Ebike Conversion

Goal: Design and build an eBike that is very fun to ride and has battery capacity for longer trips around Boston.

Background Research and Component Specification

The first problem I had to tackle was choosing a motor. I initially set up a free body diagram like I’ve done in class and solved for the force required to move the bike. This method gave me a rough estimation but luckily, I found this template online that took more variables into account (range, weight, resistance factors, etc.) and decided to choose a 500W motor.

I next had to decide what voltage to run the bike on. I knew that I wanted low-end torque over high-end speed, and according to the power law, a lower voltage results in a higher peak current and thus higher torque. Because of this, I chose to run a 24V system. The peak current came out to 20.83A.

Battery Design

Now that I knew I was using a 500W 24V motor, it was time to design the battery. The cheapest reputable lithium ion 18650 cells I could find had a nominal voltage of 3.60V, a maximum discharge of 7.5A, and a capacity of 2.550Ah. I decided that a battery capacity of 15Ah would be ideal and give me about 1.5 hours of charge under average riding conditions. To create a 24V 15Ah battery, I aligned 7 cells in series and 6 cells in parallel. In battery terms, mine would be 7s6p.

I chose a BMS that was rated for 24V/30A and followed my 7s configuration. After plenty of YouTube tutorials, I came up with the following schematic for the battery.

Building the Battery

Because this was my first time building a battery, I thought it would be best to use a battery connections kit rather than spot welding and hot gluing all the cells. This way, I could remove individual cells should I damage one, or add more cells if 15Ah proved to be too small. The image on the right shows all of the cells properly connected, as indicated by the ~24V multimeter reading.

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I then wired in the BMS and insulated the entire battery in foam to protect it from vibration or drops. The foam limits air flow and could cause a battery to overheat in some cases, but the sources I found online said a battery this small should remain cool enough. The entire battery was then wrapped in heat shrink (not pictured) to make it water-resistant.

Component Layout and Wiring

Having chosen all of my components, it was then time to decide how they would connect to each other and where they would be on the bike. I had several layouts drafted but ultimately decided this was the best one because the chain was completely free of my legs and feet. It is important to note that the components are not drawn exactly to scale. This drawing was made prior to handling the parts, so I had to estimate sizes. It will be shown later that several components were much larger than expected, thus altering the layout.

To test the wiring, I substituted the battery with a power supply and connected as according to the diagram. It worked!

Mounting the Motor

As seen in the component layout sketch, I planned to fix the motor to a rear rack that I attached to the bike frame. I held the motor down using zip ties to get an idea of where everything would line up. At this point, I had removed all pedal-drive related components of the bike as well as converted it to single speed. The video below shows the motor zip-tied to the frame and the chain attached to the freewheel and motor.

I decided that the easiest way to run the chain would be through one of the pre-cut slots on the rack, so I designed the adaptor plate to these specifications. At this point in the process, I overlooked the uneven surface on the rack which would cause future problems seen below.

Luckily, the Northeastern capstone lab had extra capacity over the summer and was able to help waterjet and tap my part.

Putting Everything Together

I designed this schematic to help with wiring the components together. In the battery box, I chose to include a 30A fuse for safety and a battery indicator. The motor controller I chose is unique in the way that it takes in charge and then sends it through the battery's discharge wires to charge it. If the motor controller didn't have this function, I would have had to wire the charge wires directly to the battery.

Once I had assembled the battery and wired in the motor controller, I realized that my initial layout was not going to work. I could not find a rigid battery box and motor controller box that would both fit in the bike frame. I was about to resort to cutting up Tupperware when my girlfriend suggested I look into travel makeup cases. By using a makeup case, I was able to combine the two housings into a single one and have it fit in the frame.

The wires run from the battery to the motor controller beneath the divider and out to the throttle and motor through the zippers in the case.

The Final Product (For Now) and What Went Wrong

During my first ride of the bike I could not exceed ~2mph; something was clearly wrong. I got off to examine the bike more closely and took the following pictures.

The motor and adaptor plate drastically shifted during the initial acceleration and could not smoothly deliver power afterward. I had initially planned for the motor nuts to sit flush against the rear rack, but the lip prevented me from doing so. To fix this problem, I added two spacers below the plate. In doing this, the downward force of the chain caused the entire plate to cantilever and be pulled over the side. The chain was no longer aligned with the freewheel and broke loose whenever additional power was applied.

Future Improvements

In order to get the motor to smoothly deliver power, I need to redesign or add to the adaptor plate. I think the easiest solution would be to add material to the rear rack to make the surface flush. This would prevent me from having to use spacers and would make the four motor nuts the four points of contact.

It is definitely disappointing that I will have to head back into the workshop, but exciting nonetheless. Even though the bike isn't 100% functional, I still learned many great skills and had a lot of fun working with my hands. I am not giving up and I am still looking forward to zooming around Boston.

Cost Breakdown and BoM