but not a lot more expensive too)
2.3)do more voltage batteries weigh more than more wattage batteries?
2.2)how much would batteries that can last 2 Hours weight if they allow me to go a) 20[Km/h] , b) 30[Km/h] , c) 40[Km/h]
2.3)Can those weight less than 3 Kg?
2.4)What would be the recharge time for that 2 Hours?
ANY help will be appreciated :thumb:
1.3: You will use ordinary bicycle brakes. Some kits offer regenerative braking in addition -- they are more expensive, and have other downsides as well.
1.4: wattage differences: more watts means your components need to be able to take the power. All of the controller's parts are built to handle specific conditions in terms of maximum volts, amps, and temperature. many hub motors are run at less than their maximum power, so a hub motor sold with a 250-watt kit might be able to handle 1500 watts (it might not). You need a battery that can handle the amount of power you are using. (in terms of amps and in terms of amp hours). Voltage difference matters for a couple reasons. High voltage means that you can put more power through a wire of a particular size. This is why it has become "standard" to run most e-bikes at 36 or 48 volts. The exact same motor can be usually be used at 36 or 48 volts-- it will run faster at 48 volts, whereas 36 volts will give better energy-efficiency, at least in some circumstances.
1.5: There are some really technical reasons why you would want to go with 12 volts or 200 volts or whatever, if you are experienced at building your own systems. If you are not, you shouldn't be concerned with voltage. # of watts (power) does matter -- lots of watts are required for fastest uphill speeds. For flat-ground speeds just look at the speed the seller says the system is capable of.
1.6: Yes -- additional gears on an ebike can work. I recommend you do not use a multi speed e-bike system unless you are interested in going faster than 40kph or going up really steep hills.
2.0: if you want cheap batteries use Lead Acid. If you want reasonably affordable lightweight batteries use LiFePO4.
2.1:voltage does not determine the weight. wattage also does not determine the weight. for a particular chemistry (such as LiFePO4), energy capacity (which is the same as watt hours... which is the same as [amp-hours times volts]) will determine the weight.
2.2:we need to make more assumptions to answer that question. let's say you're on nearly flat ground, not pedaling, on a bike with wide tires and wide handlebars, going for 2 hours, rarely stopping, and your battery is LiFePO4. Based on http://www.kreuzotter.de/english/espeed.htm you probably need:
20kph: 400 watt hours, or a 4kg battery (at a minimum)
30kph: 900 watt hours, or a 9kg battery (at a minimum)
40kph: 1850 watt hours, or a 18.5kg battery (at a minimum)
2.3: see #2 .2. "li-po" batteries cost more but give more energy per kilogram, so less than 3kg is possible in your 20kph scenario.
2.4: if you use a typical battery and a typical charger, you should expect a charge time of 4 to 5 hours. If you are really concerned about fast charge time, better numbers are possible.
thank you so much for you help man :)
Few more things... due to weigh i decided to go for 20 kph
a)So could this mean than 12v 250w motors only need fatter wires and therefore could be unreliable but have the same power as 48v 500w motors ?
b)is it possible to put more power on 250w motor and make it stronger & faster than the usual ?
3.0) conditions: nearly flat ground, NOT pedaling at all, on a bike with wide tires and wide handlebars, going for 2 hours with 20kph , 3kg LiFePO4 Battery ,3 Kg motor
could 250W Mini Hub Motors work for this conditions and should i be concerned about voltage or better what voltage could i use?
The real problem with a 12v ebike system is that you probably have to build it yourself.
A 12v 250w system is not comparable to a 48v 500w system. but the watts are the important part.
watts ARE power. therefore the following systems have the same power:
a 12v 500watt system, a 36v 500watt system, a 48v 500watt system, or a 100v 500watt system.
those are (all) twice as powerful as any 250watt system.
