The Resistor Network: Long Range Electric Longboarding

I bought a Boosted Board back in August of this year while living in New York City. It was an awesome purchase. It is fun to ride and I could often beat the subway to my destination on shorter trips. I have since moved to Silicon Valley and find the range to be lacking slightly. The board gets approximately 7 miles of range out of the box and my office is 10 miles from home.

My Boosted Board shortly after purchase.

I added 288Wh of high-discharge lithium-ion batteries to the 99Wh of batteries that came with the board. I was inspired by the Portable Electric Vehicle Youtube Channel. During the first test ride I was able to ride more than 13 miles before stopping. The fuel gauge on the remote showed more than 20% of battery remaining.

My Boosted Board after the battery upgrade :]

During this process I also designed a custom lighting system based on WS2812 LEDs and an Arduino Mini Pro.

These LEDs are bright!

Batteries, Batteries, Batteries

The Boosted Board uses Lithium Iron Phosphate (LiFePO4) batteries. In a nutshell, this battery chemistry trades capacity for higher power and longer shelf life. Boosted has a blog detailing the design process behind the selection of battery chemistry and size. The battery inside the board is a 99Wh, 12 cell LiFePO4 pack.

You may have been shocked to read that I selected Lithium-Ion cells to extend the range. I built two 92.5Wh 10 cell Li-Ion battery packs. This should immediately register as cause for concern. Li-Fe cells have a nominal voltage of 3.2V and Li-Ion cells have a nominal voltage of 3.7V. There are 12 LiFe cells in the board which gives a nominal voltage of 38.4V. My 10 cell pack has a nominal voltage of 37V. This is very close.

Verifying the internal voltage

To ensure that this could be done safely, I measured the voltage of a fully charged Boosted Board to be 40.7V. My external pack, when fully charged is also 40.7V. What this means is that I can connect the batteries in parallel. The load of the motors will be distributed across the battery packs.

In addition to adding more capacity to the board, the efficiency of the internal battery will be increased. A battery that is discharged slowly is more efficient than a battery that is discharged quickly. Win win!

Battery Assembly

I created two 10S battery packs with Samsung INR18650-25R cells. These are high-discharge lithium-ion cells with a rate of 35A.

Batteries!

I tabbed these batteries together in series along with a balance wire. These batteries are charged externally with a balancing charger to ensure that individual cell voltages are equal to one another.

Two cells and a balance cable extension

The pack was assembled with all cells next to one another to keep the profile low. I researched different options for my batteries and found that many were too tall. There are several 10S packs available from RC hobby shops such as Hobby King but they are too tall. Clearance under the board is at a premium so it is important that any additional components are as short as possible.

Tabbed cells with the balance lead attached

These cells need to have a minimal layer of protection. I used 185mm shrink tube to protect the cells and provide rigidity to the pack. I added electrical tape to the ends of the cells as well. This made it difficult to slide the shrink tube on, so only one of the packs have this feature. It would be great if these batteries had a housing, they will take a beating so close to the ground. This will have to do for now.

I settled on HXT connectors to connect the batteries to the board. They are rugged connectors with high current handling. In retrospect, I wish I had gone with something that is easier to terminate and provides better keying. Overall, they have been durable and able to withstand over 20 miles of riding so far.

Finished pack :]

I use an iCharger 1010B+ balancing charger to charge these packs. It is capable of charging at up to 300W and ensures that voltage between cells remains equal. It is quite a feature packed device, capable of charging a wide variety of battery chemistries. I have no doubt that it will be useful for more projects in the future. I have a 12V, 10A power supply that I use to supply power to the charger.

First Charge Test at 100mA, making sure the magic smoke stays inside the batteries ;]

I initially tried charging at a mere 100mA to ensure that everything was connected properly. I verified voltages and learned how the charger works. I quickly stepped it up to 2.5A. This will get the pack charged in an hour. The fan inside the iCharger spins up at this rate.

Charging at 2.5A

Board Modification

I had to modify the Boosted Board to externalize the supply rails from the internal battery pack. I picked up a multimeter and started probing around inside the Boosted Board. The ESC (Electronic Speed Controller) designed for the Boosted Board is impressive to say the least. It supports regenerative braking which allows energy used to brake the motors to be stored in the batteries.

