I wanted to build a battery pack for my next upcoming E bike project. At this point I was working on E scooters and riding around on the gas bikes I had previously built, but I wanted to transition from working with gas bikes to something fully electric. I don't have a crazy budget, so my goal was to complete an E bike that was affordable and capable of keeping up with traffic on the road.

It started with a Tesla Battery module off of a 2014 Tesla Model S 3 of my friends and I chipped in to purchase. The module came at a cost of $200 and contained a total of 444 18650 cells. Each of us received 100 cells ($0.5 Per cell) and the extra went to my friend who gave us the ride. This Module was configured in a 6s74p, 22.8V nominal voltage, providing a whopping 5.3Kwh of energy capacity. I reversed this formula to find the Ah per cell by taking 5300/22.8 to give me the total Ah of the pack which was 232Ah then dividing that by 74 which resulted in 3.14Ah per cell. This is slightly different than the 3.3Ah listed on the website.

The process for removing and cleaning the cells was a complete pain in the ass. We had to strip off the aluminum plates providing support for the module from the sides then begin to chisel and pry off the glued plastic support pieces on the top and bottom of the module. After removing the cells we had to pick off any remaining glue chunks, use acetone to dissolve the remaining glue and clean/polish the surface of the top and bottom portion of each individual cell. After cleaning each cell we did a simple voltage test as well as an internal resistance test by measuring the voltage drop through a 10 ohm resistor and on top of that we packaged the cathode area to prevent shorting with any cells while it was in a storage bin. We did that for each of the 444 cells and it took about 2-3 weeks (given our workload) to fully disassemble the pack for our use of the cells. Below is an image of the battery module being disassembled and yes we did end up loosing 2 cells in this caveman removal process. We used Vice Grips, Flat Head Screwdrivers, and Hammers to pry and break off the large tray of plastic that was glued onto the batteries.

After the cleaning and testing process for the cells, we distributed them and I got to work on my battery pack specifically

The materials for completing this battery pack includes all of the following

• Xt60, Xt30, JST 3pin, and Barrel Jack 5.5mm Connectors (Power Charge/Discharge, Ignition, & Charging Port)

•  8 (4x5 18650 Cell Holders)

• 16s 40A Li-Ion BMS

• 2p Nickel Strips

• Battery Pack and Battery Cell Heat shrink

• Spot Welder

• 14 Gauge Wire (Red & Black)

• 18650 Terminal Insulators

My first Task was to heat shrink all of the batteries. I slipped over the 18650 cell covers on all of my 80 cells and took a heat gun with a wide spread to do 20-25 cells at a time. After the heat shrinking process I rolled each one over with the heat gun to make sure the heat shrink was evenly done on all sides. After heat shrinking each cell, I laid the cells in a snake pattern onto my 18650 cell holders, alternating polarity as you go down the line.

Below are attached images of the process

After getting the battery pack arranged, my next step was to begin the spot welding process. I started with spot welding the terminal bars on and then the Series connections. The nickel bus bars I purchased was specifically designed for a rectangular battery form factor so all I had to do was cut them to size and spot weld them across. The best technique for the spot welder was to sharpen the tip and hole the portable spot welder at a 30°- 45°. I was able to get some pretty clean spot welds from that small machine and the bus bars were firmly welded together.

After spot welding across, I had to connect the 16s BMS to the pack. I first designed a mount to 3d print that would isolate the BMS from the pack to prevent any sort of contact with the board. I followed the wiring diagram that the manufacturer provided as well as understanding the base level components and how the BMS functions

Basically onboard the BMS they have a charging side and discharging side MOSFET, some resistors, and zener diodes that only allow current to flow in one direction and bleed off after that threshold has been reached. This is a common port BMS where both the charge and discharge ports connect to the same negative terminal on the BMS. The Micro controller onboard determines when to engage the bleed resistors after the voltage passes 4.2V per cell group. The balance leads then goes to each of the cell groups in series since this is a 16s BMS there will be 17 leads because there will be one extra lead for ground. After attaching all the leads in proper order, I did a quick check with the multi meter to make sure that I had voltage coming out from the common port discharge terminal. The next step was to pack everything together with a special kind of high temperature resistant tape called Kapton tape.

Below I have attached images of the spot welds, the BMS, and the wiring diagram.

I ended up choosing the XT60 connector for discharge as I am drawing a maximum of 35 Amps and the 60 in XT60 stands for 60 Amps. For charging I chose to go with a 5.5mm Barrel Jack connector as that is what came on the 67.2V charger. The charger is pushing 2 Amps so I used an XT30 connector to bridge the gap in case I were to swap to a charger with a different connector in the future.

The final thing to do was to do a quick charge and discharge test to make sure the battery was fully functional. I used the charger to fully charge the Battery to 67.2V then I hooked it up to a controller and motor setup and ran that for about 20-30 minutes to discharge the battery pack. After conducting the test, the final step was to pack up the battery with some large PVC Heat shrink to protect the battery pack from scratches and any minimal external damage. 

The battery pack is now complete, below are all the images of the process