If you are looking to go 20kph on nearly flat ground, a 250watt system will be fine. It may be better than a 500watt system for your situation, in terms of being efficient.
i had my doubts but i will got for 250w 36V-42V as it is energy efficient,im sorry but i cant find out how you measured battery weight on http://www.kreuzotter.de/english/espeed.htm ,i would like to do it myself,aslo what is this pedaling cadence,cant find any good translation (im Croatian) for this,could you please explain what is it for?
btw it shows i can go about 26,6 kph with Roadster and Height 195 cm,Rider Weight 95,Bicycle Weight 20
[TD]so it might me possible to go 25 kph http://www.bikeforums.net/images/smilies/biggrin.gif i hope
ok i think i get the cadence now,in my case its 300 if my motor has 300 RPM correct ?
still searching for battery weight formula in there (http://www.kreuzotter.de/english/espeed.htm)
now i found another question ..
i see that 24V 250W Motors are slower than 48V 250W motors,is this possible ??
by my calculations using I=P/V
24V 250w uses 10.42 Amps 24V
48V 250w uses 5.21 Amps 48V
so if i use 15 (3.2V 10 Ah)battery cells they will last for about 1.92 hours on either one of those motors ?
Let me get this straight, you are trying to get a patent for something you know nothing about?
You want us to tell you an answer to a question so that you can then claim ownership over the answer?
First, learn about the product catagory, then find a need, then do your R&D. Don't go to a group of enthusasists and they to ask for help while claiming ownership over their hard work.
The patent system has a fairly respectable tradition. However, it does create inefficiencies and as long as all are honorable the inefficiencies do not outweigh the good (the harm done to emerging technologies, like the early steam engine not withstanding). Unfortunately, patents do tend to stifle emerging technologies by making increments improvement a near impossibility. This is why patent trolls are such a problem in the computer industry, by the time a patent expires the technology is obsolete.
This guy (the OP) clearly dreams of becoming a patent troll, patenting the obvious and then waiting until someone becomes successful using that idea as part of their product, unaware of the existence of a patent on a obvious technology (like the guys who recently tried to sneak a patent on the wheel through the patent board). Patent trolls do not develop or provide new technologies, they just add inefficiency to emerging technologies.
oh. . . I see, form the post above it looks like you ate thinking of patenting Ohms law. . . good luck with that one. Tell you what, go read about batteries on wikipedia (it will have more than you need to know for this exercise) then use a thesaurus and change all the words using that. Step three is to patent the battery. Wait about ten years, find someone using batteries in a small, innovative device. Make sure he can not afford to fight your BS patent, then file a suit.
Here is the trick, settle for a very small sum, so small that he would be a fool not to settle for it. Now you have precedent, the courts have validated your 'longstanding' patent, now go for the big fish.
Yes im working on a petent as a said,but it could be a lie what if i just want this information for a regular ebike,are u willing to help now ??
anyway here is what is confusing me http://electronicsclub.info/power.htm
formula for amps is watts/volts meaning :
250W 24V (is 10.41 mamps) engine uses more AMPS less VOLTS (2 x 7.5 cells(3.2V 10Ah cells)
250W 48V (is 5.21 mamps) engine uses more VOLTS less AMPS (1 x 15 cells(3.2V 10Ah cells)
both run for 2 hours on same amount of batteries?? but 48V will go faster ??
That part you are missing is what is known as the "kV" rating of an electric motor which has to do with how its coils are wound. Basically to get the free running no load top speed of an electric motor you multiply its "kV" rating by the voltage you are running it on. Once you start putting load on the motor it slows it down some, how much it slows it down depends on how much load and the performance curve of the individual motor.
