DIY E-Bike Build

Note: Bike is missing the battery usually mounted on the rear rack in this picture.

This is a post detailing the build of my mid-drive e-bike, consisting of a home built battery, and a Baofeng BBS02 mid-drive unit. The core component, a custom lithium-ion battery pack, proved to be a significant undertaking with its own set of hurdles.

Completing the Battery Build

I finished assembly of the battery using a modified version of my v1 spot welder. I had narrowed down the root cause of failures on that board to the poor isolation of power between the control circuitry and the switching circuitry for the spot welder, this is understandable given the high transient current involved. I started by removing the power input to be shared with the welding leads. I also moved the mosfet driver to a separate PCB to further isolate that component from the switching transients. These updates allowed me to complete the assembly of the pack. I also added some copper sheets that were welded between the nickel strips, this was an attempt to increase conductivity of the pack and decrease losses though the effect was never measured.

After a few tests it started to become clear one of the cells was dying. A bank of cells was always too low and would be the first to charge, a clear indication of a dying or dead cell in a row. Some time had passed since my first welding experience and affordable spot welder alternatives were starting to pop up on Amazon/AliExpress. These welders were selling for ~$50 that was essentially identical to the one I was building myself and they included the battery / doubled as a power bank.

Not the same model, most of these welders are fairly similar.

While the Amazon welder produced slightly weaker welds than my custom unit, cranking the power to maximum got the job done effectively. With the new welder I was able to quickly finish assembly of cells in the pack. Then adding the series cells balancing wires and main power wires, and overwrapping with a large diameter heat shrink. Finally, I completed the battery assembly by integrating a Bluetooth-enabled Battery Management System (BMS).

Safety is paramount with li-ion batteries so an additional fire-resistant Kevlar fabric cover was added for extra protection. The finished battery pack is secured in a soft bag attached to the rear bike rack.

Motor and Bike Build

For the drive system, I went with a fairly standard DIY e-bike approach. I selected a Bafang BBS02 mid-drive motor rated at 750W—plenty powerful for throttle-only cruising while offering enough torque for hill climbing with pedal assist.

In retrospect, my biggest mistake was choosing a used Walmart bike from Craigslist as the platform. This decision would come back to haunt me later, as I'll explain below.

Real-World Usage

I was able to start testing with smaller rides working up to longer rides. Eventually with my longest rides being my daily commute, about 11 miles.

I found that the battery capacity was sufficient for the commute to work in the morning allowing significant use of the throttle without concern. The return trip required more conservation, particularly due to a significant hill climb. I found myself pedaling considerably more on these return journeys.

On a couple of occasions, I completely depleted the battery on the way home. The extra mass and rolling resistance was made abundantly clear and getting home required quite a bit of effort!

Bike Issues: The Weakest Link

The budget bike chassis revealed numerous limitations when paired with the powerful motor:

• Multiple chains wore out out broke under high torque, eg. climbing hills in the wrong gear
• The rear derailleur hanger bent repeatedly under similar high-load conditions
• The brakes proved inadequate for the speeds the e-bike could achieve
• The frame was slightly too small for my height
• Without rear suspension, I had to install an oversized comfort seat to compensate for road vibration

These issues highlighted how choosing a cheaper bike significantly reduced the utility of the entire setup. The constant reliability concerns made me hesitant to rely on the bike for fear of becoming stranded.

Future Plans: Upgrade or Dismantle?

I'm currently at a crossroads with this project. I haven't decided whether to upgrade the system or dismantle it to reclaim space. If I choose to upgrade, my priorities would be:

• A higher-quality bike frame and components for improved reliability
• A better mounting solution for the battery pack

The DIY e-bike journey has been educational and rewarding, despite the challenges. The electrical system performed admirably, while the mechanical platform proved to be the limiting factor. It's a clear demonstration that an e-bike conversion is only as good as its weakest component.