Cuda E with RC drivetrain

RC E-Bike Drivetrain tutorial

By Warren Beauchamp

RC based e-bike drives trains can be extremely efficient, high powered, and deliver a lot of bang for the buck. Because the RC industry is well established there are a wide range of vendors of products and they enjoy a reasonable price due to the high volumes of production. These systems are not designed for e-bike use, but can be readily adapted.

Note that building an RC e-bike drive train requires basic electronics and mechanical skills and use of a soldering iron. It also involves batteries with extremely high power density. The batteries used are very sensitive to over and under charging, and if damaged can catch fire in a spectacular manner. Also, these types of systems are not good for people looking for an e-bike appliance. These are more for someone looking for an e-bike hotrod.

Basic parts needed to build an RC drivetrain are an RC motor, ESC, servo tester, BEC, throttle, LiPo battery pack, and a gear system to reduce the motor RPM to bike wheel speeds.

Basic Terms

  • RC (radio controlled) - General term for products designed for the RC industry.
  • AC (alternating current) - as opposed to DC (direct current), AC voltage varies up and down around a zero point.
  • V (volts) -  Generally the higher the voltage that faster you go. Sort of like how fast water flows through a hose.
  • A (amps) - This is current. Generally the more current you have, the faster you get to the top speed. 1000 milliamps (MA) = 1A
  • W (watts) - This is volts times amps. Usually used to show total power used. 1 kilowatt (KW) = 1000 watts. V * A = W
  • Controller - A device that controls the motor. Simplistically, it takes power from the battery and a signal from the throttle and used that to control how much power is sent to the motor.
 

RC Motor information

Motor Wiring
Builders of 3 phase brushless RC Motors Motor drivetrains have to pay attention to the motor wiring. RC motors have 6 power wires coming out of them, one set of wires for each group of windings.

These need to be arranged one of two ways, "Delta" or "Wye". Generally Wye (also called "star") is more efficient. Wye has 1.73 times less KV than Delta (slower speed per volts), but 1.73 times more torque. In general it's a good idea to stick with Wye configuration for electric bikes as e-bikes need slower motor speed, higher torque, and high efficiency.

For Wye configuration, the 3 positive wires are tied together, and the 3 negative wires go to the controller. These 3 negative wires are the phase wires, just like in a hub motor.

For Delta, the positive from one set of windings is connected to the negative of the next set. Each connection of positive and negative makes up one phase.

In both configurations, if you connect the ESC to the motor and it spins the wrong way, just reverse two of the phase wires from the ESC.


KV - RC Motors are AC brushless motors and are rated by their KV. KV is the relationship between voltage and motor speed (rpms per volt. Multiply the motor KV times the battery voltage to find the maximum motor RPM. For instance a motor with a KV of 100 would run unloaded at 5000 RPMs at 50V. Note that RPM with the motor under load is around 70% of the unloaded value. This means that the actual max motor speed while being used is about KV * V * 0.7

Windings -  One of the main things that controls the KV of a motor is how the copper electro-magnets are wound. For example a 6 x 10 motor has 10 turns of wire on each pole, and 6 wires wound together in parallel onto each pole. A higher number of turns on each pole of the motor means a lower KV, lower RPM, and lower top speed. A lower number of turns on each pole of the motor means a higher KV, higher RPM and higher top speed.

Hall Sensors - Most e-bike specific motors use hall sensors. These are used to tell the controller what position the motor is in, so it knows when to send a pulse to the electro-magnets. RC Motors do not have any hall sensors, so at low RPM they can lose sync easily. Generally with RC bike systems you should pedal to at least 5MPH before the applying the throttle to avoid this issue. It is possible to retrofit hall sensors to RC motors but that will not be discussed here.

RC Motor Sync - For e-bike use, Inexpensive ECSs like Turnigy controllers can lose sync when accelerating under high load, at very low RPM, or at very high RPM. The loss of sync at high RPM sounds like the motor is screeching, and is accompanied by a loss of power. At low RPM it sounds like the motor is stuttering. This should be avoided as it stresses the controller. The Motor itself is also important. To help prevent the loss of sync a high quality motor like Plettenberg or AstroFlight is recommended.

Choosing an RC motor
There are several parameters to look at when choosing an RC motor for your e-bike application. The first is, inrunner or outrunner? With inrunner motors the shaft rotates. This is good for driving chains or belts. With outrunner motors the outside shell rotates. This is good for direct friction to tire drive applications.

Typical E-bike RC motors run at 8,000 to 12,000 RPM with no load. In general this means with a 50V battery pack you will want a KV of 225 or lower. With a motor like this you can use a simple 2 stage gear reduction. You will need to pick a motor that works with the reduction gearing you are using to deliver your chosen top speed at maximum RPM. Higher motor speeds can be used, but the speed reduction gearing becomes complex. RC e-bikers have used motors with much higher RPM with complex multi-stage or planetary gearing like the Neugart PLE units. The advantage of this is that the bike has much better low end torque.