The Boosted Board ESC

The supply rails deliver power from the battery at the front of the board through flat braided cabling that is routed through the deck of the longboard. I exposed a section of the braided cable to attach my external battery connector to.

Exposed supply rail from the internal battery

I used 12AWG silicone wire to connect external batteries. I find it to be flexible and easy to work with.

12AWG external battery cable

I routed these wires through the loom at the back of the board and out the side. I should note that the use of an external battery is completely optional after this modification.

External Battery Cabling

An HXT connector was used to terminate the external battery connector.

HXT Connector

I used extra-strength velcro to hold the battery to the bottom of the board. This is different than the usual variety of velcro that uses a felt portion coupled with plastic hooks. This velcro has plastic hooks on both sides that engage one another. As such, it is difficult to push together and even more difficult to break apart. I can lift the weight of the entire board through the battery.

Velcro mounting on the battery and board

I built a small Y-adapter for the HXT connectors so that two packs can be connected in parallel.

HXT Splitter

The batteries are mounted next to one another. I am currently using electrical tape to hold excess wire and balance connectors to the bottom of the board. I will come up with a more permanent solution soon, but this works for now.

Mounted battery packs

One of the most important details of this setup is that it is important to verify that the voltage of all battery packs are close to one another. It would be detrimental if one full pack was connected in parallel to a pack that is empty.

Lighting System

You may have noticed the LED lights that appeared in the pictures above. This system is designed by Third Kind. I was not impressed with many aspects of their design, namely the fact that the LED strip must be disconnected from the battery in order to recharge. I designed a lighting system based on WS2812 LEDs and an Arduino.

Power Switch and Mode Button

The lighting system runs from an internal LiPo battery that was salvaged from the Third Kind system. It was a 3S pack that I reduced to 2S. Two LM7805 linear regulators provide 5V for the LEDs and Arduino. The LEDs are split across two 5V rails to avoid exceeding the current of one of the regulators when running at max brightness.

In order to ensure that the battery does not reach an overly discharged state, the ADC is used to measure the battery voltage through a resistive divider. When it reaches a pre-determined cutoff voltage, the LEDs will switch to a low duty-cycle flashing mode to indicate that the main power should be switched off and the lights recharged. Currently charging is performed using the same 1010B+ charger used for the external battery packs.

Lighting system with the box open

The construction is very simple with point-to-point used for all components. The Arduino is wrapped in shrink tube with the programming headers exposed. I decided to stick with the Arduino bootloader for the simplicity of updates. I am not a fan of the boot delay and may consider loading code using my AVR-ISP directly.

The mode button allows switching between various animations. So far I have only wrote one, but have plans for a few more. I may also consider adding an IMU (Inertial Measurement Unit) to detect motion of the board. For example, when slowing down the LEDs could automatically fade to red and glow with a brightness that is a function of deceleration (after some signal processing and filtering, of course).

Initial testing

The lighting system looks great and is extremely bright. I was able to come up with a color of orange that matches the Boosted Board reasonably well. I call it "Boosted Orange" which in the color gamut of the WS2812 LEDs is encoded as #ff1900. I used the light_ws2812 driver from cpldcpu on GitHub. It works very well for the purpose and makes no assumptions about usage of the Arduino framework (which I don't use).

Front-facing and downward facing LEDs

Front and downward facing LEDs were added to the front of the board. This casts a beam towards the ground on the front of the board. The sharp angle between the road and board make it difficult to cast a wide beam.

Now this is a loaded deck ;]

I should note that the board is quite heavy compared to before. The good news is that the board functions the same as it did before, without the batteries. If I am going on a short trip or riding from my bus stop to the office, I can usually leave the extra batteries at home.

Lighting system cover and screws

The LED lighting system is currently charged by removing the door. I expect approximately 2 hours of runtime which means a recharge will be necessary after every couple of night rides. I should probably add a durable external connector to make recharging easier.

FTDI Programming Interface

The Arduino Mini Pro is held into the box with a small piece of high-strength velcro.

A Warning

Using any of the information contained in this article to modify your Boosted Board will void the warranty. You risk damaging the board and injuring yourself. This article is provided for informational purposes only. If you decide to perform the same modification you accept any liability associated with doing so.

Proceed with caution and always wear a helmet.