No-Sweat Biking

After realizing that skateboards are pretty damn dangerous (following my e-board build), I decided to try and make my own e-bike for the lowest amount of money possible. Although an electric skateboard may have a smaller footprint, I think that e-bikes are a lot more practical. For me, I can actually ride a bike around town without getting thrown off when the smallest of pebbles get lodged under the wheel. Additionally, the area behind the rider on a bike can be used for extra storage. 

There are a multitude of electric conversion kits for standalone bikes, but most overcharge for basic components that can be found off the shelf for lower prices. An e-bike is extremely simple. The whole system consists of a motor, battery, motor controller, and other input devices (throttle, brakes). My first decision was about which motor type I wanted to use. In general, the two main motor variants are front/rear hub and center drive. To minimize cost, I decided on a front hub motor. Although not the most powerful, a front hub is the easiest to integrate (no need to modify pedals or gears). After browsing around on eBay, Amazon, sketchy Chinese sites, Craigslist and Facebook Marketplace, I found someone selling a used e-bike kit for a lot lower than market value. The setup included a 36V 350W front hub motor, motor controller, and various accessories such as a twist throttle and e-brakes. In hindsight, I would've liked a slightly more powerful motor, but it still works well for cruising around town (for reference, 72V 1000W motors are essentially the high end of the motor spectrum in terms of power). 

Next, I started thinking of the battery setup. I knew that I needed a pack to suit the motor: supplying a nominal voltage of 36V and an amperage of around 9-10A (350W/36V). From experience building drones and model airplanes, my first choice was a lithium polymer battery. Although widely available, these batteries tend to get expensive when it comes to voltages as high as 36V (most drones run anywhere from 7 to 20ish volts). Also, LiPo batteries tend to wear quickly after many discharge/charge cycles. Because of these factors, I decided to use 18650 (18mm diameter, 65mm long, 0 for circular shape) lithium-ion batteries. These look like larger AA cells and can be found in most older laptops. Even Tesla uses li-ion batteries in their cars (although not 18650s specifically). Depending on the manufacturer, 18650 cells can last a long time and deliver a large capacity in a relatively small package. 

As I soon realized, 18650s aren't the cheapest batteries. That's when I made the decision to harvest old laptop batteries. While this can be dangerous (old cells typically aren't safest), you'd be surprised at how many perfect cells can be recovered. The only real downside with using these recycled batteries is the time it takes to check each cell. I also found it hard to get my hands on old laptop batteries as most people I knew had thrown their computers out (except my grandparents, where I got the majority of cells). After deciding on a battery layout consisting of 50 cells, a 10s5p setup, I began the long process of acquiring the cells. For the pack design, 10s5p means that there are 10 cells in series (3.6V nominal voltage of 18650s x 10 = 36V and 2000mAH capacity x 5 = 10AH - most smartphones are around 2-3AH). To test each cell, I used my drone battery charger to charge and discharge the batteries. I removed any cells that started to overheat during the process and also observed the cell voltages following charging after multiple days to make sure the batteries held their voltage. I also purchased a BMS (battery management system) that would allow me to charge the cells safely and make sure none of them burnt out. Most li-on packs are assembled using nickel strips and a spot welder to make connections. Without a spot welder and a large budget, however, I decided to use a pack assembly kit. The kit was made up of plastic battery holders with screw threads at the terminal ends so that cells could be connected by bridging these threads with nickel strips (and securing them with small bolts). Following a long process of building and checking connections (nearly ruining the whole setup by shorting two cells with a screwdriver), I wrapped the pack in several layers of foam and finished the whole thing off with some gigantic heat shrink tubing. 

Ironically, the bike was the last part of this build that I got. I managed to pick up a decent hybrid for free from a friend that was moving. After testing all the electronics, I started mounting them on the bike. I noticed how the provided e-brakes not only clamped down on the wheels when triggered but also sent a signal to the motor controller to stop the motor. Although the wires running along the frame gave it away, the front hub blended in perfectly. I rode the setup around my neighborhood and got up to 15-20 mph with minimal pedaling. Following some cable management and minor modifications, I started going on longer rides for range testing. I managed to go about 20 miles with not much physical exertion and still had about 25% battery left (voltage in this battery goes from 42V fully charged to 30V completely drained). So far, I've had no major problems with this build and it works perfectly to get around campus (surprisingly, I still have some in-person classes in the age of Zoom). The only real negatives come mostly with the bike itself. The caliper brakes aren't the best (looking to switch to disc later) and the gears don't change very fast. Other than that, the battery is a bit heavy but does provide significant range. 

In the future, I may swap in a more powerful motor, improve the braking situation, add more weatherproofing, and attach a key-start system. Not to mention, the cheap charger I bought doesn't really display charge percentage/rate so it's slightly inconvenient. Also, I had an idea to add a generator to the pedals so I could charge the battery when pedaling (although there might be too much resistance/weight). Overall, this project went a lot better than I expected and I'm genuinely satisfied with the final outcome. See pictures below. 

Motor Controller mounted below the seat

Integrated Motor in Wheel Hub

Fully sealed Custom Battery