This post is a work in progress and is not yet complete
The tutorial is written as a design guide and specifications for a DIY electric skateboard/longboard. It will introduce the various components required and how to select each component for an optimal configuration. An example build is included that shows my first prototype board and the various design decisions I have made. This guide will include links to products, drawings, and all files used.
The breakdown of this guide will be as follows:
- Parts and designs
- Implementation and build guide
Overview of an Electric Skateboard
The components needed to build and electric skateboard can be divided into three categories:
1. Components that make up an ordinary skateboard (deck, trucks, wheels, bearings)
2. Electrical components (motor, electronic speed control (ESC), remote and receiver, batteries, charging system)
3. Mechanical components that interface the two (motor mount, pulleys and belts, enclosure). There can be many more components but this is the minimum.
The batteries provide power to the ESC which is connected to the receiver/remote which allows you to control the motor; all of this is mounted to the deck and can be hidden away in an protective enclosure. The motor is connected to the ESC and is mounted to the trucks. The motor shaft has a pulley and belt system that is connected to a pulley mounted to the wheel of the skateboard, this is how the board is driven. It is possible to have motors driving two wheels however this is much more expensive as it requires two motors, two motor mounts, two ESCs, and two sets of pulleys and belts. I will discuss the benefits and tradeoffs between a single motor and dual motor setup but really it comes down to more torque for roughly %30-%40 more cost. Hub style motors where the motor is built into the wheel are also becoming popular. They can be a nice solution as they eliminate the need for many mechanical parts and give the board a cleaner profile.
Once all of the parts are selected you're going to need a way to charge the board. Any battery setup is going to consist of a battery pack made of up many individual cells which introduces the problem of keeping the cells balanced to the same voltage. To account for this there are typically two methods: a battery balance charger, and a battery management system (BMS). I will expand more on this in the battery topic but the gist is the balance charger is an external component that is connected to the battery pack with multiple connections whereas the BMS is a internal circuit board permanently connected to the battery pack and allows the use of a single laptop style charger.
To control everything a remote control is used; there are many different type of remotes but the most common and arguably most reliable is a simple RC car remote. These are cheap (<$30) use 2.4gHz radio control, are a simple plugin and play setup, and can be modified to be much smaller and aesthetically pleasing.
Overview of Components
The batteries for electric skateboards can create a lot of problems but there have been a lot of developments in this area recently and many good solutions exist. The construction of batteries for electric skateboards have gotten to the point where they're comparable to configurations used in some electric cars but on a smaller scale. The cheapest batteries for an electric skateboard prebuilt hobby RC battery packs. These packs are typically LiPo batteries that available in nice form factors, are cheap, and offer good power. They are however the most dangerous battery chemistry and have been known to cause fires. Additionally they offer relatively lower charge life cycles, typically around a few hundred compared to Li-Ion batteries which are around a few thousand. Additionally these batteries are contained in a delicate pouch so they will need to protected in a safe enclosure.
I will go over the basics of batteries and then I will break the sections into LiPo battery builds and Li-Ion battery builds.
Nominal voltage: This is the average voltage of the battery over the battery charge. So as a battery is discharged the voltage will drop, the nominal voltage is the average from fully charged to fully drained. For example a Li-ion battery typically is fully charged at 4.2V and fully discharged at 3.2 volts so the nominal voltage is listed at 3.7. This trend of a battery to lose voltage as it is discharged is called voltage sag.
Naming: Batteries are listed in the following format xSyP where x is the number of cells in series and y is the number packs of x series are in parallel.
- Example 1: A 10s3p pack will consist of 3 sets in parallel of 10 cells in series for a total of 30 cells.
- Example 2: A 10s or 10s1p pack is just a pack of 10 cells in series.
- Example 3: A 8s6p pack will consist of 6 sets in parallel of 8 cells in series for a total of 48 cells.
Series/Parallel: When the cells are run in series the voltage is accumulated and when they are run in parallel the capacity and current output is doubled and the per cell current draw is divided. So looking at example 1 of 10s3p. If each cell has a nominal voltage of 3.7V, capacity of 3000mAh, and a max continuous current draw of 20A then the pack will have 37V, 9000mAh, and a max continuous current draw of 60A. This also means that if the total draw on the pack is only 40A then each cell will have a draw of ~13A.
C rating: the C rating is just the max continuous current draw of the battery where C is a multiplier of the capacity. For example a 5Ah LiPo battery that has an advertised C rating of 20 will have max continuous discharge of 100A which is really high. It is often recommend to do your research when referring to a manufacturers C rating as sometimes the advertised C rating is much higher than the actual. Additionally the C rating is typically inversely proportional to the internal resistance of the battery so a battery with a higher C rating usually will have a lower internal resistance.
Balancing: Balancing is the act of maintaining very similar voltage and state of charge across each cell in the pack. This is very important since any battery pack for an electric skateboard is going to consist of multiple cells so without balancing a pack must stop discharging when the cell with the lowest charge is empty. So if batteries are not properly balanced this can become a huge safety issue as over discharging a lithium-based battery can result in battery failure. I will expand more on methods of keeping cells balanced but the two main methods are to use a balance charger (typically used with LiPo batteries) and a battery management system (BMS, typically used with Li-ion batteries).
The best motors for electric skateboards are brushless DC outrunners that are typically found on hobby rc helicopters and planes. These motors offer plenty of power and torque and are available in configurations that are ideal for an electric skateboard motor.
Here are the important specifications to look for when selecting a motor:
Name and Size: Outrunner motors are typically listed in the following format name-xxyy where xx is the width of the motor in mm and yy is the length of the stators in mm. For example the SK3-6364 is a 63mm width motor with a stator size of 64mm. The second number yy is can typically be seen as the size of the motor can however be careful using this number in your designs as the total length of the motor is usually more, for example the SK3-6364 measure closer to 80mm in length. Typically it is better to use larger motors where you can with common sizes being 6355, 6364, 6474. For mono drive boards using the biggest motor possible is often a good approach however with dual drive boards where there is size constrains 6355 is a common size.
