I have built an Electric Longboard and I'm going to share my experiences with you along with some tips and tricks I found neat. There are alot of guides out there with people sharing their builds so what makes this one different? I will mostly focus och the mechanical build. What motor to use, what voltage, and what transmission in a general manner.
Step 1: Battery and Voltage ⚡
Okey. When I decided on building myself an electric longboard i found this area to be overwhelming. There are alot of people on forums etc. just screaming out what to use and what to avoid but I wanted to dig deeper into the mathematics to fully understand how it all comes together.
First off we need to discuss power. Everybody wants alot of power for their board. Power is measured in Watts (P) and is calculated as the product of Voltage (U) times Current (I), P = U * I. High current results in high losses. Think of it like a river. Voltage is the width of the river and Current is the speed of the water. With a wide river, lots of water can flow even though the speed of the water is low. To have the same amount of water flowing (Power) on a narrow river, the speed of the water have to be high, splashing all over (losses). To summerize, we want to have a high voltage so that the amperage can be minimized. Losses are bad for both longitivity of the components (through heat) and range of the longboard (through low efficiency).
But beware! In Sweden where i live, voltages above 48 volt is classified as lethal. A 12S battery is about 50,4 volts fully charged.
You want to have a capacitor with a large value (typically 2000uF+) close to your ESC. This is for reducing the risk of voltage spikes on the esc when connecting it to the power supply, and also to help deliver current spikes demanded by the esc.
Here's some LiPo batteries. Get three of these and your good to go! These are thin which makes it easy to
mount under your board!
Heres some capacitors:
Having high voltages in combination with a large capacitor close to the ESC requires another type of switches than what would normaly be used in cars for example. I started out with a 12 V car switch on my e-board and it worked... For one time. The spark that hit the connector inside the switch welded it together. The instant the switch is closed, a very large current will flow to fill the capacitor. Very large currents are good for welding but nothing else. So how do we solve this problem? One solution is to use a so called "sparkless-switch". These use a normal small switch to enable the gate of one or several mosfets. It works like a charm but they are rather expensive.
Another way to go is to use thesesmall connectors. When plugging them in, a resistor of 5 ohms or so is connected for a brief moment before the switch is fully closed. For a 12S battery, it would mean that about 10 amps is flowing through the resistor for a blink of an eye before the capacitor is charged and the switch is closed. Using any online calculator you can see that it takes about 10 milliseconds to fill the capacitor to 67% whith 50 volt supply and 5 ohm series resistance. They also work like a charm and I have been using mine for some time now.
I use my spark free xt90 connector both as an on/off switch and as a safety feature. The positive wire to the ESC goes through the xt90 connector in a loop, enabling the rider to easily disconnect the Power to the esc when riding if something goes wrong.
Spark-free XT90 connectors to use as on/off switch
One thing that seems to confuse people is the amount of energy stored in batteries. When talking car batteries, amp-hours are often used and a battery with 110 Ah has higher capacity than one with 60 Ah, right? Not always. It depends on the voltage of the battery. Car batteries are always 12 V and can therefore be compared using Ah:s. But now that we are talking Lithium-Cells (cause this is probably the kind of cells that you will be using), we have a number of different voltages that people are using, ranging from 6S to 12S. BTW, this means 6 cells in series and 12 cells in series. Since the nominal voltage of a regular lithium cell is 3.7 volt, we could instead of 6S and 12S say 22,2 volt and 44,4 volt. Its just easier to say voltages in terms of how many cells are connected in series.
The best way to measure the capacity of the battery is to use the unit watt-hours Wh. This is simply the product of the Ah value and the nominal voltage of the battery. The battery for my longboard is a 5,2 Ah 9S battery, it has 173 Wh. If I drained it with 173 W it would last for 1 hour.
Electronics in a Longboard
Building a battery system for a longboard is a mess. There is not a lot of room for components and there are extensive vibrations when riding. Therefore, it is crucial to pay extra attention to isolating all cables and connections. A short can cause explosions in your lithium cells and/or ruin your components. Use heat shrink everywhere you can and be generous when applying hot glue on all open connections. When doing electrical maintenance in a building, the law requires two layers of isolation on all wires. This is a good way to go here as well!
I would really recommend you getting a kit like this. It has all dimensions you need and plenty of it!