A motor that is wound for kV=50 then on a 12V system the free spinning no load RPM of the motor would be 600-RPM
A motor that is wound for kV=100 then on a 12V system the free spinning no load RPM of the motor would be 1,200-RPM
A motor that is wound for kV=150 then on a 12V system the free spinning no load RPM of the motor would be 1,800-RPM
A motor that is wound for kV=200 then on a 12V system the free spinning no load RPM of the motor would be 2,400-RPM
A motor that is wound for kV=250 then on a 12V system the free spinning no load RPM of the motor would be 3,000-RPM
A motor that is wound for kV=50 then on a 24V system the free spinning no load RPM of the motor would be 1,200-RPM
A motor that is wound for kV=100 then on a 24V system the free spinning no load RPM of the motor would be 2,400-RPM
A motor that is wound for kV=150 then on a 24V system the free spinning no load RPM of the motor would be 3,600-RPM
A motor that is wound for kV=200 then on a 24V system the free spinning no load RPM of the motor would be 4,800-RPM
A motor that is wound for kV=250 then on a 24V system the free spinning no load RPM of the motor would be 6,000-RPM
A motor that is wound for kV=50 then on a 36V system the free spinning no load RPM of the motor would be 1,800-RPM
A motor that is wound for kV=100 then on a 36V system the free spinning no load RPM of the motor would be 3,600-RPM
A motor that is wound for kV=150 then on a 36V system the free spinning no load RPM of the motor would be 5,400-RPM
A motor that is wound for kV=200 then on a 36V system the free spinning no load RPM of the motor would be 7,200-RPM
A motor that is wound for kV=250 then on a 36V system the free spinning no load RPM of the motor would be 9,000-RPM
A motor that is wound for kV=50 then on a 48V system the free spinning no load RPM of the motor would be 2,400-RPM
A motor that is wound for kV=100 then on a 48V system the free spinning no load RPM of the motor would be 4,800-RPM
A motor that is wound for kV=150 then on a 48V system the free spinning no load RPM of the motor would be 7,200-RPM
A motor that is wound for kV=200 then on a 48V system the free spinning no load RPM of the motor would be 9,600-RPM
A motor that is wound for kV=250 then on a 48V system the free spinning no load RPM of the motor would be 12,000-RPM
For e-bike drive systems it is rare to find anything designed to use less then 24V or more then 60V and usually the motors are wound for as low of kV rating as possible while still keeping decent power output per motor size and weight (trying to wind an electric motor for much less then kV=50 starts producing large heavy motors with not much power) and its not uncommon to find e-bike systems that use gearing reductions on the motor (usually planetary gears) that have kV ratings of 100 or higher and many home-built drives using larger size RC out-runner motors go well into the 200+ kV rating range.
I have done a few 12V custom home-built builds myself and very much like them because of the voltage compatibility with a huge assortment of high quality for low price automotive electrical components from LED lights that are far brighter and far superior to almost any regular bike light for less cost to Amp gauges to show my power usage circuit breakers and fuses to being able to charge the battery on the e-bike with any common car battery charger (provided you use the right battery chemistry some battery chemistry won't like being charged with a car battery charger).
But I did have to pretty much do it completely from scratch with the one exception being the one build where I used a 24V 650watt cyclone motor head and purchased a 12-24V Kelly brushless controller that is compatible with that motor and just run it on half the volts (for half the RPMs and half the watts power output with the same amount of Amps through the controller) and then just adjusted the gearing between the motor and pedals to make up for the motor running half the speed it was designed too. Works fine although I also lost a little bit of efficiency as well because that motor doesn't run quite as efficiently at the lower voltage then it was designed to run at.
My other 12V builds I used either industrial brushed motors designed for 12V (big and heavy but okay for a cargo bike that is going to be big and heavy anyway), brush-less higher power automotive accessory motors (some of the fancier newer cars and SUVs use powerful brush-less motors for the radiator cooling fan that you can get for a decent price from a salvage yard if you look well enough and hard enough and you can re-wire them to run off an e-bike type brush-less controller that will run off that low of a voltage level Kelley 12-24V is the only one I know of), or brush-less larger size RC motors capable of running at that volatage level (usually labeled as being able to run off a 4-cell Li-Po pack) where you can either install your own Hall sensors in order to use the Kelly 12-24V controller or run it off an RC brush-less sensor-less controller using a servo tester throttle assembly or a CA-v3+ e-bike computer unit or similar which is capable of putting out an RC type throttle signal with a standard e-bike throttle plugged into it.
all i know now that if motor that has kV=250 it will produce 1V per 250 RPM :(
i cant even fin kV values on the motor on internet
I think im gonna have to go on more sites....,im searching for FORMULA to count how long will motor last on some amount on batteries (on max power)
250W 24V (15 cells(3.2V 10Ah) time ?
250W 48V (15 cells(3.2V 10Ah) time?
right now im thinking about buying this
and this batteries
okayyyy..... so 15 batteries and that motor running on max would last for how long and why ??
I was probably getting too deep for you with bringing up the kV rating. If your using off the shelf e-bike motor and building totally from scratch then you probably don't need to know or consider going that deep into things.
The motor you linked too is as far as I can tell a variant of the common internally gear reduced freewheeling hub-motors. The voltage range they are giving you is telling you the minimum voltage you need to run it off to get decent results (24v) and the maximum voltage you can run it off and get decent results (48v).
I also really like your choice of batteries to use with it. I am also a huge fan of the prismatic hard case LiFePO4 cells with the screw top terminals. Haven't used those ones specifically but I'm using ones of the same general style myself.