The Astroflight 8120-5T is a 225KV motor. 5T refers to five of turns per coil in the motor. This motor when run at 48V and using a 14:1 gear reduction with a 20" wheel give a top speed of about 30MPH. Because the RC motor doesn't have hall sensors, it is a "pedal first" system. This means you will need to pedal up to about 5 MPH to allow the motor to get to a high enough RPM to sync properly at a high load. After that, whoosh!

Here's an RC Drive Calculator to help you figure out how the tire size, motor KV, battery voltage, and gearing all interact to give you a top speed.

Next you will need to determine how much power you need. RC motors are rated in watts. It takes around 2000 watts (2KW)  to get up hills at a decent speed and to go over 20 MPH into a stiff headwind, so that should be the minimum to consider. Anything over 5000 watts is a monster motor and will require a very powerful battery and stronger than standard bicycle drivetrain components. Note that 750 watts is one horsepower.

Electronic Speed Controller (ESC)
RC Motors controllers (ESCs) are very small and light compared to traditional e-bike controllers, they are also smarter. This is nice because they are small and lightweight, but it's bad because they do not have the ability to shed heat as easily.

The RC controller (ESC) has to figure out the correct timing synchronization for each throttle position and load condition using EMF (electro motive force) feedback. Some RC controllers are better at this than others.  Castle Creations controllers are an example of a good controller. Cheaper controllers like Turnigy work fine for aircraft use, as the low RPM / High load conditions of the aircraft environment are not nearly as rigorous as those in an electric bike.

Castle Creations controllers are programmable via USB. You'll need the Castle Creations USB adaptor to connect your PC to it. This will also allow you to download the logged data so you can see pretty graphs of your motor use. Click here for some suggested settings when using the HV160 with the Astro Flight 3210 motor.

RC Controllers are generally do not support voltages higher than 50V (12 LiPo cells in series). The ESCs have heavy power wires going in and three heavy phase wires going out. They also have 3 smaller wires that connect to the throttle circuitry (servo tester).

Note that the ESC throttle wire colors in the schematic above may not match those on your ESC and servo tester, and the little plug can be plugged in two ways. In general, red goes to red and black goes to brown and if you get those right the other one will be right too.. 

Throttle Electronics
The Throttle for an RC motor is more difficult than a traditional e-bike system because RC ESCs are designed to be throttled by signals from a radio receiver. To make is work with a resistive e-bike throttle you need a "servo tester" and a "battery elimination circuit" (BEC). The BEC converts the 48V from the battery to 5V for the servo tester.  The servo tester has a throttle control potentiometer which is removed and replaced with the wires from the throttle. Use a 5K resistive throttle like the Magura.  (see schematic below). Matt Shumaker posted a throttle tutorial on the endless sphere site.

Batteries
Terms

Ah (amp-hours) - This is how much current the battery can deliver and for how long. For example, A 10Ah battery can deliver 10 amps for 1 hour at it's rated voltage. RC batteries are rated in milliamps - hours (MAH). 1000mah = 1ah
C - This is how much peak current the battery can deliver. A 20Ah battery rated at 2C can deliver 40A continuously without frying.
S and P (series and parallel) - Individual e-bike batteries are generally small cells with lower voltage and current. To get more voltage and current the batteries are hooked together in series (for more voltage) and parallel (for more current).

LiPo Batteries
RC LiPo Batteries require respect. It's a lot of power in a little package. In the past treating them badly could result in flames, but the newer LiPos are a bit more forgiving. Luke on endless sphere notes: Never exceed 4.3v per cell and never discharge below 2.7v per cell. To extend the battery life and improve reliability, charge the cells up to 4.15V and don't discharge below 3.0V If you abide by these guideline they should behave. RC LiPo Batteries are nominally 3.7V per cell, but generally run closer to 4 volts each. When you buy RC battery packs they are rated by "S" and "C" and by the milliamp-hours (5000 milliamps is 5 A).

S is the number of batteries in the pack, arranged in series.
C is how fast the battery can be discharged. 6S packs are about 24V.

12S pack = 50V hot off the charger.

To build a pack that is 10Ah and 48V requires four of these 6s 25C 5000Mah (5ah) Lipo packs. This battery pack will supply 48V at 10 amps for one hour, or up to 250 amps(!) in bursts. These packs are about $70 each, or about $240 for all 4. This pack would only weigh about 7 lbs.
 

Here are four 6S (22.2V) 5Ah RC battery packs, an RC ESC, and some basic electronics like a fuse, a switch, and connectors. To build an RC system you will need to make connectors to hook this stuff together.

Be very carful to not touch the red (+) and black (-)  wires together. This will make a huge spark which can burn fingers, damage eyesight, and damage the expensive batteries and electronics. Your goal is to keep the magic smoke inside the batteries and electronics!

These batteries will be combined in series and parallel to build a 44.4V, 10Ah battery pack.

Note that all individual battery packs should be balance charged before connecting them together in parallel to ensure that all the cell voltage match.