Kv: the motor velocity constant. Simply this is the number of rpm per volt applied. For example a 190Kv motor will reach a top speed of 7980rpm with a fully charged 10s(42v) battery (190*42=7980). This is probably the most important specification to consider when selecting a motor as it needs to work with the batteries selected, the esc, and will impact acceleration and top speed. It is suggested to select your batteries, esc, and your desired top speed before selecting a motor in order to choose an optimal Kv. Typically for electric skateboard you'll want a motor with a Kv in the range of 160Kv to 300Kv. Personally I thing that 190Kv is often a good selection for most
How this affects the ESC: Some ESCs are only rated up to a certain ERPM (Electric RPM) which is the the number of the number of pole pairs of the motor * the rpm of the motor. For example the Turnigy 6364 190kv motor has 14 poles, so 7 pair poles, with a 10s battery will give a max ERPM of 190427 = 55,860. The VESC's upper limit is 100,000 ERPM however some users have found this number to be closer to 60,000 ERPM. Exceeding the ERPM is both inefficient and can damage the ESC.
How this relates to batteries: Using the max ERPM of the esc and dividing by the pole pairs of the motor will give an ideal max RPM for the motor. For example using the VESC many agree that 60,000/7 = 8,570 is an good number to aim for. Using this information we can then select the best battery setup for out motor, or select the best motor for our battery. For example:
- 8570/12s(50V) = 171Kv
- 8570/10s(42V) = 204Kv
- 8560/8s(33V) = 260Kv
This demonstrates why it is important to choose the right motor for your battery setup. If you chose a 280Kv motor and then decided that you wanted to run a 12s battery pack you would either need to limit the motors speed which would leave you with overall less torque than if you had selected a motor with less Kv.
How this relates to top speed and acceleration: So now that we know that the ideal max RPM is around 8570 it is easy to see how this will affect our top speed, gearing ratio, and acceleration. If we want our top speed to be 45KPH and have 83mm wheels we can now determine how we will need to gear our board.
(0.083m * Pi * (8560rpm/60) * 3.6)/45kph = 2.98
This calculation takes our target speed of 45kph and divides it with the theoretical speed produced without any gearing reduction of roughly 136kph
This gives us a gearing ratio of 2.98 so 1:3. So if we want our top speed to be 45kph then we will need to run a gearing ratio of 1:3 (36/12, 42/14) where the motor speed is being reduced by a factor of 3 to the wheel speed which will give us good torque and acceleration as well as top speed. Ideally you want to aim for a gearing/reduction ratio greater than 3. Once the batteries and motor have been selected to give us our max RPM of 8,570 we can play with the gearing ratio and the wheel size to make slight adjustments to top speed and acceleration.
Note on top speed and safety: When first building a board you'll probably want to maximize your potential top speed. I'm pretty comfortable with speed but I can honestly say that anything faster than 35kph is terrifying. Additionally when electric skateboarding wear a helmet. There can be a lot of unexpected forces that could throw you off and generally it's a lot easier to fall than you'd expect. Here's a picture of my helmet after a fall I had during a slow speed test run.
Max Voltage(V): The max voltage the motor can take will determine the largest size battery pack we can use with the motor. For example if a motor is only rated to 36V then ideally we should not use a battery larger then 10s. There is a little room for play with this number but you really don't want to push it too much. For example I used to run a 12s battery with a SK3-6364 which is only rated to 36V and I never ran into any problem other than a loss of efficiency as the motor ran quite hot.
Max Current(A): This will determine that maximum load the motor can handle, if this is not listed then divide the listed motor wattage by the max voltage (ex: 2000w/36V = 55A). This typically will be greater than 50 however sometime can be a bottleneck in the skateboard design. If you're able to limit the current with the ESC then you can keep your motor safe but setting the max motor current to this number.
Note on torque and power: It is often hard to find information regarding the torque of the motors as this somewhat difficult to reliably measure. The power in watts however is a good number to go by when determining the motor size with 1hp being equal to 745.7watts.
Speed Controller (ESC)
Powering the electric motor is a little more complicated than just turning the power on/off. The speed controller is what interfaces the battery and the motor and provides the ability to control the motor's speed, direction, and even regenerative braking. There are only a few commonly used options for ESCs with the two main choices being hobby RC car ESCs and the open source VESC. The VESC is an open-sourc ESC designed by Benjamin Vedder and is widely considered the best choice for electric skateboards. Although expensive it provides many great features and the capability to completely tune the controller for your setup; overall I believe the the VESC is well worth it.
Some noteworthy features of the VESC are:
- USB connection and desktop app: The VESC is developed with a desktop application that is used to configure its settings whereas most ESC require using a programming card. There are also good options for connecting your phone to the VESC via bluetooth being developed by the community; this enabling you to change settings and monitor power and range.
- FOC control: Most speed controllers operate using brushless DC control (BLDC) which is basically very fast switching of each phase of the motor. FOC controls the motor through modulation of each phase, this creates more torque and significantly less noise at the cost of a slightly lower top speed. Of course with the VESC everything can be configured so either mode can be used.
- Voltage input of 8V – 60V (3S to 12S LiPo)
- Current draw of 50A continuous with 240A bursts.
- Small form with no heatsinks or fans.
- Low voltage cutoff (LVCO): The VESC is configured with a soft to hard LVCO which will cutoff power when the battery voltage is too low. This enables the VESC to act as an additional safety for avoiding over-discharging of the batteries.