• Adam Bender

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

Samsung 30Q 18650 lithium battery cells in a shipping box

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

Using a multimeter to read the voltage of an 186500 lithium cell

3. Mark with a sharpie the positive and negative side of the batteries for easy identification later.

Marking the polarities of 18650 lithium battery cells for easy identification

4. Use the handy 18650 cell holders to hold all the cells together. Just press fit each cell into the right position

Pressing 18650 lithium battery cells into plastic holders to secure them in place

5. Push the plastic cell holders onto the opposite side to finish fully secure the cells together.

Adding the top plastic cell holder to secure 18650 lithium cells

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

Cutting nickel busbar using the 18650 lithium cell holder as a template for the cut length

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

Sunnko spot welding being used to weld nickel busbar strips to 18650 lithium battery cells

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.

A group of 8 lithium 18650 battery cells that have been spot welded together using nickel strips

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.

Adding nickel busbars to connect 18650 lithium cells together in series

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.

using a file to clean up the welding electrodes on a spot welder used to weld 18650 lithium cells to nickel strips

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.

A 18650 lithium battery pack held up in front of a spot welder that was used to make the battery

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.

Using tape to hold nickel busbar strips to a lithium battery pack

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.

Soldering silicone wire to nickel busbars that will be used as battery outputs for a lithium pack

14. Spot weld on this end battery connection.

Silicone wire leads coming off of a battery pack made of 18650 lithium cells

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.

Using a multimeter to measure the output voltage of an 18650 lithium cell battery pack

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.

Soldering large gauge silicone wire to a battery management system used on a lithium ion battery pack

17. Connect all the balance lines of the BMS to the serial connections of the battery

Cutting balance leads that are used with a BMS to control the charge of a lithium ion battery pack

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.

Soldering balance leads from a BMS to a lithium ion battery pack made of 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.

A DIY lithium battery pack made of 18650 cells. A BMS and electronic control unit sit on top of the battery pack

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.

A homemade lithium 48V battery pack with BMS and electronic control switch. Cells are 18650

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.

Soldering an XT90 connector to silicone wire

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.

Adding heat shrink to a soldered silicone wire and XT90 connector to protect against shorts

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.

Adding heat shrink to a battery pack made of 18650 lithium cells to provide mechanical protection

24. Install your new battery!

A custom 18650 lithium battery pack sitting inside of an electric scooter

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

Sunnko spot welder used to make custom lithium battery packs with 18650 cells

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

Pure nickel busbar strips used with a spot welder to join 18650 lithium cells together electrically

Cell Holders: https://amzn.to/2EBNG3Y

plastic 18650 lithium ion cell battery holder

XT90 Connector (Used in this build): https://amzn.to/2KiQneq

XT90 connector used in a lithium battery pack

Battery Life Display: https://amzn.to/2VWeEJM

Digital power gauge used to tell the output of a lithium ion battery pack

Chargeport: https://amzn.to/2YX6EKl

chargeport used to charge a lithium ion battery pack

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

Silicone wire on a reel


Electronic Switch: https://amzn.to/2VWAufS

Electronic on off switch used in a custom lithium battery pack

18650 Battery Cells: https://www.imrbatteries.com/samsung-30q-18650-3000mah-15a-flat-top-battery/

Samsung 30q 18650 lithium ion battery cells

Awesome DIY battery book: https://amzn.to/2MoFEC1

Book on how to make your own lithium batteries

Don't forget to check out part 2 of this tutorial as well, link here

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