And a glue-gun!
And Another one:
Charging your battery
I've made another guide on how to charge your lithium cells. Please go to: How to Charge your Lithium Battery
- Use as high voltage as you can to reduce the current, but beware of shocks.
- Have a large capacitor (2000uF+) in parallel and close to your ESC.
- Use a spark-free switch or connector.
- Charge and discharge your cells properly to increase lifetime and safety.
- Use double layers of isolation on all cables and use hot glue on all open connections.
Step 2: Transmission Ratio, Motor KV and Speed
Depending on the goal of your build you will want different characteristics. Do you want it for the daily commute or do you want to outrun road bikers? The easiest way, I think, to explain the relation between voltage, motor kV, transmission ratio, speed and torque is to make an example.
Let's say you have decided on using a 9S battery (like mine). The voltage is about 35 volt and let's say I want a top speed of 35 km/h. My longboard have wheels with 69 mm diameter. You could make all calculations by hand or you can use this app for example. Mix and match motor kV, transmission ratio and find the combo that will work for you. Most transmissions out there are between 1:2 to 1:3 so there is not much wiggle room there. Motors comes in alot of different kV ratings however, so it may be easier to first settle for a transmission and then chose the motor. In the example above, a 1:3 transmission is used and a 245 kV motor. The top speed is about 37 km/h in theory.
The Power rating of my motor is 2700 W and thats way more than I use it for. About 1300 W is my max setting on my ESC and that gives me just the perfect amount of torque. But if you weight more than me (63kg) you might want more power. However, if your using a one wheel drive like me, you'll find that you need a really sturdy transmission to handle that kind of torque. More of that under the step "My personal experiences". If you got the resources, go for a dual motor build. Better traction, less wear on each transmisson, less unwanted turning when braking/accelerating.
Remember that if you want a high top speed, you will have to input alot of current in the motor to get good torque at slow speeds. It is a trade-off.
I have now switched to using chain instead. So much more reliable. The first belt I used wore down in a few weeks. Just slipped during slightest acceleration. With the chain mounted, no more slipping. Some say that it is more noisy, but I don't hear any of it while riding. The chain need lubrication once in a while but its worth it. The transmission ratio is now close to 1:2 so I need to input more current to get the same acceleration as before, my esc is set at 50A for now. If I were to build a new board, I would definitely go for a chain right away! I bought this one: Chain
Benjamin Vedder has made an excellent guide on this subject. Here it is!
Step 3: My Personal Experiences
Where I live there are almost no smooth roads. I quickly found that my 69mm 82A wheels are both to small and too hard for a pleasant ride. I will upgrade to at least 80mm and preferably 78a or less in the future. This affects the top speed and low speed torque of the board so I may have to change either the motor (or its kV rating by ching from a delta configuration of the windings to a star configuration) or the transmission ratio.
I also found that the Htd3m (10mm wide) belt is way too weak and handle way to little torque. Belt slips at every hard acceleration and braking, resulting in quick wear. As a quick and easy fix for this I will upgrade the motor pulley and the belt to fit a 15mm belt. 50% larger contact area may solve the belt slip problem. The motor pulley also has a few more teeth then the one I have installed now.
If I were to redo all purchases of components from the start, I would buy a motor with a lower kV rating and get a htd5m belt (they can handle more torque!) or a chain instead. The top speed of my current build is according to the calculator about 37 km/h. And that i found to be just perfect for me.
HTD 5m transmission kit
HTD 5m motor pulley:
HTD 5m wheel pulley
I have ordered 90 mm 76a wheels and a transmission with chain. I will update when they are installed and report on my experiences with those.
The 90 mm 76A wheels that i bought are the best! For me anyway ;) Smooth riding even though there's rough asphalt or cobble stones. Combined with the chain I got myself a reliable commute board. The downside of big wheels and changing transmission ratio is that I need to input lots of current to get the amount of acceleration I want. In some future I may rewire the motor from a D-connection to a Y-connection, lowering the kv-rating by the square-root of 3 (~1.73). That should reduce the amount of current needed by the same factor.
Another great upgrade I've done is to replace the box for the battery pack with one made of glass and carbon fibre. The old was made of SAN-plastic and was too brittle. It was a lot of work (and money) making the new one, but it was definitely worth it!