250W 36V basic speed numbers

[Max1980]

Like a few others on the forum I just picked up this rear hub. Bafang G020.250.DC 13. I'm trying to figure out some basic numbers regarding speed. Purely interested in the numbers.

I found an equation on YouTube - RPM x Wheel Size x 0.003

I assume you use the Load RPM and not the No Load number? For this motor that would be 205 (or 245) so in its original guise

205RPM x 27.5" x 0.003 = 16.9mph

Then if run at 48V - 16.9 x 48/36 = 22.55mph

I'm assuming this number is at the original 15A which seems to be the norm for UK Ebikes. Other ratings give;

17A = 25.4mph = 816W

20A = 30mph = 960W

Are these all about right or am I messing something up here?


I've just watched a video of someone pulling to 30mph on a 250W 36V motor with 48V battery but only pulling 600W. He doesn't mention The controller Amps but how is that possible? Shouldn't the watts be way higher?

 

[sjpt]

You can see power needed vs speed at

An interactive model-based calculator of cycling power vs. speed

An interactive calculator that lets you explore the relationship between cycling power and speed.

www.gribble.org

With it's default parameters that give 540w needed for 30mph.

Of course, that's motor output power, not battery power. A typical motor is around 80% efficient at optimum revs, so that would be 480w output or just under 29mph with his default parameters.

 

[Sturmey]

You need to understand the difference/relationship between power, torque and rpm.. Its the magnetic field of the current (amps) that produce the torque. Its the voltage (emf/ electric motive force) thats necessary to push the current through the motor coils (and overcomes the back emf which increases with speed and allows for speed/rpm. (put very simply).
Uncontrolled, a simple motors sort of automatically takes in more current under load as the motor speed drops, back emf drops, potential difference between motor and battery increases etc. Thats why they use graphs to analyse/explain.
Good article below might explain some things. (Bear in mind they use bigger motors in the USA)

Futility of Motor Power Ratings

"There's no such thing as a rated watt!" Read here for why we don't advertise a simple power rating for the motors we sell. Also, have a look at our Youtube video where Justin talks about the power to propel bicycles in <a href="https://www.youtube.com/watch?v=ALde6zhLPs0">"School of Watts"</a>.

ebikes.ca

 

[Bonzo Banana]

Like a few others on the forum I just picked up this rear hub. Bafang G020.250.DC 13. I'm trying to figure out some basic numbers regarding speed. Purely interested in the numbers.

I found an equation on YouTube - RPM x Wheel Size x 0.003

I assume you use the Load RPM and not the No Load number? For this motor that would be 205 (or 245) so in its original guise

205RPM x 27.5" x 0.003 = 16.9mph

Then if run at 48V - 16.9 x 48/36 = 22.55mph

I'm assuming this number is at the original 15A which seems to be the norm for UK Ebikes. Other ratings give;

17A = 25.4mph = 816W

20A = 30mph = 960W

Are these all about right or am I messing something up here?


I've just watched a video of someone pulling to 30mph on a 250W 36V motor with 48V battery but only pulling 600W. He doesn't mention The controller Amps but how is that possible? Shouldn't the watts be way higher?

Normal nominal wattage for a UK/EU is 250W which at 36V gives you 7A (250 shared by 36) so a normal controller will provide that current to be legal but may peak considerably higher. Voltage dictates speed but current is torque which is needed to maintain that speed of 16.9mph or 22.55mph so will need more or less current depending on weight of rider and inclination of road etc. So current could be all over the place but your first calculations make sense for maximum speed but not sure how you would work out current which should vary a lot. 7A at 48V is a baseline of 336W. A controller with peak 17A output at 48V is 816W but it won't be able to do that for very long and lets face it, the Bafang motor is quite small and probably doesn't dissipate heat that well plus you have nylon cogs which would wear fast at high wattage output. So you probably want to be conservative how much power you put through it, exceptional high current on steep hills will clearly massively increasing the wear on the gearing.

If you take the commonly used 10Wh for every 1km used which is rough estimate used by the industry for range that gives 60km or about 40 miles for a 600Wh battery. Average speed of an ebike is about 16mph so that gives you about 2 1/2 hours riding. So 40% of your battery capacity is being used per hour except 20% is typically reserved to protect the battery. So you have 480Wh so you are averaging about 200Wh being used per hour of a 600Wh battery. 200Wh shared by 48V is clearly about 4A and with 36V about 5.5A although in reality 48V would typically use a little more current, its only a rough rule.

It's quite possible on a 7A controller current will be considerably less a lot of the time but when it needs it can peak higher for shorter periods. You wouldn't be able to run a 17A peak controller at that current for very long its not designed that way.

