Which battery to match motor?

[mailmanx]

Hello!

I'm about to buy an e-conversion kit for my bike:

http://www.ev-power.eu/EVBike-Kits-1-1/E-bike-conversion-kit-EVBike-36V-500W-Front-28.html?cur=1

I also need a battery. What spec's should the battery have to best match this 500w motor?
A 36V/10Ah battery is 360w and won't feed the motor with enough power, am I right?
Would a 36V/14Ah be better, or a 48V/10Ah?

Thanks

 

[dogman dan]

That kit has a typical DD motor able to tolerate up to 1500w fairly well.

It has a 22 amps controller, often called 20 amps in specs. Very typical. If you run it at 36v, you will get about 800w peak power. It will tolerate 48v, which would give about 1200w peak.

The key thing here is to match the amps of the controller, to the discharge rate of the battery. Typical Ebike batteries have a discharge rate of about 2c, and it's recommended by many to not discharge at 2c any more that you must. 1.5c max is better.

What the heck did that mean? Say you have a typical 10 ah battery with a 2c discharge rate. That means that discharging it at 10 amps is 1c. 2c is 20 amps. You will have a 22 amps controller. If you try to discharge it at 1.5c max, that's only 15 amps. See the problem?

I tend to recommend a 15 amp hour size battery for this type of kit. With 15 amp hours size, 1c is now 15 amps, and 2c is 30 amps. 1.5c is right about 22 amps. This give you the headroom you need for climbing hills, or maybe if you just weigh a lot.

So get a 15 ah size battery, OR, select a battery type that has better c rate. Headways for example, are supposed to handle 5 c. They don't, but a 10 ah headway pack can definitely handle 3c. Again, 30 amps, and no problem.

Also, a 15 ah size gives you the range to make the bike a much more practical tool for replacing a car. My rule of thumb for 36v bikes is get 1 ah per mile range you need. If you use 36v, a 20 ah size is really nice, giving you 20-35 miles of range depending on your speed. 20 miles at 25 mph.

 

[mailmanx]

Thanks!

 

[mailmanx]

What would a good rpm be for a 28 inch wheel on mostly flat roads?

 

[mlt34]

What would a good rpm be for a 28 inch wheel on mostly flat roads?

The RPM will be entirely controlled by your battery voltage. Motors have a kV rating, which means how many turns they get per volt. A 10 kV motor would get about 10 rpm per one volt, or 360 RPM at 36V.

In your situation, you're kind of stuck since the motor has a fixed kv. If you were shopping around for motors though, you could choose a motor and winding with the right rpm to match your desired speed.

Here's a formula I made to allow me to quickly calculate the speed of any motor on any wheel size. If I know the RPMs, I multiply that number by the wheel size in inches and then by 0.003 and the answer is the speed in mph (use 0.0048 instead of 0.003 if you want it in km/hr). For example, a 270 rpm motor on a 26 inch wheel would equal 270*26*0.003=21 mph.

make sense?

 

[wesnewell]

Brushless hub motors don't really have a volt rating that means anything. They have a watt rating, and it should be the max watt rating for 24/7 operation unless stated as peak, which would mean never to exceed that wattage to the motor. The controller is what you match the battery to. The components in the controller are what dictates the max voltage you can use with that particular controller. Most 48V controllers can take 63V without going over specs. Most 72V controllers can take 100V without going over specs. Some 36V controllers may be limited to 50V while others may be 63V or higher depending on the quality of the controller. Controllers have watt ratings too and that should be the 24/7 rating also, not peak watts. To find peak/max watts you have to know the max amp rating of the controller. Multiply the max amp rating times battery voltage to get the max watts the controller will put out. A 40A controller with 50V battery will put out the same max wattage as a 20A controller with a 100V battery (2000W), but the one with 100V will be twice as fast rpm wise, so it which reach higher no load speeds. Here's a gnumeric speed calculation template that will let you find the exact no load speed on any wheel size and motor rpm. Change extension to gnumeric fro linux users.Whatever for windows if it even works in their spreadsheet. Sorry, like my cleaning lady, I don't do windows.
View attachment speedcalc.zip
Or create your own.
Column A1= Wheel size, B1=pi (3.1416), C1=circumference (A1*B1), D1=motor rpm, E1=MPH ( =C1*D1*60/12/5280), F1=KPH ( =E1*1.6093)

 

[mailmanx]

Here's a formula I made to allow me to quickly calculate the speed of any motor on any wheel size. If I know the RPMs, I multiply that number by the wheel size in inches and then by 0.003 and the answer is the speed in mph (use 0.0048 instead of 0.003 if you want it in km/hr). For example, a 270 rpm motor on a 26 inch wheel would equal 270*26*0.003=21 mph.