Here's a schematic of the batteries and RC drive system. Not shown are the battery balance charge wires

Parts shown:

  • Turnigy (Hobby King) 5000mah 6S 22.4Vnbatteries

  • Castle Creations BEC

  • Castle Creations Servo Tester

  • Castle Creations Phoenix ICE HV160 ESC (controller)

  • Astro Flight 3210 RC motor

  • Magura throttle

This schematic shows two sets of batteries that have been soldered together in parallel. Alternatively, connectors can be used to connect the batteries in parallel.

Want to build a 20Ah pack at 48V? then you need eight of these 5ah 22.4V packs, you have to connect four in parallel, then combine two of those paralleled packs in series.

I used the 70 amp Anderson connectors, but it's easier just to use the same style of bullet connectors that come with these batteries.

To properly solder 10 gauge wires together, first press them together and do not twist the strands. Then take a long piece of solid copper wire, like phone wire, and wrap it tightly around the wires. This holds it all together and makes it much easier to solder. After the connection is soldered, cover the connection with some nice thick heat shrink tubing, and use a cigarette lighter or small butane torch to shrink the tubing.


LVC (Low voltage Control) - To ensure that the LiPo batteries are not damaged by over discharging, the system needs some type of low voltage control (LVC). There are several alternatives that monitor each cell individually, like  This LVC or a low voltage monitor like this one. Fortunately the Castle Creations controller I will be using have a low voltage control that can be programmed in, which can either shut the motor down completely or just reduce the power when the battery level drops below a pre-set voltage.

Battery Charging/Balancing
There are two methods of charging LiPo batteries, balance charging and bulk charging. Balance charging charges each cell individually, this usually involves charging each parallel group of cells separately. Bulk charging allows you to charge the whole pack at once, which can be done without taking it apart. Todays LiPo cells do not need to be balance charged every time they are charged. They can be bulk charged about 10 times between balance charges.  After several discharge/charge cycles you will learn how well your packs stay in balance.

Typical RC E-bikers have both charging systems and they usually just bulk charge and then periodically balance charge. This takes some of the pain out of charging as bulk charging is much easier than balance charging.

All individual battery packs should be balance charged before connecting them together in parallel to ensure that all the cell voltage match. You may need to perform a couple balance charge / discharge cycles to get the individual cells to within 0.1 volts of each other.

Here's a Bantam e-station sitting on a Meanwell 12V supply, charging a single battery pack.  I had to use the manual to figure out how to use the e-station but it was pretty straightforward.

All cells are now either 4.20 or 4.19 volts. I had to perform a couple balance charge / discharge cycle to get the individual cells to within .1 volts of each other.

Here's a schematic showing how the pack is balance charged using a single 6S battery charger. Each of the two paralleled 22.4V LiPo packs are attached directly to the balance charger. Each of the two 22.4V 10A packs is charged separately.

Like almost all RC chargers, the  Bantam e-station BC6-DC, requires a 12 volt DC power supply. These power supplies are available from various sources but the most popular and reliable for us e-bikers are the MeanWell power supplies, which are available on e-bay. 

To avoid having to solder the balance connectors together in parallel like I did,  this harness can be used to connect the charging leads of the sets of batteries that are in parallel.

Bulk chargers need to have an HVC (high voltage control) that shuts down the charging when the pack is charged. For a 12S pack like this one, the charger should shut off when the pack reaches a conservative 49.8V (4.15V per cell). I am being even more conservative and only charging to 49V.

The method shown here is a basic bulk charging configuration. My primary bulk charger is a 600 watt BMSBattery.com LiPo charger, calibrated to 49V. It automatic shuts off when it reaches 49V.

I also use a use a modified MeanWell S-350-48 power supply to charge the battery at work. It does not automatically shut off, so it can't just be left to charge overnight, but it drops to very low current when it gets close to the set voltage, so if is safe to leave for several hours.

 

It's a good idea to protect the batteries from damage. This picture shows an  aluminum box from Mouser. Be sure to use foam or something to fill in any gaps so the batteries don't rattle around.

One nice thing about the Anderson connectors is that they offer panel mount options.  I used that ability to mount them to one end of the box. This allows detachable output for the motor wires (left side). On the right is the fuse. Disconnecting this Anderson connector disables the battery pack, so effectively this is the on/off switch. 

A normal switch could be used, but DC voltage puts much more stress on a switch than AC due to it's tendency to arc when switching on or off under load. This can be avoided by never switching under load , but some arcing will occur just to charge the capacitors in the ESC. Also it's very hard to find a small switch rated higher than 16amps. 

There are many ways of getting the power from the motor to the wheels. Here's a video of the Shumaker RC drive reduction unit being tested on my basement floor.

Another method is to use an outrunner RC motor (outside of motor spins) as a friction drive against the tire.

Another method is to use a tiny cog on the motor and a giant one on the wheel.

 

   
Credits: I have learned a lot over the past year from Matt Shumaker and from the folks at the Endless Sphere e-bike forums. Thanks to everyone who helped teach me. Learning is a never ending process. The more you know, the less you know you know.

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