Also bear in mind if the controller thinks you need more torque it will drop voltage to increase torque for hills, so you have far less speed but more torque for hills, those are typically the times when the controller is delivering high current. This is what the controller is doing its converting higher voltages into lower voltages but with more current this is different to the current drawn from the battery, you may have 48V 20A drawn from the battery but the controller lets say might be sending 12V 80A to the hub, so its only a quarter of the speed but with much more torque.

As your current demands go up to their highest levels your speed goes down. At 816W you are probably well below 10mph in theory although I guess there is a point where going fast perhaps with a mild incline could peak your current requirements. 816W is a huge amount of power that would likely discharge a 600Wh battery in less than 30 minutes as in theory you should be able to get 45 minutes based on 600Wh but it would be such a high drain rate for those cells they would get exhausted much sooner and that battery pack would have a very short life.

I think your battery pack is 13 cells in series with 4 cells in parallel based around Samsung 2600mAh batteries or similar, so the pack is happy at a discharge rate about 10.4A output but can peak for continuous output at 20.8A but that is really a high discharge rate for those cells so 17A would be very close to that. It's a good battery pack but you have to be conservative a bit. You would probably want to current restrict the controller a bit, perhaps max out at about 13-15A to give the battery pack a long life and to make the discharge rate more efficient on those cells. I don't know what the maximum comfortable discharge rate is but I know many ebikes have used low capacity battery packs and they have had a high failure rate because the current was just too high. They seemed to be getting less than 200 charge cycles sometimes less than 100.

Ultimately you have to consider both the motor and the battery pack. It's great to get non-legal speeds but its not so great if either the motor or battery pack fails in 3 months. I think some sensible speed restrictions and current restrictions in the display settings could mean a ebike that lasts many years without issues. Some ebike owners clearly have stronger motors and much higher capacity battery packs that can be pushed much further.

 

[Max1980]

Normal nominal wattage for a UK/EU is 250W which at 36V gives you 7A (250 shared by 36) so a normal controller will provide that current to be legal but may peak considerably higher. Voltage dictates speed but current is torque which is needed to maintain that speed of 16.9mph or 22.55mph so will need more or less current depending on weight of rider and inclination of road etc. So current could be all over the place but your first calculations make sense for maximum speed but not sure how you would work out current which should vary a lot. 7A at 48V is a baseline of 336W. A controller with peak 17A output at 48V is 816W but it won't be able to do that for very long and lets face it, the Bafang motor is quite small and probably doesn't dissipate heat that well plus you have nylon cogs which would wear fast at high wattage output. So you probably want to be conservative how much power you put through it, exceptional high current on steep hills will clearly massively increasing the wear on the gearing.

From what I've read on the SWXH it'll do 750W continuously no problem. 750W is enough to get up highgate hill, at 20mph without pedalling. According to Gribble it's 850W to get up the steepest part of Swains Lane - 14% gradient at 15mph without pedalling. As a worst case scenario, under 1 mile that sounds fine. Does that feel about right or are those numbers off?

I have however read about some melting hall sensors wires at 1000W plus and getting hot at 900W. I believe the wires aren't rated for more than 20A so that explains it.

Also bear in mind if the controller thinks you need more torque it will drop voltage to increase torque for hills, so you have far less speed but more torque for hills, those are typically the times when the controller is delivering high current. This is what the controller is doing its converting higher voltages into lower voltages but with more current this is different to the current drawn from the battery, you may have 48V 20A drawn from the battery but the controller lets say might be sending 12V 80A to the hub, so its only a quarter of the speed but with much more torque.

I had no idea controllers worked like that, thanks.

If you take the commonly used 10Wh for every 1km used which is rough estimate used by the industry for range that gives 60km or about 40 miles for a 600Wh battery. Average speed of an ebike is about 16mph so that gives you about 2 1/2 hours riding

I've seen 25Wh/Mile throttle only. That should give about 20miles right?

 

[Max1980]

You can see power needed vs speed at

An interactive model-based calculator of cycling power vs. speed

An interactive calculator that lets you explore the relationship between cycling power and speed.

www.gribble.org

With it's default parameters that give 540w needed for 30mph.

Of course, that's motor output power, not battery power. A typical motor is around 80% efficient at optimum revs, so that would be 480w output or just under 29mph with his default parameters.

Amazing thank you.

 

[Sturmey]

Re 'Gribble' and 'watts', dont forget that the function of the electric motor is to convert the electric power (watts = volts x amps) input into mechanical power output (watts =Torque x RPM x 0.104 ). And of course they both are not the same in that many electrical watts can be lost in heat. So its the motor mechanical output that counts and can only be measured with a dynamometer. Hub motors can be about 80% efficent with light loads but this figure quickly fall if heavily loaded and slow. So to get your 750 mechanical watts, you probably need well over 1000 electrical watts with a small motor.