Surely other variables are needed, like battery effect and type of controller etc?

I'm trying to find components that will make the bike go about 30-40 km/h on flat ground. The areas here are mostly flat paved roads, no steep hills but light slopes here and there. Range is not so important, most solutions will be enough anyway.

I'm having trouble finding the right mix of components to achieve this. I don't want to end up with a stupid solution where for example the battery has enough energy to drive a city but the motor is too weak to make use of it etc, things like that. I want the best optimal mix of components.

At the moment I'm looking at this:

Battery: 36V/15Ah
Motor: BPM 36V350W
Controller: dont know

Would that battery+motor combo be okay for what I'm after?

 

[wesnewell]

Welcome to ES****Do this before your first post or now (it's retroactive)*****
Please go to the User Control Panel, select Profile, and then enter your city, state/province, and country into the Location field (country minimum) and save it. This will help people help you. Example: Wylie, TX, USA. Without knowing what country you are in it's hard to make any recommendations and you will waste your time and others. Thank you.

 

[mlt34]

Here's a formula I made to allow me to quickly calculate the speed of any motor on any wheel size. If I know the RPMs, I multiply that number by the wheel size in inches and then by 0.003 and the answer is the speed in mph (use 0.0048 instead of 0.003 if you want it in km/hr). For example, a 270 rpm motor on a 26 inch wheel would equal 270*26*0.003=21 mph.

Surely other variables are needed, like battery effect and type of controller etc?

That's the amazing thing, the other parts don't really matter so much in terms of speed.

There are only 4 parts that make an electric bike: motor, controller, battery and throttle.
The throttles don't really affect speed once you're at wide open throttle (WOT)
The controller doesn't really affect top speed, unless it has a three speed switch option or speed limiting option. The controller affects ACCELERATION based on the current limit. Generally, more amps = more power = faster acceleration.
The motor and battery alone control the top speed. An electric motor has an intrinsic RPM speed per voltage. That's why my formula above works. Give it an ol' college try!

 

[mlt34]

At the moment I'm looking at this:

Battery: 36V/15Ah
Motor: BPM 36V350W
Controller: dont know

Would that battery+motor combo be okay for what I'm after?

So far you look like you're on the right track for 30-40km/hr. if you get a kit with that motor, you'll get an appropriately matched controller. Something in the realm of 15-22 peak amps is what you're looking for in the controller, but it depends on your battery. A larger battery (higher AH) can handle higher current draw. 36V10Ah should be fine if you aren't looking for range. 15AH is better for battery health and will get you a good bit more range too.

To determine the speed of the ebike though, you'll need to show us which BPM winding you are considering. You can ask the vendor what "code" the motor is. The BPMs are usually something like a Code 10 or Code 12, which relates to their RPM/volt ratio.

 

[mailmanx]

To determine the speed of the ebike though, you'll need to show us which BPM winding you are considering. You can ask the vendor what "code" the motor is. The BPMs are usually something like a Code 10 or Code 12, which relates to their RPM/volt ratio.

This is the kit I'm looking at: http://www.bmsbattery.com/ebike-kits/345-bafang-350watts500watts-bpm-motor-e-bike-kit.html

Doesn't say anything about Code in the description, guess I'm gonna have to email them.

Also, if you look just above the green Add to cart-button you'll see that I must chose rpm as well.
And at the bottom of the description it says:

High speed means low torque. Low speed means high torque. RPM means "Revolutions Per Minute". 201RPM is often used for 24", 26", 700C and 28" rim. 328RPM is often used for 20" and 24" rim.

My wheel is 28 inch so according to the description I should go for the 201RPM. However, after googling some people suggest 328RPM instead to get a higher speed. Sounds nice, but don't I lose a lot of torgue then?