How large of diameter wheel are you planning to have the motor laced into? Reason I ask is because twice the voltage means twice the RPM and its a question of whether you should wire up 16 of those cells you linked too in an 8s2p wiring configuration to provide 25.6V to the motor (just a little over 24V which is fine) or if you should wire up 15 of those cells you linked too in a 15s wiring configuration to provide 48V to the motor
In either case your looking at a total battery pack capacity of 480-512 watt hours which gives you an actual real word usable capacity of 384-409.6 watts hours. Now let me explain that calculation:
Watts = (total volts) x (total amps). Which if you wire 15 cells all in series means 48V (firstname.lastname@example.orgV) multiplied by 10 amp hours which is the capacity of the cells. Or if you wire 16 cells in an 8s2p configuration that is 25.6V (email@example.comV) multiplied by the total amps which is double because you are doing a two cell parallel arrangement so that is 20 amp hours (2@10Ah).
Then to get the actual real word usable capacity you have to take some percentage of that rated total. For Lithium (will have "Li" somewhere in the name) chemistry batteries that is about 80% since that battery chemistry family doesn't like being totally drained all the way down to nothing and it hurts the batteries and makes them wear out fast to do so. For lead acid chemistry its 50% and for NiMH batteries its about 95% which at first glance would make it look like NiMH would be best of all until you consider that Lithium batteries hold much more power for the same size, weight, and even cost to more then make up for that little extra 15%.
So long story short knowing all this means your real world actual usable battery power of the kind of battery you are looking at using is going to be approximately 400 watt hours of capacity which means you could pull 400 watts from the battery with a motor and the battery would last one hour or you could pull 200 watts from the battery with a motor and the battery would last two hours or you could pull 800 watts from the battery with still another motor and the battery would last only a half of an hour.
The motor you lined too according to its rating will pull about 180 to 250 watts from the battery depending on how much its loaded down (and also how much the controller you use limits how much power the motor pulls and how much pedaling you do so the motor doesn't have to work as hard, etc . . .)
So, if you set-up your controller to allow the motor to pull its maximum power and you loaded it down so that it was always working hard your battery would last about 1.6 hours running it like that (400 watt hours / 250 watts = 1.6 hours). On the other hand if you kept the motor from working hard so it wasn't pulling as much power your battery could last a little over two hours (400 watt hours / 180 watts = 2.22 hours).
Whether you want to run the motor on 25.6V with a 8s2p battery wiring set-up or on 48V with a 15s battery wiring set-up will depend a lot on what size wheel your going to use with it and how fast you want to go and match that up to the motors RPM range which I can help you with the math on that. You have already said how fast you want to go just need to know what wheel size you want to use with it. 20", 26", & 700C are the most common sizes, what you want to use? What size does the bike you want to put it on already use?
I am not 100% sure for the wheel size it might be a lot smaller or little bigger xD
but I might go for 40 cm/15.74inch diameter(with tire) wheels and that is about 7" spokes i guess
i am concerned about RPM due to very small wheel size,according to my poor math :D
400 RPM for 30 Kph and 265 RPM for 20 Kph
(40 cm/15.74inch) diameter with tire
1.)can i make more RPM with sensors ?
2.)could i remove the 3 gears inside and attach the stator to rotor directly?
3.)is it necessary to have speed-changing gears ?
4.)i would like to go to minimum of 20 kph but 30 or 40 kph is fun too :rolleyes:,okay maybe just 30 :D
If your going to go with a small wheel then you want to run the higher 48V to make the motor spin faster to get decent speed out of the smaller wheel which must spin faster then a larger one to go the same speed. (Sounds like you already figured that out yourself)
The motor in question is rated for 180-370 RPM across its acceptable voltage spectrum which is 24V to 48V so the top end of that RPM range (370-RPM) is going to be on the highest voltage (48V) and the low end of the RPM range (180-RPM) is going to be on the lowest voltage (24V).
According to my quick numbers 370-RPM on a 40cm diameter wheel (outside tire tread diameter) will give you 27.9 Kph free running speed which you will probably have to pedal and put some of your own into to reach but it should do 20 Kph motor only with the motor loaded down a little no problem.
