How To Build Your Own Lithium Battery (Part 1 of 2)
Updated: Jun 10, 2019
Don't forget to check out part 2 of this tutorial as well, link here
In this tutorial, I'll walk through step by step how I build a 48 cell lithium battery pack out of 18650 cells. I'll talk about the mechanical structure, how I welded the cells together, setting up the battery management system, and the pushbutton on-off switch. Stay tuned for the next video, where I do a deep dive on the engineering design that went into this battery.
1. Here are the batteries! I selected Samsung 30Q 18650 cells
2. Lay the batteries out in a tray, and measure the voltage of each one to make sure that none are dead. The voltage should be between 3 to 4V
3. Mark with a sharpie the positive and negative side of the batteries for easy identification later.
4. Use the handy 18650 cell holders to hold all the cells together. Just press fit each cell into the right position
5. Push the plastic cell holders onto the opposite side to finish fully secure the cells together.
6. Using the cell holder as a guide, cut nickel busbars strips to length. Cut the strips just a hair shorter than the width of the plastic holder. The nickel cuts with normal scissors
7. The cells are joined together with a spot welder. Do a few test welds to make sure the nickel strips are well secured to the cell tops. A good weld should hold up to a medium strength tug
8. Here is a finished section of the battery pack. Cells are joined in parallel first. Each cell should have 2 spot welds (each spot weld makes 2 marks). Do 1 weld on each cell, and wait a minute or so before welding the second spot on the cells in order to minimize heat generation.
9. Next add the nickel busbar strips that join the cells in series. Use the same approach of 1 weld per cell, wait a minute, and come back to make a second weld. If necessary, add a secondary nickel strip to the serial connection in order to boost up the current carrying capability.
10. If the spot welder starts making bad welds, or starts to look really black (oxidized), then give it a quick file. The weld tips are made of copper, so they'll clean right up.
11. Finish welding up the rest of the cells in a similar fashion. It's easier to build the pack up in smaller sections, and only join them at the very end.
12. Lay down nickel strips on the last parallel cell group on the battery (This needs to be done for both the plus and minus output ends). Using painters tape, lift off the busbar.
13. Solder on the output wires to these busbars. We used the tape method, since soldering generates a lot of heat, and heat is bad for the cells. By doing this step offline, we don't risk damaging the cells.
14. Spot weld on this end battery connection.
15. Check the battery voltage from end to end at this point to make sure it's what was expected. If it's not the correct voltage, work your way through all the connections and make sure nothing was missed.
16. Solder up the battery output connection to the battery management system (BMS). The BMS makes sure the cells are charged, and discharged, without causing any damage to the pack.
17. Connect all the balance lines of the BMS to the serial connections of the battery
18. These will have to be soldered on, so make sure the wire is joined far from the cell ends to minimize the heat absorption by the 18650 cells.
19. Starting to look like a battery pack! At this point, an electronic switch was wired in series with the output lines to allow the whole pack to be turned on and off. You'll notice I have painters tape over the battery ends, this is done to reduce shorting out the cells if I accidentally drop a tool, or anything conductive, on the ends of the cells.
20. To help hold things together, zip tie the pack together in a few spots. This keeps all the wires held in tight, and provides extra mechanical support.
21. On the final output lines, solder on the connector that will be used. Make sure sufficient solder flows into the joint. It'll take some time for the soldering iron to heat up the connector and cable, since they're both quite large.
22. Slide some heat shrink over the ends to reduce the shorting risk, and as well provide some strain relief to the solder job. Colored heat shrink will also help identify the positive and negative outputs.
23. Use a large piece of heat shrink over the whole pack as the final layer of protection. This makes sure the cell ends won't short out to foreign objects, and provides further mechanical support.
24. Install your new battery!
Part 2:
Want to learn the details and engineering behind the build? Check out part 2 of this article where I do a deep dive on the design.
Links
(Amazon are affiliate links and won't cost you a thing, but will help me make new content):
Spot Welder (Used this exact one for the build!): https://amzn.to/2QvoyAL
Small Pack Nickel Strips (this size fits perfectly in cell holders): https://amzn.to/2Qpcl0o
Large Pack Nickel Strips (this size fits perfectly in cell holders): https://amzn.to/2EBN1zz
Cell Holders: https://amzn.to/2EBNG3Y
XT90 Connector (Used in this build): https://amzn.to/2KiQneq
Battery Life Display: https://amzn.to/2VWeEJM
Chargeport: https://amzn.to/2YX6EKl
16AWG Wire: https://amzn.to/2W2tsGw
14AWG Wire: https://amzn.to/2MgYhIc
12AWG Wire: https://amzn.to/2YUFC6b
10AWG Wire: https://amzn.to/30RJgzF
8AWG Wire: https://amzn.to/2MiThCH
22AWG for signals lines: https://amzn.to/2W2tsGw
Electronic Switch: https://amzn.to/2VWAufS
18650 Battery Cells: https://www.imrbatteries.com/samsung-30q-18650-3000mah-15a-flat-top-battery/
Awesome DIY battery book: https://amzn.to/2MoFEC1
Don't forget to check out part 2 of this tutorial as well, link here