Will my bike be powerless in a steep uphill with a 328RPM option on 28 inch? My gut feeling says 328 should be okay since I'm mostly on flat ground, but I'm far from sure

Little off topic: why do they say 700CC and 28 inch above, isn't that the same thing?

 

[dogman dan]

Actually false about the torque, but they say that because understanding how it really works is very hard.

If you want more speed, go with the 328 rpm motor. It's fast, so you will have your speed. It will be less efficient on hills, and if you must do a lot of starts and stops in city riding. It will waste more of your watts into heat when starting up, or climbing very steep hills. So losing that few hundred watts does, in a way, mean less torque. It's complicated to explain, and I usually also oversimplify and screw it up.

201 RPM (at 36v) is quite a bit slower than the 328 though. So it will do better on range if you have a lot of stop and go, and just being slower will get you more range. But it might be too slow for your needs.

Solution, run the 201 motor at 48v, adding about 6-7 kph.

EM3 ev is a good vendor, and has some good motors in all rpm ranges. His 7t direct drive motor kit and a 36v battery would nail your speed range just right.

 

[mailmanx]

201 RPM (at 36v) is quite a bit slower than the 328 though. So it will do better on range if you have a lot of stop and go, and just being slower will get you more range. But it might be too slow for your needs.

Solution, run the 201 motor at 48v, adding about 6-7 kph.

A 48V350W motor? Or do you mean 500 W, 48V500W seems to be more common?

 

[wesnewell]

Look through these and get what you want. They have performance data on each one. You'll need about 300 rpm minimum for 40kph in 28" wheel.
http://www.leafbike.com/products/diy-bike-conversion-kit/28-inch-electric-hub-motor-kit/c-70/
I'd recommend this one with 12s lipo as the best option.
http://www.leafbike.com/products/diy-bike-conversion-kit/28-inch-electric-hub-motor-kit/28-inch-48v-1000w-rear-hub-motor-bike-conversion-kit-606.html

 

[mlt34]

201 RPM (at 36v) is quite a bit slower than the 328 though. So it will do better on range if you have a lot of stop and go, and just being slower will get you more range. But it might be too slow for your needs.

Solution, run the 201 motor at 48v, adding about 6-7 kph.

A 48V350W motor? Or do you mean 500 W, 48V500W seems to be more common?

He is referencing a 48V500W motor (if you are looking at the BMSbattery website). 48V on a BPM is a really rewarding experience. The 201 RPM at 48V would give you a nice speed, great range and an overall impressive package.

 

[mailmanx]

He is referencing a 48V500W motor (if you are looking at the BMSbattery website). 48V on a BPM is a really rewarding experience. The 201 RPM at 48V would give you a nice speed, great range and an overall impressive package.

Yeah, it's just that I was planning for a front motor and 500W is not recommended for that? This is the reason I'm thinking about 350W instead.

 

[mlt34]

He is referencing a 48V500W motor (if you are looking at the BMSbattery website). 48V on a BPM is a really rewarding experience. The 201 RPM at 48V would give you a nice speed, great range and an overall impressive package.

Yeah, it's just that I was planning for a front motor and 500W is not recommended for that? This is the reason I'm thinking about 350W instead.

Ah ok, you're absolutely right, you don't want to put a 48V BPM in the front fork. Even though its called a "500W" motor, it will actually be pushing over 1000 watts peak depending on the controller they pair with it (a standard 20A or 22A controller would put out a peak 1000W). I put a BPM on 36V in a front steel fork and it still twisted right through a stainless steel (albeit chinese) torque arm. You'll want to make sure you use a good torque arm to keep that from happening.

 

[mailmanx]

Ah ok, you're absolutely right, you don't want to put a 48V BPM in the front fork. Even though its called a "500W" motor, it will actually be pushing over 1000 watts peak depending on the controller they pair with it (a standard 20A or 22A controller would put out a peak 1000W). I put a BPM on 36V in a front steel fork and it still twisted right through a stainless steel (albeit chinese) torque arm. You'll want to make sure you use a good torque arm to keep that from happening.

I've only found one dealer selling a Bafang bpm 48V 350W front motor, problem is the rpm has changed and is now 280 which changes everything? What do you think of this? Motorcode is 48V350W16(11).

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2012-5S-Q4AN.2MJXJ