If on the other hand if you went the other direction and used a fat 700c tire (about 70cm diameter) then you would be looking at 48.8 Kph free running speed on 48V something which would require the rider to be extremely strong and pedal very strongly to keep going fast enough to not bog the motor down well below its intend RPM on the other hand running on 24V with that big of a wheel on that motor would bring the free running speed down to only about 23.75 Kph which is a little on the slow side especially if you consider that without pedaling and just the motor only its going to be actually slower then that as the motor slows down under load. Running on 35.2V or 38.4V using either 11 or 12 cells in series (and possibly doubling up in parallel to 22 or 24 cells if you really wanted the capacity for longer run time) would probably be ideal for a big 700c wheel size with that motor putting the free running speed at about 36 Kph or so.
I think your idea of going with a smaller wheel size and running at top end 48V is probably the best plan. A 40cm wheel size (16" size in US measurement) is just a little small unless your trying to do a folding electric bike or something. I would personally be much more likely to build on a 20" wheel size (about 48cm with a decent width BMX type tire) which is a very popular wheel size at least over here in the U.S. and the most common small wheel size and would produce a 33.5 Kph free running speed on 48V for that motor by its ratings which is probably about right for you giving you 20 Kph or better on flat ground with just the motor and allowing you to add your own pedal power to bring it up to around 30 Kph which will also unload the motor and reduce its power draw at the same time. Thus by pedaling on you will both increase speed and increase your range on the battery which is about the ideal set-up.
You will need to gear the pedal gearing to match. Looks like that motor is designed to take a six speed freewheel which if you get one with a 13t high gear and combine that with a 48t chain-wheel on the pedal cranks should allow you to pedal along with the motor up to full speed without your pedaling cadence reaching or exceeding 120-RPM which you want to keep it a little under that for effective pedaling. It depends on how good of a pedaler you are but for a normal person ideal pedaling cadence is about 90-RPM or just under and the better of an athlete you are the higher that goes with Olympic level cyclist athletes being most comfortable and producing the best pedal power at about 105-110 RPM pedal cadence. Most normal people can't effectively pedal at 120-RPM or faster cadence and put out useful power and actually push into the pedals, its even hard for Olympic level cyclists athletes to pedal effectively above 120-RPM and actually push into the pedals to produce useful power.
Long story short, 20" wheel on 48V system with a 48t chain-wheel on the cranks and a 13t or even 12t small cog high gear on the rear six speed spool (I'd be tempted to see if I could make a 7-speed spool fit but if they say it won't then it probably won't) so I could pedal strongly up to the motors free running speed of a little over 30 Kph would work for me. Since I'm much more of a mid-drive guy and not so much into the hub-motors wouldn't be my ideal set-up but using that motor and a 48V system that would probably be what I would do or something close to it.
yeah something like a folding bike .. that is the problem because it will need to have 40cm diameter(with tire) or smaller
might you know any way to increase RPM? , i am thinking about changing the gears inside the motor itself
Q: if the motor is making 5 spins/second and i make 7 spins/second than is the motor slowing me down ??
Not on a motor like that with a freewheeling assembly inside of it (although you would be wasting power by just having the motor spin and be doing nothing since your pedaling faster then the motor so if you ever are pedaling faster then the motor take your finger off the throttle and don't waste power).
As to modifying the gears inside the motor, not a good plan unless you have your own fabrication and machine-shop at your disposal.
You might consider overvolting it and running it on hotter voltage then it was designed for (60V maybe?) but you have to be very careful with that because its a good way to burn up a motor. You would probably need a programmable controller to do that so you can set the amp limit lower to offset the higher voltage to keep from burning up the motor. If the motor is designed for 250-watts of power. That's 5.2 Amps at 48V (48x5.2=250) and you would have to use a programmable controller and set it down to only like 4 Amps to keep from frying it at 60V and even then it isn't a guarentee because you are still running the motor beyond its intended design specs. Some people have done it successfully and others have burned up motors, its kind of less science and more "black magic" which is a combination of skill and luck once you start running things beyond their specs whether you will get away with it or not.