 

[Bonzo Banana]

From what I've read on the SWXH it'll do 750W continuously no problem. 750W is enough to get up highgate hill, at 20mph without pedalling. According to Gribble it's 850W to get up the steepest part of Swains Lane - 14% gradient at 15mph without pedalling. As a worst case scenario, under 1 mile that sounds fine. Does that feel about right or are those numbers off?

I have however read about some melting hall sensors wires at 1000W plus and getting hot at 900W. I believe the wires aren't rated for more than 20A so that explains it.



I had no idea controllers worked like that, thanks.



I've seen 25Wh/Mile throttle only. That should give about 20miles right?

That could well be true but the rule of 10Wh per kilometre I'm sure is meant more as a rough guide to pedelec type ebikes in Europe and the UK and as an approximate figure. The issue with throttle only is then the weight of the rider massively effects range because a heavy rider often has strong leg muscles and can assist more in power than a light rider but not enough to compensate for their greater weight however with throttle only and they aren't assisting at all then the motor and battery have to do all the work so some small light ebikes with light riders might get 20km but stick a heavy rider on there and it could be 6km.

 

[Bonzo Banana]

I had no idea controllers worked like that, thanks.

To be honest now that I re-read what I've written 12V at 80A wouldn't be possible as the cable I think is only capable of about 40A max from controller to hub motor. It won't scale as much as I've stated it will stay within safe parameter's but controllers do work by lowering voltage and increasing current to produce more torque. This is probably more about protecting the battery so its quite possible when you see a 7A controller that peaks at 15A that 15A could be at a reduced voltage so current demands of the battery are far less than you realise. Hence why averaged wattage consumption is actually quite low on ebikes.

This is one area where mid-drive motors are far superior, they have the controller inside the motor assembly typically and this means for the short distance between the controller board and motor they could have a much higher current connection. I personally don't like mid-drive motors that much for general cycling I think they are overly mechanically complicated and wear out the drivetrain at an excessive rate but the way they can scale power is definitely a huge advantage off-road although I still don't understand how they can be classed as legal 250W motors when their peak current demands are so high.

 

[Nealh]

On throttle one would expect the wh/m or wh/km figures to be about double or more, so quite expensive depleting the battery far sooner.

 

[Bonzo Banana]

On throttle one would expect the wh/m or wh/km figures to be about double or more, so quite expensive depleting the battery far sooner.

It really depends on how you use it, the whole point of the throttle is it gives you precise linear control only at the points you want it, you might want it all the time or you might only use it for the steepest hills it gives you that flexibility. How does the average person with a throttle ebike use it I guess is the big question and that is anyone's guess. It could be those who are doing short trips use it generously and those who are riding long trips use it sparingly. You might say with 5 power levels you get 50km, 40km, 30km, 20km and 10km based on the power level but a throttle would be more like 5-100km lets say with the same battery.

 

[Nealh]

Your average throttle from my experince isn't linear or gradual in use, they are either all or nothing and little affect trying to feather them.
I no longer fit them to my bikes as find they are just more clutt ron the bars then being really useful, a good able rider shouldn't need one.

 

[Bonzo Banana]

Your average throttle from my experince isn't linear or gradual in use, they are either all or nothing and little affect trying to feather them.
I no longer fit them to my bikes as find they are just more clutt ron the bars then being really useful, a good able rider shouldn't need one.

Even on a bike where the throttle isn't linear it still gives instant control of power for hills if that is the only time you use it. Is that a common issue on ebikes poor throttle control? Is it mismatched throttle to the controller or the controller not handling throttles very well? I don't get the good able rider shouldn't need one comment, that makes no sense to me at all.

Anyway most controllers I think have something like 1-4.2V for throttle control with 0V being off I think but controllers may default to different ways of interpreting throttles if you have a display which controls the controller. I think the basic violamart kits handle throttles well and probably all the basic ebike kits that rely more on throttle control. They don't always handle PAS as well, sometimes all or nothing. My experience is limited I have to admit. I don't currently run a ebike with a LCD display linked to the controller to understand throttle issues but have read about parameters to set to get decent throttle control when you are using a LCD display so presumably they don't always work well by default. Surely a throttle that is all or nothing is faulty or badly setup. The chinese have millions of e-scooters with no pedals using KT controllers and can't imagine all those vehicles have all or nothing throttles sounds like mass carnage.

 

[Nealh]

The cheap kits like voilamart use speed control so PAS is all or nothing as max current is walloped in the motor in all PAS levels.
e-scooter using throttle only will have different programming for a graduated controlled throttle to be used to deliver power, the same programming doesn't appear that evident in the lishui or KT's for ebikes I have used.
The two markets for ebike and scooter control appear to be different.