Again,thanks for helping me out on this :thumb:, well i am afraid of doing the high voltage but from this picture i see it might be possible to jamm the 3 reduction gears
and im less afraid of that xD but than again it looks like it will got 5x faster :/ and what about the kV values you mentioned :twitchy: , okay now i have to go see if i will buy the whole kit or part by part,im from Croatia so the shipping costs me but not on small item parts and im thinking about buying only the parts i need
2.)Controller 48v 250w (i have problems finding them its usually 36V250W and 48V350W :(
4.)And the motor
well i will need about 10 days for this, check other sites than ebay and what all to buy,how it all works etc
Has 370 RPM so for now i will not touch RPM for a longer time
for beggining i want to use batteries with only 30 Minute duration on 48V250W motor on max power
can i make that with this ones http://www.ebay.com/itm/LithiumFerru...item4ac7e6979e
or do i have to buy smaller batteries ?
here´s a reward for deer members that posted here
If you only need 30-min of duration those batteries are larger then you need and you could easily get away with batteries half their size.
i will go find small batteries than i can double if needed,that good ?? xD
could i use this kind http://www.ebay.com/itm/4pc-x-3-2V-L...item540174bf82
does this mean i have 1Ah x (250/48) = 0.72 x 60 =11,52 Minutes?
now if i buy 45 it will work ???
and what if i put 17 (3,2vx17) instead 15 per pack,would it make RPM better cause damage or nothing ?
If your going to build a pack from cylindrical cells you need to buy ones that have solder tabs already pre-micro-welded to their ends like this:
Or you can also buy cylindrical cells with screwterminal ends as well, like this:
Basically unless you have your own micro-spot-welding capabilities you need to have cells with solder tabs or screw top terminals because you need to be either able to solder the wires on them or screw the wires down to them. You can't just solder to the bare end of a cell and spring type terminals are unreliable at best for a bicycle power pack with all the vibration and the high power draw can take the spring out of the springs.
I have seen some simple contact sustained compression pack assemblies work but I don't like to trust that either with all the vibration and such the pack will be exposed too over time.
You might also take a look at low capacity pre-built packs in your desired voltage instead of building your own pack. Most are more expensive then buying the cells and putting your own pack together but not always and sometimes you can find a deal.
I know that usually you can get away with 16s instead of 15s and get in one extra cell and run 51.2V instead of 48V on a controller and motor designed for 48V. I've done it myself without issue but I haven't tried bumping up to 17s myself so can't verify that works. You will get a little more RPM with the one extra cell but not a huge amount more.
BTW i have a thought of improvising the hub motor(when i´l have money for experimenting) you can try it yourself if you are interested,poking around 45° holes for air ventilation,but that is another story...
I hope i will not have any questions for some time so you can rest from it : D
edit:got one already,not sure witch one do i use then putting more batteries,its small difference but i would like to know the correct formula
do i take the voltage of the batteries or just the motor´s max voltage,or let me put it this way,will the motor run on same amps with 16 batteries??
1.) 1N = 1 battery 1Ah 3,2V
a ) (16N/15N)/(250W/51,2V)*60min = 13,1072 minutes of working
b ) (16N/15N)/(250W/48V )*60min = 12,288 minutes of working
Tohot, you seem to have the misconception that the nominal power rating of a motor has anything to do with how much power it can put out <at least for a short time>. A "500 watt motor", can easily put out 1500 watts given the right controller. In fact in can put out a great deal more power than the nominal rating until it burns out from heat, the controller amp limit is reached, or the battery is trashed from a too high amp draw, exceeding the C rating of the battery. So plugging in the voltage of the battery into Ohm's law, as a way of calculating some "operating amperage" of the motor is quite useless. The amperage varies continuously as the motor operates, spiking to very high levels, say 40 amps, when torquing the motor at a stop sign with no pedalling. The nominal rating is also useless other than an extremely rough guide to the power of the motor. For example, my Conhis motor (same as a Golden motor Pro hub) is a so-called "500 watt motor". My Cycleanalyst routinely shows power over 1300 watts on this motor, given the 30 amp controller I use, and the 48V Headway pack I use. This is confusing to the newbie. That motor could probably produce over 2000 watts, if the phase wires were upgraded, and a high powered controller was able to push 50 amps into it. Another example: my Cute100 geared brushless motor is a "250 watt motor". I usually see many spikes to 700 watts on this motor (as seen on CycleAnalyst), with a 15 amp limited controller and 48V Headway. I wouldn't put any more than 750 watts into it though, on a continuous basis - the gears would be at risk inside the motor. There is also further refinement of the power issue when it comes to "phase amps" vs "battery amps"...can't say I know enough to untangle those concepts. The CA measures battery amps.
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